Lawn Seed Calculator UK

Grass Seed Coverage Rates by Type

Different grass varieties require different amounts of seed per square metre. The table below shows the recommended seeding rates for the most common grass types, for both new lawns and overseeding.

How to Use This Lawn Seed Calculator

Getting the right amount of lawn seed doesn't need to be complicated. Here's how to use this tool and get an accurate result every time.

Step 1: Measure Your Lawn

Use a tape measure or pace out the length and width of the area you want to seed. For irregular shapes, break the area into rough rectangles and add them together. If you already know your total square metreage (or square footage), enter any length and width combination that equals that area.

Step 2: Choose Your Grass Type

Perennial Ryegrass is the most common choice in the UK — it's hard-wearing, germinates quickly, and looks great in most lawns. If your lawn gets a lot of shade, Fine Fescue is a better option. In warmer climates (US South), Bermuda Grass is the standard.

Step 3: New Lawn or Overseeding?

Starting from bare soil? Select "New Lawn" — you'll need more seed to establish full coverage. If you're thickening up an existing lawn with thin patches, "Overseeding" uses about half the amount. For small bare spots, "Patching" uses a higher rate to ensure quick coverage in concentrated areas.

Step 4: Add 10% Extra

Our calculator automatically includes a 10% buffer. This accounts for uneven spreading, seed that birds eat, and areas near edges where you'll want heavier coverage. It's always better to have a little extra than to run short mid-project.

After seeding, you'll want to feed your lawn — use our lawn feed calculator to work out the right amount, or our fertiliser calculator for general garden feeding.

How to Overseed a Lawn — Step by Step

Overseeding is the fastest way to thicken up a tired, patchy lawn without starting from scratch. It works by sowing new grass seed directly into an existing lawn, filling in bare spots and introducing stronger grass varieties. Here is exactly how to do it in the UK.

When to Overseed

The best time to overseed in the UK is early September to mid-October. Soil is still warm from summer, autumn rain keeps it moist, and there is less competition from weeds. Spring (March to April) is the second-best window, though you will need to water more if April is dry.

Step 1: Mow Short

Cut your lawn to 25mm (1 inch) — shorter than normal. This lets sunlight reach the soil surface where the new seed needs to germinate. Collect the clippings rather than mulching them.

Step 2: Scarify

Rake out thatch (dead grass and moss) with a spring-tine rake or powered scarifier. This creates gaps in the surface for seed to make contact with soil. Scarifying is the single most important step — seed sitting on top of thatch will not germinate.

Step 3: Aerate (Optional but Recommended)

If your lawn is compacted (common on clay soils or high-traffic areas), spike the surface with a garden fork or hollow-tine aerator. Push the fork in 75–100mm deep every 100–150mm. This improves drainage and gives roots space to grow.

Should I Mix Grass Seed with Topsoil When Overseeding?

This is one of the most common questions gardeners ask — and the answer depends on the condition of your lawn.

For general overseeding (thickening up a lawn that is mostly grass), you do not need to mix seed with topsoil. Simply scatter the seed over the scarified surface at 15–20g per square metre and rake it in lightly. The seed needs light to germinate, so burying it under a thick layer of soil can actually reduce germination rates.

For patching bare spots, mixing seed with a thin layer of topsoil or lawn topdressing (3–6mm deep) gives much better results. The soil holds moisture around the seed, protects it from birds, and creates a better growing medium than bare, compacted ground. Mix roughly 1 part seed to 4 parts sieved topsoil by volume, spread the mixture evenly over the bare patch, and firm it down gently with the back of a rake.

Never bury seed deeper than 6mm. Grass seed needs light to germinate. A common mistake is covering seed with 10–20mm of topsoil — this is too deep and most seed will fail. If you are using topsoil, think "dusting" not "covering".

Step 4: Spread the Seed

Use the calculator at the top of this page to work out how much seed you need. For overseeding, the rate is typically 15–20g per square metre — about half the rate for a new lawn. Spread evenly by hand or with a seed spreader, going in two passes at right angles for the best coverage.

