Gardeners Toolkit
Free UK lawn seed planner

Grass Seed Calculator

Calculate seed weight and whole packs for a new lawn, overseeding or patch repair using the application rate printed for the seed mix you are actually sowing.

New lawn + overseedingEditable g/m² rateWhole-pack planning
Diagram showing grass seed applied over a measured lawnarea × g/m²seed

The rate on the bag is part of the calculation

Grass-seed mixtures are not interchangeable. A new lawn and an overseed can use different rates, and the recommended figure can change between products. This calculator keeps the rate editable so your product label — not a generic website default — remains the final authority.

Irregular lawn?

Use the Garden Area Calculator for curves and cut-outs, then enter the final m² below.

Calculate grass seed

Measure the lawn, choose the job, then replace the example application rate with the rate printed on your seed pack.

Rate helpNew lawn or overseed?
Step 1

Measure the area to seed

Extra measured lawn areasAdd any other sections you already know in square metres.
Step 2

Choose the job and enter your product rate

g/m²
35 g/m² is an example starting value for a new lawn. Replace it with your product label.

RHS advice says the weight per square metre is given on the packet and can vary with the seed mix. A current Johnsons Lawn Seed example uses 35 g/m² for a new lawn and 20 g/m² for overseeding; those values are useful examples, not universal rates.

Your seed requirement

Total seed required—

Calculated from your lawn area and the application rate currently entered.

Lawn area—
Project type—
Application rate—
Seed in grams—
Whole packs—
Pack size—
Check the bag before ordering

The calculation is only as accurate as the rate entered. Seed mix, coating and intended use can change coverage, so use the manufacturer's stated rate for your exact product.

Seed quantity sorted — choose the right lawn job

The next step depends on whether you are establishing bare ground or improving grass that is already there.

New lawn or overseeding?Compare preparation, rate, timing and aftercare.Compare turfSee how many rolls an instant lawn would require.Need soil preparation?Calculate a measured topsoil volume before sowing.Recheck the lawn areaCombine curves, triangles and exclusions accurately.

How the grass seed calculation works

Calculation: lawn area (m²) × application rate (g/m²) = grams of seed. Divide by 1,000 for kilograms, then divide by your pack weight and round up to whole packs.

The important variable is the application rate. RHS guidance for a new lawn specifically tells gardeners to use the weight per square metre given on the seed packet because it can vary with the mix. That is why this tool never locks you into one “correct” rate.

Grass seed application rate calculation diagram100 m²×35 g/m²rate=3,500 g = 3.5 kgArea × label rateThe formula is simple; choosing the correct product rate is the part that needs care.
Comparison between bare soil for a new lawn and existing grass for overseedingbare soilnew lawnexisting lawnoverseedNew lawn and overseeding are different jobsThe label may specify different rates because seed is being established into different starting conditions.

Worked examples

100 m² × 35 g/m²3.5 kg
200 m² × 25 g/m²5 kg
5 kg needed3 × 2 kg packs
Pack ruleRound up

Why seed rates vary

Seed mixtures contain different species, cultivars, seed sizes and sometimes coatings. The job also changes: sowing onto prepared bare soil is different from adding seed into an established lawn. Current supplier examples show separate new-lawn and overseeding rates, and specialist calculators increasingly expose that choice instead of hiding it. Always replace any example value with the instructions on the product you are using.

Measuring for patch repair

Do not enter the size of the entire lawn if you are only repairing a few bare patches. Measure the actual repair areas, add them together and calculate the seed for that combined area. If the whole lawn is very poor, RHS advice notes that completely re-seeding or re-turfing may be more appropriate than patching isolated areas.

Common seed-calculation mistakes

  • Using a rate from another seed mix. Coverage can differ, so copy the rate from the pack in front of you.
  • Confusing overseeding with a new lawn. Select the actual job and check its matching rate on the label.
  • Measuring beds and paths as lawn. Calculate only the surface you will seed.
  • Rounding seed weight down to fit a pack size. The calculator rounds purchasable packs upward so the planned application rate is still achievable.

Sources behind the practical guidance

For new lawns, the RHS lawn-from-seed guide explains that packet rates vary by mix and recommends sowing in two passes for even distribution. The RHS lawn-repair guide distinguishes patch repair from complete reseeding. Product-specific example rates on this page are cross-checked against current Johnsons Lawn Seed guidance, but your own product label remains authoritative.

See Gardeners Toolkit methodology for unit conversions, rounding and calculator limitations.

Need instant cover instead?

Compare the measured lawn area with whole-roll turf quantities before deciding which route suits the project.

Open Turf Calculator