Fertiliser Calculator

How Much Fertiliser Do I Need?

So your plants look hungry, the fertiliser aisle's confusing, and you don't want to burn anything. Here's the honest version.

By Gary Hodson · 15+ years UK gardening experience · Rates cross-referenced against RHS, Met Office and UK industry standards · Last reviewed 23 April 2026

Fertiliser is one of those garden jobs where the bag's instructions can be worse than doing nothing. Too little and the borders stay pale all summer. Too much and the tips of the grass or the leaves of your tomatoes yellow off within a week — that's fertiliser burn, and it takes four to six weeks to grow out once you've caused it. Most labels assume you know what "per square metre" means on your actual plot, which most gardeners don't, which is why the calculator at the top of this page exists.

Enter your area in metres (length × width for a rectangle, split L-shapes into rectangles, round irregular plots to the nearest square metre), pick your product, and it returns the exact weight to apply and the number of packs to buy. The rest of this page is the context that makes the number trustworthy — particularly around timing, NPK ratios, and the one thing you really want to avoid, which is feeding anything that's already stressed by heat, cold, or drought.

What the NPK ratio on the bag actually means

Every bag of fertiliser carries three numbers, usually separated by dashes — 7-7-7 on Growmore, 10-10-10 on a general-purpose granular, 14-2-5 on a spring lawn food. The N is nitrogen (leaf and stem growth). The P is phosphorus (roots and establishment). The K is potassium (flowers, fruit, and cold hardiness). A balanced feed has roughly equal numbers and is safe for most borders. A high-N feed drives lush leafy growth — lawns in spring, salad crops, greenhouse tomatoes in early growth. A high-K feed is for fruit, flowers, or autumn lawn hardening. You don't need to memorise every ratio. Matching the N:K balance to the season and the plant does 90% of the work.

The other number that matters and isn't always on the front of the bag is the application rate in g/m². Standard granular feeds run 30–70 g/m² depending on product. Blood, fish and bone is typically 70–100 g/m². Chicken manure pellets go higher at 100–150 g/m². Liquid concentrates are measured in millilitres per 10 L of water instead. The calculator handles all these units, but the rate is why two similar-looking bags can do very different jobs — always check the rate before you spread anything.

When to feed, and the months to skip entirely

The UK feeding year has two windows. March to September is the main push — lawns, veg, flowering borders, and container plants all benefit from feeding every four to eight weeks during active growth. Inside that window, stop feeding two to three weeks before the end of each crop's season (late tomatoes, for example, don't need nitrogen in late August). September to October is the autumn window — a last high-K lawn feed to toughen grass against frost, and nothing for perennials or trees already going dormant.

November to February: don't feed anything. Grass is dormant, soil biology is slow, and fertiliser applied to cold or waterlogged soil leaches straight through into the watercourse — wasteful and, in some catchment areas, technically against the rules. The single biggest timing mistake UK gardeners make is "giving the lawn a boost" in late February before soil temperatures are reliably above 5 °C. You're feeding nothing that can absorb it, and you risk burn once growth finally restarts.

Fertiliser burn — what actually causes it

Fertiliser burn is a salt problem, not a nutrient problem. The concentrated mineral salts in feed draw moisture out of plant roots when too much is applied, leaving scorched brown patches on lawns, yellowed tips on shrubs, or crispy edges on container plants. The three triggers are over-application (30–50% more than the product calls for is the usual culprit), applying in drought (no soil moisture to dilute the salts), and applying to wet foliage (granules stick to leaves and scorch them directly). All three are avoidable: measure accurately with the calculator, feed before expected rain or water it in yourself within twelve hours, and don't feed bone-dry soil.

If you've already over-applied — brown stripes on the lawn, yellowing leaf tips on the borders — soak the area heavily for 24 to 48 hours to flush the excess salts through the root zone. Most mild burn recovers in three to four weeks. Dead patches (where the grass pulls away easily with no root resistance) need reseeding — our grass seed calculator works out the seed quantity for bare spots.