Step 5: Topdress (Optional)

For the best results, scatter a thin layer (3–5mm) of lawn topdressing or sieved compost over the seeded area. This protects the seed, retains moisture, and improves soil-to-seed contact. Use our soil calculator to work out how much topdressing you need.

Step 6: Water

Water lightly but frequently — twice a day in dry weather — for the first 2–3 weeks. The soil surface must stay moist until seedlings are established. Once the new grass is 50mm tall, reduce to watering every 2–3 days. After 6–8 weeks, the new grass should be fully established and you can return to normal mowing.

Overseeding Rates by Grass Type

After overseeding, give your new grass a boost with the right feed — our lawn feed calculator works out exactly how much you need.

Once your lawn is established, make the most of your garden — our UK planting calendar shows what vegetables to grow each month.

Frequently Asked Questions

Types of Grass Seed for UK Lawns

Choosing the right grass seed mix is the single biggest factor in whether your lawn succeeds or struggles. I see this mistake constantly — people grab the cheapest bag off the shelf without checking if the mix actually suits their garden. A shade-tolerant fescue blend will die in full sun, and a ryegrass-heavy mix will thin out under trees. Here is what you actually need to know about each type.

Hardwearing / Family Lawn Mix

This is what most UK gardeners should buy. A hardwearing mix is typically 60-80% perennial ryegrass with 20-40% strong creeping red fescue. The ryegrass handles foot traffic from children, dogs, and garden furniture, while the fescue fills in gaps and tolerates closer mowing. Sow at 35-50g per square metre for a new lawn. Perennial ryegrass germinates in just 5-10 days, so you will see results quickly. This type thrives on most UK soils and is the standard choice for back gardens, play areas, and any lawn that gets regular use.

Fine / Ornamental Lawn Mix

If you want that bowling-green finish, you need a fine lawn mix. These are 80-100% fescue — usually a blend of chewings fescue, slender creeping red fescue, and browntop bent. The result is a dense, fine-textured lawn that looks stunning when mown at 15-20mm. The trade-off is that ornamental mixes are less hard-wearing, need more frequent mowing (twice a week in summer), and recover slowly from damage. Sow at 25-35g per square metre. Germination takes 10-14 days, and the lawn needs a full season to establish properly. Only choose this if you are genuinely committed to regular maintenance.

Shade-Tolerant Mix

Under trees, next to fences, north-facing gardens — shade is the number one reason lawns fail in the UK. A shade-tolerant mix uses hard fescue and creeping red fescue, both of which photosynthesise efficiently in low light. Some premium mixes add a small percentage of wood meadow grass (Poa nemoralis), which is specifically adapted to woodland conditions. Sow at 30-40g per square metre. Even with the right seed, shaded lawns need extra care: raise the mowing height to 40-50mm, reduce foot traffic, and keep fallen leaves cleared in autumn so the grass gets whatever light is available.

Drought-Resistant Mix

With UK summers getting drier — and hosepipe bans becoming more common — drought resistance matters more than it used to. Tall fescue is the star here. Its roots grow 150-200mm deep compared to 50-100mm for ryegrass, allowing it to access moisture that other grasses cannot reach. A good drought mix blends 60-70% tall fescue with 30-40% perennial ryegrass for wear tolerance. Sow at 35-45g per square metre. The downside is that tall fescue has a coarser blade than fine fescue, so the lawn will not look as manicured. For most family gardens, that is a worthwhile trade.

Wildflower Meadow Mix

This is not technically a lawn seed — it is a deliberate shift away from mown grass towards biodiversity. A wildflower meadow mix contains 80% native wildflower seeds (ox-eye daisy, bird's-foot trefoil, red clover, yarrow) and 20% fine grass. The critical difference is the sowing rate: just 4-5g per square metre, because wildflowers need space and low competition to establish. Wildflower meadows need poor, unfertilised soil — never add fertiliser or lawn feed, as the nutrients will favour grass over flowers. Mow once in late summer (August), remove the cuttings, and let the area develop naturally.

