Lawn Seed Calculator & Coverage

What Goes Into the Numbers

The formula behind a lawn seed calculation is straightforward: multiply the lawn area by the seed rate, then add a waste allowance. Area comes from length times width in metres, giving you a figure in square metres. Seed rate is expressed in grams per square metre, and the two values multiplied together give the total grams of seed. Divide by 1,000 to convert grams to kilograms, then multiply by your waste factor (typically 1.10 for 10% waste) to get the final weight.

The seed rate you choose depends on whether you are creating a new lawn or overseeding an existing one. New lawns need a heavier application because there is no existing grass to fill in bare patches. The recommended rate for a new lawn sits between 30 and 50 grams per square metre, depending on the seed mix. Fine ornamental blends sit at the lower end, while hardwearing rye grass mixes need the higher rate to establish a dense, resilient sward.

Overseeding an established lawn uses a lighter rate of 15 to 25 grams per square metre. If you prefer instant coverage rather than waiting for germination, the turf roll quantity estimator handles that alternative approach. The existing grass already provides ground cover, so you are only filling thin spots and thickening the turf. Applying the full new-lawn rate over existing grass wastes seed and can cause overcrowding, which leads to weak, leggy growth.

Once the calculator produces a kilogram figure, it rounds up to whole bags. Most retail grass seed comes in 1 kilogram bags, so even if you only need 1.32 kilograms, the result shows 2 bags. This rounding ensures you have enough seed for the job without a shortfall partway through sowing.

What the Numbers Mean

The results panel shows three values. Lawn area is the total coverage in square metres, calculated from the length and width you entered. This is the base figure that drives the rest of the estimate.

Seed needed is the weight of grass seed in kilograms, including the waste factor. Waste accounts for seed lost to wind drift during sowing, uneven distribution, and bird predation before germination. A 10% allowance is standard for most domestic lawns.

Bags needed rounds the kilogram figure up to the nearest whole bag. The same rounding-up logic applies to carpet and underlay estimates indoors, where you cannot buy partial rolls. Grass seed is sold in sealed 1 kilogram bags by most UK retailers, so partial bags are not an option at the till. If your calculation shows 1.32 kilograms, you still need 2 bags. The leftover seed stores well in a cool, dry place for up to two years, making it useful for patch repairs later.

Overseeding results look lower than new-lawn results for the same area because the seed rate is roughly half. That is expected. Existing turf already covers most of the ground, so the seed only needs to fill gaps and thicken sparse areas rather than establish full coverage from bare soil.

Practical Tips for Planting

Good soil preparation makes the difference between a patchy result and a dense, even lawn. If your ground needs fresh topsoil before sowing, the topsoil depth and volume calculator covers how much to order. Before sowing, rake the soil to a fine tilth so the surface is crumbly and level. Remove any stones larger than 20 millimetres and break up compacted clumps. A flat rake or landscaping rake works well for this.

Sow the seed in two passes for even coverage. Measure out half the total seed and walk in parallel lines along the length of the lawn, scattering by hand or using a spreader. Then take the second half and walk perpendicular to your first pass, covering the width. This cross-hatch technique prevents stripes and thin patches.

Consider these points before and during sowing:

• Check the weather forecast for at least five consecutive days without heavy rain or frost.
• Avoid sowing in strong wind, as seed drifts onto paths and flower beds.
• Lightly rake seed into the top 5 to 10 millimetres of soil after sowing to improve soil contact.
• Firm the surface with a light roller or by walking on boards laid flat across the lawn.

After sowing, the single biggest factor is moisture. Water with a fine mist setting on a hose or sprinkler. Heavy jets wash seed into puddles and create bare patches. Keep the seedbed consistently moist for the first two to three weeks. In dry spells, that may mean watering twice a day in short bursts rather than one long soak.

When to Adjust These Numbers

Shaded areas under trees or beside fences need a shade-tolerant seed mix, and these blends perform best at a higher application rate of 40 to 50 grams per square metre. In deeply shaded spots where grass struggles entirely, bark mulch or ground-cover planting may give a better long-term result. The reduced light means slower germination and thinner growth, so a denser sowing compensates.

High-traffic areas such as play lawns and paths between borders benefit from a hardwearing rye grass mix at the upper end of the new-lawn range (40 to 50 grams per square metre). Rye grass germinates faster and tolerates foot traffic better than fine fescue blends, so it fills in quickly even under regular use.

Patch repair differs from general overseeding. If you are filling a bare patch left by a removed shrub or repaired drain trench, use the full new-lawn rate rather than the overseeding rate. There is no existing grass in a bare patch, so it needs the same density as a brand-new lawn. Increase the waste factor to 15% for very small patches where proportionally more seed bounces off the edges.

Sowing season is the next biggest factor in germination success. In the UK, autumn sowing between September and mid-October gives the best results. Soil is still warm from summer, moisture levels are rising, and weed competition is declining. Spring sowing from late March to May is the second-best window. Avoid summer sowing unless you can water daily, and never sow into frozen ground in winter.

Sloping ground loses more seed to runoff and gravity. On any gradient steeper than about 1 in 4, increase the waste factor to 15% and consider pegging a light erosion-control mesh over the seed until germination anchors the roots. Once the lawn is established, surrounding borders benefit from defined planting — the hedge spacing calculator helps plan boundary hedging alongside a new lawn.