Grass Seed Calculator

The right amount of seed per square metre depends on the mix, and the RHS says the rate is printed on the packet.¹ We only give a number where a source states one. For a standard lawn mix that is the Johnsons' Quick Lawn rate, which the calculator uses for every lawn grass type.⁸ For a wildflower-and-grass mix it is the RHS rate.³

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 because 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", as you'll need more seed to establish full coverage. If you're thickening up an existing lawn with thin patches, "Overseeding" uses 20g per square metre against 35g for a new lawn (the Johnsons' Quick Lawn rates).⁸ For small bare spots, "Patching" also uses 20g per square metre⁸, which sits inside the RHS repair rate of about 15–25g.² Not sure seed beats turf for your plot? Our turf or grass seed comparison sets out cost, timing and shade side by side.

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.

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 autumn. The RHS sowing window is September to November, and it says lawn seed germinates best in early autumn and mid-spring (March to April).¹ In spring you may 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.⁶

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.⁶

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.⁶

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 the rate on the packet (Johnsons' Quick Lawn says 20g per square metre for overseeding) 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 gives much better results. The RHS asks for "a light sprinkling" and gives no depth. Our own suggestion, as GardenCalc, is about 3mm: enough to cover the seed without burying it.² The RHS reason for the cover is to hide the seed from birds. Work out your seed quantity with the calculator above, choosing "Patching Bare Spots" and entering the patch size (the RHS puts the usual repair rate at about 15–25g per square metre), add it to the sieved topsoil you need for a 3mm layer, 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. That 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, Johnsons' Quick Lawn says 20g per square metre, against 35g for a new lawn.⁸ Spread evenly by hand or with a seed spreader, going in two passes at right angles for the best coverage.²

Step 5: Topdress (Optional)

Scatter a light sprinkling of lawn topdressing or sieved compost over the seeded area, as the RHS advises. Our rule of thumb is about 3mm.² The RHS reason is to hide the seed from birds. Use our soil calculator to work out how much topdressing you need.

Step 6: Water

Water lightly once a day for the first six weeks, and skip days when rain has done the job.⁵ In hot, dry spells, add a second light watering so the surface never dries out.⁷ Once the seedlings are growing, gradually water less often but more deeply.⁷ After the first cut, autumn-sown grass usually needs no more mowing until spring. Spring-sown grass gets cut about once a week, with the blades lowered bit by bit.¹

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.

Choosing the right grass seed mix is the single biggest factor in whether your lawn succeeds or struggles. The common mistake is grabbing 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. The RHS describes a general-purpose lawn mix as hard-wearing grasses that stand up to foot traffic, children and pets, usually perennial ryegrass, tall fescue, red fescues and browntop. It grows quite fast and needs regular mowing.¹ Any perennial ryegrass in it should be a turf or fine ryegrass, not a cheap agricultural type. Sow at the rate on the packet.

Fine / Ornamental Lawn Mix

If you want that bowling-green finish, you need a fine lawn mix. The RHS says these are usually Chewing's fescue, slender creeping red fescue, strong creeping red fescue and browntop. They form a dense, soft lawn and can be mown low for a neat finish, but they are slow-growing and wear less well than a general-purpose mix.¹ Sow at the rate on the packet.

Shade-Tolerant Mix

Under trees, next to fences, north-facing gardens: shade is one of the commonest reasons UK lawns thin out and fail. The RHS says most shade mixes, for light to medium shade, contain hard fescue, slender creeping red fescue, strong creeping red fescue and browntop. These are fine-leaved grasses and usually not very hard-wearing.¹ Sow at the rate on the packet. Even with the right seed, shaded lawns need extra care: mow a little higher, reduce foot traffic, and keep fallen leaves cleared in autumn so the grass gets whatever light is available.

Drought-Resistant Mix

Drought-tolerant mixes are sold for dry, sunny gardens. The RHS notes that if you read the ingredients, most special mixes like these turn out to be similar to the general-purpose, fine or shade mixes above.¹ So check the species list on the bag and sow at the rate on the packet.

Wildflower Meadow Mix

This is not technically a lawn seed. It is a deliberate shift away from mown grass towards biodiversity. The difference that matters most is the sowing rate. The RHS says a wildflower-and-grass mix is usually sown at 5g per square metre, far less than a lawn mix.³ Follow the instructions on your mix for feeding and cutting.

Sowing grass seed is straightforward, but the details matter. Skip the soil preparation or sow at the wrong time and you can waste a whole bag of seed. Follow these steps and far more of the seed you paid for will come up.

Step 1: Prepare the Soil

Remove any existing weeds and stones, plus any 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.¹ Seeding a lawn that is already bumpy? Our guide on how to level a bumpy lawn covers thin top dressing layers and lifting turf to fill hollows.