How to Sow Grass Seed — Step by Step

Sowing grass seed is straightforward, but the details matter. I have seen people waste entire bags of seed by skipping soil preparation or sowing at the wrong time. Follow these steps and you will get germination rates above 85%, which is about as good as it gets in real-world conditions.

Step 1: Prepare the Soil

Remove any existing weeds, stones, and debris from the area. If the soil is compacted, dig it over to a depth of 150-200mm with a fork or rotavator. Then rake it to a fine tilth — the surface should feel crumbly, with no lumps larger than a grape. Firm the soil by treading it (walking over the entire area on your heels), then rake level again. This sounds tedious, but seed-to-soil contact is everything. Seed sitting on lumpy, uneven ground will dry out and fail. Use our soil calculator if you need to add topsoil to build up levels.

Step 2: Check Soil Temperature

Grass seed needs soil temperatures of at least 8-10 degrees Celsius to germinate. In most of the UK, that means the earliest you can reliably sow is mid-March in the south and early April in the north. Use a soil thermometer pushed 50mm into the ground, or check morning temperatures — if you can comfortably hold a handful of soil without it feeling cold, it is warm enough. The best months for sowing grass seed in the UK are April-May and September-October.

Step 3: Divide Your Area into Sections

Weigh out the total seed you need using the calculator at the top of this page, then divide both the seed and the lawn into equal sections. For example, if you have a 40m² lawn needing 1.8kg of seed, mark off four 10m² sections and weigh 450g for each. This prevents the classic mistake of running out of seed halfway across the lawn with half the bag already used.

Step 4: Sow in Two Passes

For each section, take half the allocated seed and sow it walking in one direction (north to south, for example). Then take the remaining half and sow it walking perpendicular (east to west). This two-pass method gives far more even coverage than a single pass. A handheld seed spreader helps enormously here, but you can do it by hand if you practise the wrist-flicking motion a few times first.

Step 5: Rake In Lightly

After sowing, rake the entire area very gently with a spring-tine rake. The goal is to cover the seed with no more than 5mm of soil — just enough to hide it from birds without burying it too deep. Grass seed needs light to germinate, so anything deeper than 6mm will dramatically reduce your success rate. After raking, firm the surface again with a light roller or by treading on a plank laid flat on the soil.

Step 6: Water Gently

Water the entire area with a fine spray — a garden sprinkler on its lightest setting or a watering can with a rose attachment. Never use a jet of water, as it will wash seed into clumps and leave bare patches. Keep the top 25mm of soil consistently moist for the first 2-3 weeks. In dry weather, that means watering lightly twice a day (early morning and late afternoon). Once seedlings are visible, reduce to once daily.

Step 7: First Mow

Wait until the grass reaches approximately 75mm (3 inches) tall before the first mow. Set your mower to its highest setting — around 50mm — and only remove the top third of the blade. Never scalp new grass. Use a rotary mower rather than a cylinder mower for the first few cuts, as cylinder mowers can pull young seedlings out of the ground. After the first mow, gradually reduce the cutting height over the next 4-6 weeks until you reach your preferred lawn height (usually 25-40mm for a family lawn).

Overseeding vs Starting a New Lawn

One of the most common questions I get asked is whether to overseed an existing lawn or rip it up and start again. The answer depends on how much healthy grass is left and how much effort you are willing to invest.

When to Overseed

Overseeding is the right choice when your lawn still has 50% or more healthy grass coverage but has developed thin patches, bare spots, or areas where moss has taken over. Overseeding is also the best option after scarifying in autumn, as the process of removing thatch inevitably thins the lawn out. The sowing rate for overseeding is 15-25g per square metre — roughly half the rate for a new lawn. This lower rate accounts for the fact that existing grass already occupies much of the space. Preparation is simpler too: mow short, scarify to remove thatch, and sow directly onto the exposed soil. Results are visible within 2-3 weeks and the lawn should look fully restored within 6-8 weeks.

