Lawn Seed Calculator

Grass seed application rates depend on whether you are creating a new lawn or overseeding an existing one. For a new lawn the typical rate is 35 to 50 g per m², while overseeding to repair patches or thicken thin areas needs only 15 to 25 g per m². Premium mixes built around ryegrass tend to use the higher rates and fine fescue mixes the lower ones. Sowing too heavily wastes seed and crowds the seedlings so that none of them establish properly, while sowing too thinly leaves bare patches that weeds colonise first.

The calculator takes your lawn area and your purpose and estimates the total weight of seed needed. Seed type matters as well: hard-wearing utility mixes suit families and pets, fine ornamental mixes suit low-traffic lawns, and shade-tolerant blends cope with dappled light under trees. Each carries a slightly different recommended sowing rate. On a 100 m² plot that difference is the gap between roughly 5 kg of seed for a new lawn and 2 to 3 kg for an overseed.

Timing does more for germination than any product claim. The best windows in the UK are April to May and September to October, when soil temperatures sit above 8 to 10°C and rain is reliable. Most retail mixes germinate in 7 to 14 days at a soil temperature of 8 to 15°C, and the ground has to stay moist for 2 to 3 weeks after sowing. Retail packets of 1 to 2.5 kg cover roughly 25 to 50 m², so a small repair job rarely needs more than one box.

Mix choice sets the price as much as the performance. A general purpose blend of perennial ryegrass, fescue and bent costs £15 to £40 per kg and establishes quickly. Hard-wearing family mixes with a high ryegrass content run £20 to £50 per kg and tolerate children and dogs. Shaded-lawn blends built on creeping red fescue and smooth-stalked meadow grass cost £25 to £60 per kg and grow more slowly. Fine ornamental bowling-green seed, heavy on fescue with no ryegrass at all, is £40 to £80 per kg and demands intensive care. Wildflower meadow mixes of about 80% grass and 20% native wildflowers cost £30 to £100 per kg and are cut once or twice a year.

Preparation is where a new lawn is won or lost. An existing sward has to be killed off with glyphosate where that is still permitted, or rotovated in. Bare soil should be dug over 100 to 150mm deep with stones and roots taken out, raked level, trodden firm with a light heel-down all over, then raked again to a fine tilth. Aim for a pH of 5.5 to 7.0 and lime an acidic clay. Poor ground benefits from 25 to 50mm of screened topsoil at £40 to £80 per m³ delivered, rolled lightly with a garden roller or by the plank-and-feet method. Budget a weekend for 100 m².

Sowing itself is quick. Mix the seed half and half with sand so you can see where it has landed, then sow half the total walking north to south and the other half walking east to west, which crosshatches the coverage and hides any missed strip. Rake in lightly to a depth of 1 to 2cm and water with a fine spray rather than a gushing hose, which floats seed into drifts. Keep the soil moist until the grass is established. Take the first cut when the sward reaches 75mm, dropping it to 50mm and never removing more than a third of the height.

New grass needs protecting, so allow light foot traffic only for 4 to 6 weeks and full use after 8 to 12 weeks. After that the lawn settles into an annual rhythm. March and April are for scarifying, aerating, top-dressing and a high-nitrogen feed. May and June mean weekly mowing, selective herbicide where it is genuinely needed, and overseeding thin patches. Through July and August you either water in drought or let the lawn brown off, since it recovers in autumn. September brings a feed lower in nitrogen and higher in potassium, more overseeding and a light scarify. October and November are for the last raised-blade cut and regular leaf raking, and in winter you stay off frosted or saturated turf. Feeding 100 m² costs £20 to £40 a year.