Which type of fertiliser for which job

For a typical mixed UK garden, three products cover 90% of what you'll need. Growmore (7-7-7) is the reliable general-purpose granular — borders, veg, shrubs in spring, and a lighter mid-summer top-up. Blood, fish and bone is the slow-release organic alternative — four to eight weeks to release, perfect for planting new roses and shrubs where you want gentle feeding without the burn risk. Chicken manure pellets are the cheapest nitrogen per kilo if you've got a lot of leafy crops to feed, but they smell fierce for 48 hours after application — apply on a still day with rain forecast.

Lawns need a specialised feed rather than general-purpose granular: our dedicated lawn feed calculator covers spring, summer, and autumn formulations separately because the NPK balance shifts by season. For greenhouse tomatoes, buy a tomato-specific liquid feed (high K, around 4-5-8 or similar) rather than using garden-grade fertiliser — the nutrient ratio matters once flowering and fruiting start. Whatever you pick, use the calculator above to get the exact weight before you open the bag.

Fertiliser Application Rates

Different fertilisers need different application rates. Use this guide alongside the calculator to choose the right product and dose for your needs.

How to Apply Fertiliser Evenly

Getting an even spread matters as much as using the right amount. Patchy application leads to stripes on lawns and uneven growth in borders.

For lawns

A handheld or wheeled spreader gives the most even coverage. If spreading by hand, divide your total fertiliser into two halves — apply the first half walking north-to-south, then the second half walking east-to-west. This crosshatch pattern avoids missed patches and double-dosing.

For beds and borders

Weigh out the correct amount and scatter evenly by hand, then lightly fork or rake it into the top few centimetres of soil. Water well afterwards — dry granules sitting against plant stems can cause burns.

Handful guide

A level adult handful of granular fertiliser weighs roughly 35–40 grams. Two handfuls is about 70g — enough for one square metre at the Growmore rate. The calculator shows a handful estimate in the results to help you spread without scales.

Weed and Feed Calculator — How Much Do I Need?

Weed and feed products combine a lawn fertiliser with a selective herbicide that kills broadleaf weeds (dandelions, clover, daisies) without harming the grass. Most UK weed and feed products are applied at 35g per square metre — the same rate as a standard spring lawn feed.

When to Apply Weed and Feed

Apply in April to June when weeds are actively growing and the grass is growing strongly. The herbicide needs to stick to weed leaves to work, so:

• Apply when the grass is dry but the soil is moist
• Do not apply if rain is expected within 24 hours (washes herbicide off leaves)
• Do not apply during drought, frost, or extreme heat
• Wait 3 days after mowing before applying, and do not mow for 3 days after
• Do not apply more than twice per year

Popular UK Weed and Feed Products Compared

How Much Weed and Feed for Common Lawn Sizes

At 35g/m², here is a quick reference for typical UK lawns:

Tip: For lawns over 80m², the large 14kg boxes or bags are significantly cheaper per square metre. Select the specific product in the calculator above to get your exact amount.

Already dealt with the weeds? Our lawn feed calculator covers standard spring, summer, and autumn feeds without herbicide.

Feeding your veg plot? Our vegetable sowing calendar shows exactly when each crop needs planting and feeding. Check what to plant in March for this month's timing.

Frequently Asked Questions

Types of Fertiliser for UK Gardens

Walk into any garden centre and the fertiliser aisle can feel overwhelming. I have been gardening in the UK for over fifteen years, and I still think the labelling is needlessly confusing. Here is the plain-English breakdown I wish someone had given me when I started.

Understanding NPK Ratios

Every fertiliser bag shows three numbers separated by dashes — for example 7-7-7 or 20-5-10. These are the NPK ratio, and they tell you the percentage by weight of the three main plant nutrients:

N (Nitrogen) drives leaf and stem growth. A high first number means lush, green foliage. This is what makes your lawn thick and dark green. P (Phosphorus) promotes strong root development and helps plants establish after transplanting. K (Potassium) strengthens cell walls, improves disease resistance, and boosts flower and fruit production. A high third number is what you want for tomatoes, roses, and autumn lawn hardening.