When to Start from Scratch

If more than 50% of your lawn is weeds, moss, bare earth, or coarse grasses (like annual meadow grass or Yorkshire fog), overseeding will not fix the problem. You need to start fresh. Kill the existing vegetation with a glyphosate weedkiller, wait 2-3 weeks, then rotavate or dig over the top 150mm of soil. Level, firm, and rake to a fine tilth before sowing. The sowing rate for a new lawn is 35-50g per square metre, depending on the mix. A new lawn from seed takes 8-12 weeks to establish fully, and you should stay off it as much as possible during that time. If you want an instant result, consider laying turf instead — it costs more but gives you a usable lawn within 2-3 weeks.

Expected Results Timeline

5 Common Grass Seed Mistakes (and How to Avoid Them)

I have helped hundreds of people troubleshoot failed lawns, and the same five mistakes come up over and over again. Every one of them is completely avoidable.

1. Sowing Too Thick

More seed does not mean a better lawn. When you apply significantly more than the recommended 35-50g/m², the seedlings are packed so tightly that they compete for light, water, and nutrients. The result is weak, spindly grass that is highly susceptible to fungal diseases like red thread and fusarium patch. Overcrowded seedlings also develop shallow root systems because they are spending all their energy growing upwards rather than downwards. Stick to the rates — our calculator above gives you the exact figure for your area and grass type.

2. Sowing on Cold Soil

Grass seed needs soil temperatures of 8-10 degrees Celsius to trigger germination. Sowing in February or early March — when air temperatures might feel mild on a sunny day but the soil is still cold from winter — is a waste of seed and money. The seed will sit dormant on the surface, exposed to birds and wind, and may rot before the soil warms up enough. In most of the UK, the soil does not reach a reliable 8 degrees until mid-March at the earliest. Use a soil thermometer or simply wait until you can see consistent daisy growth in your neighbours' lawns — daisies germinate at similar temperatures.

3. Not Watering Enough

Germinating grass seed needs consistent moisture for 2-3 weeks after sowing. The top 25mm of soil must stay damp — not waterlogged, but never allowed to dry out completely. If the soil surface dries out even once during germination, the emerging root (radicle) desiccates and dies. That seed is then wasted. In dry spring weather, you may need to water lightly twice a day. Set a reminder on your phone if necessary. This is the stage where most DIY lawns fail — people sow the seed, water it once, and then forget about it.

4. Mowing Too Soon

New grass seedlings are fragile. Their root systems are shallow and their stems are thin. Running a mower over them before they reach 75mm (3 inches) can rip young plants straight out of the ground, especially on damp soil. Wait until the grass is at least 75mm tall, then set your mower to its highest setting (around 50mm) and only take off the top third. Use a sharp blade — a blunt mower tears rather than cuts, which damages young plants and leaves them vulnerable to disease.

5. Using the Wrong Mix for Your Conditions

A shaded garden planted with a full-sun ryegrass mix will thin out within the first year. A south-facing lawn on sandy soil planted with moisture-loving fescue will brown off every summer. Before you buy seed, honestly assess your garden conditions: how much sun does the area get (full sun, partial shade, full shade)? What is the soil type (clay, loam, sand)? How much foot traffic will the lawn receive? Then choose a mix designed for those conditions. Check our comparison table above if you are unsure which type suits your garden best.

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How Much Grass Seed Do I Need Per m²?

The amount of grass seed you need per square metre depends on two things: what you are doing (new lawn, overseeding, or patching) and the conditions of the area. Here are the rates that work in practice across UK gardens.

New lawns: 35–50g per m². This is the standard sowing rate for establishing a lawn from bare soil. A typical UK lawn mix (ryegrass + fescue) sits at the 35–45g end. Pure ryegrass mixes need slightly more — 40–50g/m² — because individual ryegrass seeds are larger and heavier than fescue seeds, so you get fewer seeds per gram.

Overseeding: 15–25g per m². Because you already have existing grass covering most of the area, you need roughly half the seed rate. Use 15g/m² for lawns that are mostly healthy with just mild thinning. Use 25g/m² for lawns with visible bare patches between existing grass plants.

Shady areas: 40–50g per m². Shade-tolerant mixes (fine fescue, hard fescue) germinate more slowly and grow less vigorously than ryegrass, so a slightly heavier sowing rate compensates. Even with the right seed, expect thinner coverage under heavy shade — reduce your expectations and raise the mowing height to 40–50mm.

