Grass seed calculator
Grass seed is sown at a rate in grams per square metre printed on the box. Multiply by the area and you have the amount to buy — a little more for a new lawn, less for overseeding.
Your measurements
Printed on the seed box.
Result
- Grass seed needed
- 3.0 kg
The rate depends on the mix (sports, ornamental, shade).
How the calculation works
- Seed (kg) = area (m²) × seed rate (g/m²) ÷ 1,000.
Worked example
For 100 m² of new lawn at 30 g/m² you need 100 × 30 ÷ 1,000 = 3 kg of seed.
What the calculator assumes
- New lawns are usually sown at 25–40 g/m²; overseeding an existing lawn at 15–20 g/m².
- The exact rate is on the box and depends on the mix: sports, ornamental or shade.
When to adjust the result
- Seed only germinates well on prepared ground: dug or tilled, levelled, rolled and kept moist for the first 2–3 weeks.
- The best sowing periods are spring and early autumn, when the soil is warm and rain is frequent.
Getting new lawn seed to take
Seed fails far more often from poor preparation than from the wrong quantity. Remove weeds and stones, rake to a fine, level tilth, firm the soil by treading or rolling and rake lightly again so the seed sits in shallow grooves. Sow half the seed in one direction and half across it for even cover, then rake it in to about 0.5–1 cm and roll.
The best sowing windows are late summer to early autumn, when the soil is warm and rain is more regular, and mid-spring. Keep the surface constantly moist — light watering once or twice a day — until the grass is about 5 cm tall, and make the first cut only when it reaches 7–8 cm, taking off no more than a third.
Frequently asked questions
How much grass seed per square metre?
Around 25–40 g/m² for a new lawn and 15–20 g/m² for overseeding — check the box for your mix.
When is the best time to sow grass?
Early autumn or spring, when the soil is warm and moist.
Why is my new lawn patchy?
Usually uneven sowing, seed washed away by heavy watering, birds, or dry spells during germination. Overseed the patches with the same mix and keep them moist.
Can I sow grass seed in summer?
It is possible with daily watering, but heat and drought make germination unreliable. Late summer and early autumn give much better results.