How much grass seed per m²?

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.

When is the best time to sow grass seed?

September is ideal in the UK — soil is warm and autumn rain helps germination. Spring (March-May) is the second-best window.

How long does grass seed take to germinate?

Most grass seed germinates in 7-21 days depending on soil temperature. Fine fescues may take longer. Keep soil consistently moist.

Should I use seed or turf?

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.

Can I sow grass seed on top of existing grass?

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.

GardenCalcs

Lawn Seed Calculator — How Much Grass Seed Do I Need

Calculate how much grass seed you need for a new lawn or overseeding.

Length

Length of the lawn area in metres

Width

Width of the lawn area in metres

Seed TypeChoose 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 (optional)

Price per kg of seed — leave at 0 to skip the cost estimate

New lawn (vs overseeding)

On = bare-ground new lawn (full rate); off = overseeding an existing lawn (reduced rate)

Include 10% wastage allowance

Adds a margin for uneven spreading and edge waste — adjust the percentage in Advanced Options

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.

Frequently Asked Questions

Related Calculators

Turf Roll Calculator

Work out how many turf rolls you need to lay a new lawn.

Lawn Feed Calculator

Calculate the right amount of lawn fertiliser for your grass area.

Lawn Area Calculator

Measure your lawn area including irregular shapes and obstacles.

Overseeding Calculator

Calculate seed rates for patching and thickening existing lawns.

Lawn Overseeding Calculator

Calculate seed quantity, preparation steps, and timing for lawn overseeding.

Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.