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Lawn Seed Calculator — How Much Grass Seed Do I Need
Calculate how much grass seed you need for a new lawn or overseeding.
Length of the lawn area in metres
Width of the lawn area in metres
Choose the mix that suits the lawn's use — fine/ornamental for show lawns, hard-wearing for play areas, shade for under trees
Price per kg of seed — leave at 0 to skip the cost estimate
On = bare-ground new lawn (full rate); off = overseeding an existing lawn (reduced rate)
Adds a margin for uneven spreading and edge waste — adjust the percentage in Advanced Options
Percentage of extra seed to allow for uneven broadcasting, edges and birds — 10% is a sensible default
How We Calculate This
Seed quantity = Area × Application Rate. Rates vary by seed mix type and whether this is a new lawn or overseeding.
New lawn rates: General 35g/m², Fine 25g/m², Hard-wearing 40g/m², Shade 30g/m², Wildflower 5g/m².
Overseeding an existing lawn uses a lower rate — about 25g/m² for a general mix (roughly two-thirds of the new-lawn rate), since established grass already covers most of the ground. The wastage allowance (default 10%, adjustable in Advanced Options) covers uneven spreading and edge waste.
Working figures for this are collected in our Lawn Seed Rates reference table.
Frequently Asked Questions
For new lawns: general purpose 35g/m², fine/ornamental 25g/m², hard-wearing 40g/m², shade 30g/m². For overseeding an existing lawn, use about 25g/m² for a general mix — roughly two-thirds of the new-lawn rate, not half.
September is ideal in the UK — soil is warm and autumn rain helps germination. Spring (March-May) is the second-best window.
Most grass seed germinates in 7-21 days depending on soil temperature. Fine fescues may take longer. Keep soil consistently moist.
Seed is cheaper and offers more variety choices. Turf gives instant results but costs more. Seed is best for large areas; turf for smaller areas or slopes.
Yes, this is called overseeding. Mow short, scarify first, then broadcast seed. Rake in lightly and keep moist. Best done in September in the UK.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.