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Sow in spring (April-May) or early autumn (September), when soil is warm and moist: autumn is the professionals' pick. Split the seed in half and sow in two passes at right angles for even coverage, rake in lightly, and keep moist until established.
What The Grass Seed Calculator Works Out
- Lawn area: length multiplied by width in square metres. For an L-shaped lawn, add two rectangles together or enter the total as one long strip.
- Seed needed: the area multiplied by the sowing rate: 35 g/m2 for a new lawn, 25 g/m2 for overseeding an existing one, up to 50 g/m2 for bare-patch repairs. A 50 m2 lawn sown from scratch takes 1.75 kg. More is not better: oversown grass crowds itself into weak seedlings prone to damping off.
- Extras: a pre-seed fertiliser raked in at about 35 g/m2 feeds the seedlings without scorching them. Germination shows in 7 to 21 days; keep the surface moist, give the first cut on a high blade at 5 to 8 cm, and hold off selective weedkillers for six months.
- Seed rate override (advanced): use the rate printed on your own box, which varies by mix: fine fescue and bent mixes are often sown at 25 to 35 g/m2, hard-wearing ryegrass mixes at 35 to 50 g/m2, and shade mixes at the top of their range.
- Area to deduct (advanced): subtracts beds, paths, patios and sheds inside the measured rectangle so you do not buy seed for ground that will never be grass.
- Packs to buy (advanced): seed is sold in 500 g to 20 kg packs; the calculator rounds up to whole packs and shows what is left over for patching later. Keep the spare dry and cool and it stays viable for a season or two.
- Seed cost and turf comparison (advanced): whole packs at your price, against turf at a price per m2 for the same area. Typical UK prices are GBP 8 to GBP 15 per kilo of seed and GBP 3.50 to GBP 6 per m2 of turf delivered, so seed usually costs a fifth to a tenth as much but needs a warm, moist sowing window and patience.
- Per sowing pass (advanced): half the total seed, because the even-coverage trick is to sow two passes at right angles, ideally using a marked cane grid or a spreader calibrated on a path first.
- Rate comparison table (advanced): seed, packs and cost for all three sowing rates on your area, so you can see what overseeding the thin bits instead of starting again would save.
Source and further reading: RHS: lawns from seed.