Hard-wearing areas: 50–70g per m². Play areas, paths alongside lawns, and areas around washing lines take heavy foot traffic. Sow at a higher rate to establish a denser sward that resists wear. Use a hardwearing ryegrass-dominant mix and expect to overseed these areas annually.

Our grass seed calculator at the top of this page handles all the maths. Enter your lawn area and grass type, and it gives you the exact weight in grams and kilograms, including the 10% extra buffer for waste and bird loss.

Overseeding Your Lawn — A Complete Guide

Overseeding means sowing new grass seed directly into an existing lawn. It is the single most effective way to thicken up thin, tired grass without ripping everything out and starting again. If your lawn is patchy after winter, worn down by foot traffic, or thinning after scarifying, overseeding is the answer.

Why Overseed?

Lawns naturally thin over time. Foot traffic compacts the soil, summer heat stresses the grass, and moss and thatch crowd out healthy plants. After a few years, even a well-maintained lawn develops bare patches and thin areas. Overseeding introduces fresh, vigorous grass plants that fill these gaps and compete with weeds and moss. It also lets you introduce improved grass cultivars — modern varieties bred for better disease resistance, drought tolerance, and colour retention.

When to Overseed in the UK

September to October is the best window. The soil is still warm from summer (typically 12–15°C at the surface), autumn rainfall keeps it naturally moist, and declining daylight reduces weed germination. This combination means your new grass seed germinates quickly, establishes strong roots before winter, and faces minimal competition.

March to April is the second-best window. Soil temperatures need to reach at least 8–10°C before grass seed will germinate reliably. In southern England, this happens around mid-March; in northern England and Scotland, expect early to mid-April. Spring overseeding works well but carries two risks: late frosts can damage young seedlings, and a dry April means you will need to water daily.

Avoid May to August. Summer heat stresses both existing grass and new seedlings. Soil temperatures above 25°C cause many grass species to go dormant rather than grow. Without reliable irrigation, seed sown in summer often fails entirely.

Step-by-Step Overseeding Process

1. Mow the existing lawn short (25mm). This lets sunlight and water reach the soil surface where the new seed needs to germinate. Collect the clippings — do not mulch, as loose clippings block seed-to-soil contact.

2. Scarify to remove thatch. Use a spring-tine rake on small lawns or a powered scarifier on larger areas. Scarifying tears out the layer of dead grass and moss that sits between the living grass and the soil. This step is non-negotiable — seed sown on top of thatch will not germinate.

3. Aerate if the soil is compacted. Push a garden fork 75–100mm into the ground every 100–150mm, or use a hollow-tine aerator. Aeration is especially important on clay soils and high-traffic lawns. It improves drainage, allows air to reach the roots, and creates pockets where seed can settle into the soil.

4. Spread seed at 15–25g per m². Use our calculator above to work out the exact amount for your area. Apply in two passes at right angles — first north-to-south, then east-to-west — for the most even distribution. A handheld seed spreader makes this much easier than sowing by hand.

5. Topdress with a thin layer of compost. Scatter 3–5mm of lawn topdressing or sieved compost over the seeded area. This protects the seed, retains moisture, and improves germination rates significantly. Do not apply more than 5mm — you want a light dusting, not a burial.

6. Water daily for 2–3 weeks. The soil surface must stay consistently moist. Water lightly twice a day in dry weather using a fine spray. Never use a strong jet — it washes seed into clumps and creates bare patches. Reduce to once daily once seedlings are visible.

7. First mow when grass reaches 50mm+. Set the mower to its highest cutting height and only remove the top third of the blade. Use a rotary mower for the first few cuts — cylinder mowers can pull young seedlings out of the ground. Avoid mowing on wet soil, as the mower wheels will compress the surface and damage new roots.

Common Overseeding Mistakes

Overseeding in summer heat. Soil above 25°C prevents germination in most UK grass species. Even if you water religiously, the heat stress kills young seedlings before they can establish. Wait for September.

