GardenCalc

Fertiliser Calculator

Work out exactly how much fertiliser you need for your lawn, beds or allotment. No guesswork, no scorched grass.

Calculate Your Fertiliser

Choose your fertiliser type, enter your area dimensions, and we'll tell you how much to apply and how many packs to buy.

Your Results

Area
—
Total Fertiliser
—
Weight
—
Packs Needed
—

Get Your Free Lawn & Garden Care Calendar

We'll send you a personalised monthly guide on when to plant, feed, and maintain your garden — based on your region.

How Much Fertiliser Do I Need?

This free fertiliser calculator works out exactly how much feed your lawn, garden beds or allotment needs. Select from popular UK fertiliser types — Growmore, lawn feed, blood fish & bone, chicken manure pellets — or enter a custom application rate. Get instant results in grams, kilograms and pack counts.

Over-fertilising wastes money and can burn your grass or plants. Under-fertilising means poor growth. Get the dose right first time with accurate measurements instead of guesswork.

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.

Fertiliser Rate (g/m²) Typical Pack Best For
Growmore 70 1.5 kg General purpose — veg, flowers, fruit
Lawn feed (spring/summer) 35 2.5 kg Greening up lawns, promoting growth
Lawn feed (autumn/winter) 70 2.5 kg Strengthening roots for winter
Blood, fish & bone 70 3 kg Organic slow-release, borders & veg
Chicken manure pellets 100 3.5 kg Soil conditioning, veg plots
Pelleted organic compost 100 5 kg Soil improvement, new planting
Sulphate of ammonia 35 1.5 kg Quick-release nitrogen boost

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.

Frequently Asked Questions

How much fertiliser do I need per square metre?
It depends on the product. General-purpose granular fertilisers like Growmore are typically applied at 70g per square metre. Lawn feeds vary from 35g/m² for spring feeds to 70g/m² for autumn treatments. Chicken manure pellets are heavier at around 100g/m². Always check the pack for the recommended rate and use the calculator to get an exact total.
How much Growmore do I need for my garden?
Growmore is applied at 70g per square metre (about 2oz per square yard). For a 10m² bed, you need 700g. A standard 1.5kg box covers about 21 square metres. For large allotments, a 10kg bag covers roughly 143 square metres. Enter your dimensions in the calculator above for an exact answer.
When should I fertilise my lawn in the UK?
Apply a high-nitrogen spring/summer lawn feed from March to September, typically every 6–8 weeks during the growing season. Switch to an autumn/winter feed (low nitrogen, high potassium) from September to November to toughen grass for winter. Never fertilise frozen or waterlogged ground, and always water in if rain isn't expected within 48 hours.
Can you put too much fertiliser on a lawn?
Yes — over-fertilising causes 'fertiliser burn', where grass turns brown or yellow in patches. Too much nitrogen promotes lush, weak growth that's vulnerable to disease. Always measure your area and follow the recommended application rate. If you accidentally over-apply, water the area heavily to dilute the fertiliser. Use the calculator above to get the correct amount first time.