What this calculator works out
This calculator multiplies lawn area by an application rate, adds a waste allowance, and returns the weight of fertiliser needed in kilograms.
Rate and timing both matter. The same lawn needs a different product in spring and autumn, and applying the wrong one at the wrong time does more harm than not feeding at all.
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Before you apply
- Take the rate from the product. It varies widely between granular feeds, and applying at another product's rate risks scorching.
- Spring and summer feeds are high in nitrogen to drive leaf growth. Autumn feeds are low in nitrogen and higher in potassium to harden the sward for winter.
- Apply to moist soil, and water in if there is no rain within a couple of days. Granules sitting on dry grass scorch it.
- Never apply to frozen or drought-stressed grass.
The three numbers on a fertiliser bag are nitrogen, phosphorus and potassium as percentages. Nitrogen drives leaf growth and colour; phosphorus supports root development; potassium improves hardiness and disease resistance. A spring lawn feed is nitrogen-heavy; an autumn one shifts towards potassium, which is why they are not interchangeable.
How this calculator works
Area times rate, converted to kilograms:
Kilograms = area × rate in g/m² × (1 + waste allowance ÷ 100) ÷ 1,000The waste allowance covers spreader calibration and overlap. Even application matters considerably — a spreader run twice over the same strip can scorch it while leaving the next strip underfed.
Worked example: an 80 m² lawn
Using the default figures — 80 m² at 35 g per square metre with a 10% allowance:
- Base quantity: 80 × 35 = 2,800 g
- With allowance: 3.08 kg
Just over three kilograms, which is a single small bag. The more important number is 35 g/m² — spread by hand, that is a surprisingly thin scattering, and the instinct to apply more generously is what causes scorching. A calibrated spreader, or marking out the lawn and weighing the quantity per section, is worth the effort.
Common mistakes
- Applying by eye. Hand spreading is far less even than it feels, and overlaps scorch.
- Using a spring feed in autumn. Soft nitrogen-driven growth going into winter invites disease.
- Applying to dry grass without watering in. Granules scorch on contact.
- Feeding a drought-stressed lawn. Wait for rain and recovery first.
- Ignoring moss and thatch. Feeding a lawn choked with either wastes the fertiliser.
Frequently asked questions
How often should I feed a lawn?
Two to three times a year covers most domestic lawns: a spring feed around April, optionally a summer feed, and an autumn feed in September or October. More frequent feeding produces faster growth and more mowing without much improvement in quality. An unfed lawn on decent soil is perfectly viable, just less lush.
What is the difference between spring and autumn feeds?
Spring and summer feeds are high in nitrogen for leaf growth and green colour. Autumn feeds are low in nitrogen and higher in potassium and phosphorus, encouraging root development and hardiness rather than soft top growth that would be damaged by frost and disease over winter.
Should I use a weed and feed product?
They are convenient but blunt — the herbicide is applied across the whole lawn whether or not weeds are present there. Spot-treating weeds and feeding separately uses less herbicide and lets you time each correctly, since the best moment to feed is not necessarily the best moment to treat weeds.
Does fertiliser run off into watercourses?
It can, particularly if applied before heavy rain or onto hard surfaces. Nutrient run-off contributes to algal growth in rivers and lakes. Applying the correct rate, sweeping granules off paths and drives back onto the lawn, and avoiding application before heavy rain all reduce it substantially.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- Royal Horticultural Society — lawn feeding, timing and product selection
- GOV.UK — fertiliser use, nutrient run-off and water quality guidance
Sources are checked at publication and can change — how I choose and check references.