Step 2: Check Soil Temperature

The RHS puts the minimum soil temperature for good germination at 7°C⁴, and Johnsons says grass seed generally needs around 8–10°C or higher.⁷ The RHS sowing window in spring is March to April. 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. Our garden weather forecast gives a short-window soil temperature estimate for your postcode if you want a starting point before you get the thermometer out. The best months for sowing grass seed in the UK are March-April 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 the calculator gives about 1.5kg of seed for a 40m² lawn, mark off four 10m² sections and weigh out a quarter of the seed for each. This prevents the classic mistake of running out of seed halfway across the lawn with half the bag already used.¹

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 a light sprinkling of soil over the seed (about 3mm is our own suggestion), 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 moist. Water lightly once a day for the first six weeks, unless rain has soaked it.⁵ In hot, dry spells, add a second light watering.⁷

Step 7: First Mow

Wait until the grass is 50–75mm tall before the first mow. Firm the soil lightly first, then cut a couple of days later. Take off no more than the top third of the blade. Never scalp new grass. The RHS prefers a cylinder mower with sharp blades and the front roller taken off, so the young grass is not flattened. If you sowed in autumn, the RHS says that first cut is usually the last until spring.¹ If you sowed in spring, cut about once a week and lower the blades bit by bit until you reach your usual height (usually 25-40mm for a family lawn).⁷

A common question 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. Johnsons' Quick Lawn gives 20g per square metre for overseeding, against 35g for a new lawn, because existing grass already fills much of the space.⁸ Preparation is simpler too: mow short, scarify to remove thatch, and sow directly onto the exposed soil.

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 as long as the product label says, then rotavate or dig over the top 150mm of soil. Level and firm the soil, then rake to a fine tilth before sowing. Johnsons' Quick Lawn gives 35g per square metre for a new lawn; check the rate on your own packet.⁸ The RHS advises using a new lawn from seed as little as possible for about eight months. If you want an instant result, consider laying turf instead. It costs more but gives you a lawn straight away.

Expected Results Timeline

These are five common reasons a new lawn fails. 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 rate on the packet, the seedlings are packed so tightly that they compete for light and water and starve each other of 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

The RHS puts the minimum soil temperature for good germination at 7°C⁴, and Johnsons says grass seed generally needs around 8–10°C or higher.⁷ 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. Use a soil thermometer or simply wait until you can see consistent daisy growth in your neighbours' lawns, as daisies germinate at similar temperatures.

3. Not Watering Enough

Germinating grass seed needs steady moisture from the day you sow. 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. Water once a day for the first six weeks⁵, and add a second light watering in hot, dry spells.⁷ 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.⁷

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 50mm can rip young plants straight out of the ground, especially on damp soil. Wait until the grass is 50–75mm tall, then take off no more than the top third. Use a sharp blade. A blunt mower tears rather than cuts, which damages young plants and leaves them vulnerable to disease.⁷

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.

From budget lawn repair to premium ornamental seed, here are grass seed products widely stocked in the UK, with what each one suits. We earn a commission if you buy through these links — it does not change the price you pay, and it does not change what we recommend.

Affiliate links: GardenCalc may earn a commission if you buy through the partner links on this page, at no extra cost to you.

Just seeded your lawn? Once it's established, let a robot mower keep it perfect. These are the UK's top picks for 2026:

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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.

Timing matters more than most people realise. The same bag of seed sown in September will do far better than the same seed sown in July. Here is what each season actually means for your lawn.

Spring (March–April)

Soil is warming up from winter, natural rainfall is usually sufficient, and daylight hours are increasing, which is all good for germination. The earliest you can reliably sow is mid-March in southern England, when soil temperatures first reach the RHS 7°C minimum. 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. The soil is still warm from summer, autumn rain keeps the ground naturally moist, and weed germination slows as daylight decreases. The RHS says autumn-sown seedlings can get established before the first hard frosts, then root in well through the winter.¹ If you can only sow once a year, make it September. The RHS window runs to the end of November, but the soil cools through the month. Its October advice is to sow in warmer regions and says it is getting too late in cooler places, so we would only sow in November in milder areas, with the soil at 7°C or above.¹⁴

Summer (June–August)

Avoid sowing in summer unless you can water reliably. The RHS says lawn seed germinates best in early autumn and mid-spring, when the soil is warm, there is plenty of moisture and the weather is neither too cold nor too hot.¹

Winter (December–February)

Too cold. Johnsons says winter conditions in the UK usually prevent reliable establishment, and the RHS puts the minimum soil temperature for good germination at 7°C.⁷⁴ Sow in spring or autumn instead.

Most lawns are not perfect rectangles. Curved borders, flower beds in the middle, paths, sheds or 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, which leads 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.