Not watering enough. The number one reason overseeding fails. If the soil surface dries out during the first 14 days, the emerging root tip (radicle) desiccates and the seed is dead. In dry weather, you need to water lightly every morning and evening — not once, not occasionally, but every single day.

Mowing too soon. New grass seedlings need at least 3 weeks before the first mow. Cutting them earlier tears shallow roots out of the ground. Wait until the new grass is at least 50mm tall, then mow on the highest setting.

Skipping scarification. Scattering seed on top of a mossy, thatchy lawn is wasted effort. The seed cannot reach the soil, so it sits on the surface, dries out, and dies. Scarify first — it is the single most important step.

Grass Seed Types Compared

Different grass seed types suit different gardens, soil conditions, and levels of use. This comparison covers the five most common categories sold in UK garden centres and online.

If you are unsure which type suits your garden, a ryegrass + fescue mix is the safest choice. It handles moderate shade, reasonable foot traffic, and most UK soil types. Only go specialist (shade mix or drought-tolerant) if your garden has genuinely challenging conditions.

When to Sow Grass Seed in the UK

Timing matters more than most people realise. The same bag of seed sown in September will outperform the same seed sown in July by a factor of three or four. Here is what each season actually means for your lawn.

Spring (March–May)

Soil is warming up from winter, natural rainfall is usually sufficient, and daylight hours are increasing — all good for germination. The earliest you can reliably sow is mid-March in southern England, when soil temperatures first reach 8°C. In northern England and Scotland, wait until early to mid-April. The risk with spring sowing is a late cold snap in March or a dry spell in April that requires daily watering. Overall, spring is a solid second-choice window that works well for most gardeners.

Autumn (September–October)

This is the best time to sow grass seed in the UK, full stop. Soil temperatures are still 12–15°C from the summer warmth, autumn rain keeps the ground naturally moist, and weed germination slows as daylight decreases. New grass has 6–8 weeks to establish roots before winter dormancy, then comes back strong in spring. If you can only sow once a year, make it September.

Summer (June–August)

Avoid sowing in summer unless you have reliable irrigation. Soil temperatures above 25°C trigger dormancy in cool-season grasses (ryegrass, fescue — the types used in UK lawns). Even with daily watering, the combination of heat and intense sunlight desiccates young seedlings before they can establish. Summer-sown lawns have germination rates of 20–40% compared to 80–90% for autumn-sown lawns.

Winter (November–February)

Too cold. UK soils sit between 2–6°C from December to February — well below the 8°C minimum for grass seed germination. Seed scattered in winter sits dormant on the surface, exposed to birds, frost, and waterlogging. Some may eventually germinate in March when the soil warms, but you will lose 50–70% of the seed to waste. Save your money and sow in spring or autumn.

How to Calculate Grass Seed for Irregular Lawns

Most lawns are not perfect rectangles. Curved borders, flower beds in the middle, paths, sheds, and patios all complicate the calculation. Here is how to get an accurate figure for any shape of lawn.

Break It Into Rectangles and Triangles

Sketch your lawn on paper and divide it into simple shapes. Measure each rectangle (length × width) and each triangle (½ × base × height). Add the areas together for your total. For example, an L-shaped lawn is just two rectangles side by side. A lawn with a curved border can be approximated as a rectangle plus or minus a triangle.

Subtract Hard Surfaces

Measure any paths, patios, raised beds, or shed bases that sit within the lawn area. Calculate the area of each and subtract it from your total. A common mistake is forgetting to subtract these — leading to buying 15–20% more seed than you actually need.

Add 10% for Waste and Edges

Even with careful measurement, some seed is always lost. Birds eat it, wind blows it off the edges, and spreading is never perfectly even. Adding a 10% buffer ensures you have enough to finish the job without a return trip to the garden centre. Our calculator above adds this automatically.

Use the Calculator for the Maths

Once you have your total area in square metres (or square feet), enter it into our grass seed calculator at the top of this page. You can enter any length and width combination that multiplies to your total area — for example, if your irregular lawn measures 37m², enter 37 as the length and 1 as the width. The calculator will give you the exact seed weight for your chosen grass type and sowing method.

Grass Seed FAQ