A balanced fertiliser like Growmore (7-7-7) gives equal parts of each nutrient — a safe, general-purpose choice when you are not sure what your soil needs. A spring lawn feed might be 20-5-10 (heavy on nitrogen for growth), while an autumn lawn feed might be 5-5-15 (heavy on potassium to toughen roots before winter).

Fertiliser Comparison Table

Organic vs Synthetic — Which Should I Use?

Organic fertilisers (blood, fish & bone, chicken manure, bonemeal) release nutrients slowly as soil microbes break them down. They improve soil structure over time and are less likely to cause fertiliser burn. The downside is they are slower to act — you will not see results for 2-4 weeks — and they are bulkier per nutrient delivered. I use organic feeds on my borders and veg patch where I want long-term soil health.

Synthetic (manufactured) fertilisers deliver precise NPK ratios and can act within days. They are ideal for lawns where you want a quick green-up, and for container plants that need regular feeding. The risk is over-application — too much synthetic fertiliser scorches roots and can leach into groundwater. I use synthetic lawn feed in spring for that rapid result, then switch to organic chicken manure pellets on my raised beds.

For most UK gardens, a combination of both works best. Use our compost calculator to work out how much organic matter to add alongside your fertiliser for the best soil improvement.

How to Fertilise Your Garden — A Practical Guide

I have made every fertiliser mistake in the book — scorched lawns, leggy tomatoes, yellowing roses. Here is what I have learned about feeding different parts of a UK garden, with the specific measurements that actually work.

Feeding Your Lawn

Lawns are the most common thing people fertilise, and the most common thing they get wrong. The golden rule is 35g per square metre for spring and summer feeds, applied every 6-8 weeks from March through to September. For a typical 50m² back lawn, that is 1.75kg per application — about one standard box of lawn feed.

In spring (March to April), use a high-nitrogen feed with an NPK around 20-5-10. This promotes thick, green growth after the winter dormancy. I apply mine as soon as the grass starts actively growing — usually late March in the south of England, early April further north. Our lawn feed calculator gives you the exact amount for your lawn size.

In autumn (September to November), switch to a high-potassium feed around 5-5-15 at 35g/m². This toughens the roots and improves cold tolerance without pushing soft growth that frost will kill. I make my last autumn application in October and then leave the lawn alone until March.

Always water your lawn after applying granular feed if rain is not forecast within 48 hours. Dry granules sitting on grass blades in sunshine will burn them brown.

Feeding Borders and Flower Beds

For established borders, I apply Growmore (7-7-7) at 70g/m² in March and again in June. Scatter the granules around the base of plants — not directly against stems — and fork lightly into the top 2-3cm of soil. Water well afterwards.

For roses specifically, I use a dedicated rose feed with higher potassium (something like 5-5-10) at 35g per plant, applied in April and again after the first flush of flowers in July. I scratch it into the soil around the drip line, about 30cm from the stem.

Newly planted borders benefit from bonemeal (3.5-15-0) at 70g/m² worked into the planting hole. The high phosphorus promotes root establishment through the first season. I mix it with the backfill soil rather than sprinkling on top.

Feeding Your Vegetable Patch

Vegetables are hungry plants. Before sowing or planting out in spring, I work Growmore into the soil at 70g/m² — that is about two level handfuls per square metre. For heavy feeders like courgettes, pumpkins and sweetcorn, I increase this to 100g/m².

Once crops are growing, I side-dress with a balanced feed every 4-6 weeks. For tomatoes and peppers, switch to a high-potassium liquid feed (like tomato food at 5ml per litre of water) once the first fruits set. Feed tomatoes twice a week when they are fruiting heavily — this is the single biggest thing that improves tomato yield in my experience.

Check our planting calendar for the exact sowing and feeding windows for 30+ UK vegetables.

