Grass Seed Calculator – How Much Grass Seed Do I Need?
Measure the lawn, pick the job, and we'll work out the kilos of seed and the packs to buy — plus how much lawn feed you need across the year.
Grass Seed & Lawn Feed Calculator
What are you doing?
35 g/m² over prepared, raked soil. Firm the ground first, then rake a fine tilth — seed hates sitting on lumps.
Measure rectangles length by width. Split an L-shape into two rectangles, or measure a circle straight across its widest point.
Measure your lawn
15 m²
Odd-shaped garden? Split it into rectangles, triangles and circles — we add them up for you.
You need
0.58kg of seed
15 m² at 35 g/m², including 10% spare
- Lawn area
- 15 m²
- Seed before spare
- 0.53 kg
- Packs to buy
- 2 × 0.5kg
- Total bought
- 1 kg
How we worked this out
Your shopping list
2 × 0.5kg packs of grass seed
Covers about 14 m² at 35 g/m².
£10.00
Grass seed needed at a glance
| Lawn area | New lawn (35 g/m²) | Overseeding (20 g/m²) | Patching (50 g/m²) |
|---|---|---|---|
| 10 m² | 0.35 kg | 0.2 kg | 0.5 kg |
| 25 m² | 0.88 kg | 0.5 kg | 1.25 kg |
| 50 m² | 1.75 kg | 1 kg | 2.5 kg |
| 100 m² | 3.5 kg | 2 kg | 5 kg |
| 200 m² | 7 kg | 4 kg | 10 kg |
How this calculator works
Grass seed is sold by weight but spread by area, so every calculation starts with the size of your lawn. We add up each section you measure, multiply by the sowing rate in grams per square metre, and divide by 1,000 to get kilograms. A 10% spare allowance is added by default, because you will always waste a little at the edges, in the spreader hopper and on patches you go back over.
The sum is simply area (m²) × rate (g/m²) ÷ 1000 = kilos of seed. We then work out the cheapest sensible mix of UK pack sizes — 500g, 1kg, 1.5kg, 5kg, 10kg and 20kg — by filling with the biggest packs first and rounding the last one up. Buying one 5kg box is almost always cheaper per kilo than ten 500g boxes.
The lawn feed tab works the same way. Granular feeds are typically spread at around 35g per m², so a 100m² lawn needs about 3.5kg per application and roughly 10.5kg across three feeds a year. Liquid feeds are quoted as coverage per bottle instead, so we divide your area by the bottle coverage and round up. Always check your own product label — rates vary a lot between brands and a heavy hand on a granular feed will scorch stripes into the grass.
Worked example: reseeding a 50m² back lawn
A back lawn of 10m by 5m is 50m². Starting from bare soil, that is 50 × 35 ÷ 1000 = 1.75kg of seed, or 1.93kg once you add 10% spare. In UK pack sizes that is one 1.5kg box plus one 500g box, or simply a single 5kg bag if you want plenty left over for patching next spring.
If the same lawn is thin rather than bare, overseeding at 20 g/m² only needs 1kg — about £18 of seed rather than £35. Cut the lawn to 25mm, scarify out the dead thatch, sow, then topdress with a thin layer of sandy loam so the seed sits in contact with soil rather than on top of moss.
Tips from the trade counter
- Sow in early September if you can. Warm soil, autumn rain and slowing weed growth make it the easiest window in the UK. March to April is the backup, but expect to water daily through any dry spell.
- Cut to 25mm before overseeding. Long grass shades out the seedlings. Scarify hard enough to see soil — seed that lands on thatch never roots.
- Don't feed in drought or frost. Granular feed needs rain or watering within two days, and a feed applied to frozen or bone-dry grass burns it.
- Keep weed & feed away from new grass. Wait until the new sward has been mown three times, otherwise the weedkiller takes out the seedlings too.
- Water little and often for the first fortnight. The top 10mm must stay damp. Once seedlings are 50mm tall you can switch to a heavier soak twice a week.
- Buy one size up if patching later. Seed keeps for two to three years in a cool, dry, mouse-proof tub, so a bigger bag rarely goes to waste.
Before you sow
Prepare the ground properly: dig or rotavate, remove stones and old roots, firm it by treading, then rake to a fine tilth. Seed sown onto compacted builders' soil will germinate and then die back in the first dry week.
If you are laying new topsoil first, 100–150mm is the usual depth. Our topsoil calculator will work out the tonnes and bulk bags for you.
Frequently asked questions
How much grass seed do I need per square metre?
About 35g per m² for a brand new lawn, 20g per m² for overseeding an existing lawn, and up to 50g per m² when patching bare spots. Wildflower meadow mixes are far lighter at around 5g per m².
When is the best time to sow grass seed in the UK?
Early September to mid-October is the best window — the soil is still warm, rain is reliable and weeds are slowing down. March to April is the next best, but a dry spring means you will have to water.
Can I sow too much grass seed?
Yes. Overdoing the rate makes seedlings compete with each other, which gives a thin, weak sward that is prone to disease. Stick close to the label rate rather than emptying the bag.
How long does grass seed take to germinate?
Usually 7–14 days in warm, damp conditions. Below about 8°C soil temperature it can sit dormant for weeks, which is why sowing in November rarely works.
Should I feed and seed at the same time?
Use a dedicated pre-seed feed, which is high in phosphate for root growth. Never use weed & feed on new seed — the weedkiller will kill young grass. Wait until the new lawn has been cut three times.
How many times a year should I feed my lawn?
Three applications is a sensible default: spring, summer and an autumn feed high in potassium. Four is fine on a lawn that gets heavy use.
Do I need to cover the seed?
Rake it lightly into the top 5–10mm and firm it down. A thin dressing of lawn topdressing helps on exposed sites, and netting keeps birds off small patches.