Feeding Container Plants

Containers are a special case because nutrients wash through with every watering. Compost in pots runs out of feed after about 6 weeks. After that, you need to supplement with either slow-release granules mixed into the top layer (one application in April lasts until August) or liquid feed every 7-10 days through the growing season.

For hanging baskets and window boxes, I use a liquid feed at half the recommended strength but twice as often — this gives a steadier supply without the feast-or-famine cycle. A 1-litre bottle of concentrated tomato food at around 5ml per litre of water will last a full season for 10-15 containers.

UK Seasonal Fertilising Calendar

How Much Does Fertiliser Cost in the UK? (2026 Prices)

Fertiliser prices vary hugely depending on the type, brand, and pack size. I have tracked what I spend each season, and the biggest lesson is that buying larger packs saves a surprising amount per square metre. Here are typical UK prices as of 2026.

Money-saving tip: A full year of lawn feeding (three applications at 35g/m²) for a 50m² lawn costs roughly £21-£25 using mid-range products. Buying the large 400m² coverage boxes brings this down to around £15. For veg patches and borders, a single 10kg bag of Growmore at around £16 covers 143m² — enough for most allotments for the entire season.

If you are also improving your soil structure, consider adding compost alongside your fertiliser. Our compost calculator works out how much you need and what it will cost. For new lawns, pair feeding with the right amount of grass seed to get the best establishment. Building new beds or topping up borders? Use our soil calculator to work out exactly how much topsoil you need before you start feeding.

5 Common Fertiliser Mistakes (and How to Avoid Them)

I have made every one of these at some point. Each one cost me time, money, or a dead patch of lawn. Here is how to avoid repeating my errors.

1. Over-feeding Your Lawn

The most common mistake I see is doubling up on fertiliser because "more must be better." It is not. Applying 70g/m² of a spring lawn feed when the recommended rate is 35g/m² will scorch the grass within 48 hours. You will end up with yellow-brown stripes that take 4-6 weeks to recover. Always measure your lawn area first — our calculator does this for you — and weigh the fertiliser rather than guessing by eye. A level adult handful is approximately 35-40g, which is roughly the right amount for one square metre of lawn feed.

2. Using the Wrong NPK for the Season

Applying a high-nitrogen spring feed (20-5-10) in October is counterproductive. Nitrogen pushes soft, lush growth that frost will damage overnight. In autumn, you need the opposite — a feed high in potassium (5-5-15) to harden the grass and strengthen roots for winter. I mark my spring and autumn feeds with different coloured tape so I never grab the wrong bag from the shed.

3. Feeding During Drought or Extreme Heat

Granular fertiliser needs moisture to dissolve and reach the roots. Spreading it on bone-dry soil in a July heatwave means it just sits there, concentrated, waiting to burn any plant tissue it touches when the next rain arrives. If you must feed during a dry spell, water the area thoroughly the evening before, apply the fertiliser in the morning, and water again immediately afterwards. Better yet, wait until the weather breaks.

4. Ignoring Soil pH

If your soil pH is below 5.5 or above 7.5, most nutrients become locked up regardless of how much fertiliser you apply. Before spending money on feed, test your soil pH with a simple kit (around £6 from any garden centre). Most UK garden soils sit between 6.0 and 7.0, which is fine. If yours is acidic (below 6.0), add garden lime at 200g/m² in autumn — but never at the same time as fertiliser, as they react with each other. Leave at least 4 weeks between liming and feeding. If your soil needs improvement beyond pH, check our soil calculator for guidance on adding topsoil.

5. Not Watering In After Application

This one catches people every year. You scatter your lawn feed on a sunny afternoon, feeling productive, then forget to water. The granules sit on the grass blades in direct sunlight and the concentrated salts burn the leaf tissue. The result is brown spots exactly where each granule landed. Always water within 24 hours of applying granular fertiliser if rain is not expected. I set a phone reminder now because I have been caught out too many times.

More Fertiliser Questions Answered