GardenCalcs.uk
Overseeding Calculator — Thicken Your Lawn
Calculate seed rates for patching and thickening existing lawns.
Total area you plan to overseed, in square metres (length × width of the lawn).
Light = minor thinning (30% of new-lawn rate); Moderate = noticeable gaps (50%); Heavy = mostly bare or full renovation (70%).
General/hard-wearing mixes contain ryegrass (fast germinating); fine and shade mixes are fescue-based (smaller seed, slower to establish).
New lawn rate for general purpose seed (default 35 g/m²)
New lawn rate for fine/ornamental seed (default 25 g/m²)
New lawn rate for hard-wearing seed (default 40 g/m²)
New-lawn rate for shade-tolerant (fine fescue) seed. Default 30 g/m² reflects fescue's smaller, denser seed; some growers raise this to 40-50 g/m² because shade mixes germinate slowly.
How We Calculate This
Overseeding rates are a fraction of new lawn rates, depending on lawn condition.
Light overseeding (minor thinning): 30% of new lawn rate. Moderate: 50%. Heavy renovation: 70%.
Combined with the seed type rate (General 35g/m², Fine 25g/m², etc.) this gives the grams per m².
Frequently Asked Questions
Overseeding is sowing grass seed over an existing lawn to fill thin patches, improve density, and introduce stronger grass varieties.
September is the best time in the UK. The soil is warm, autumn rain helps germination, and there is less weed competition. Spring is the second-best option.
Yes. Mow short, scarify to remove thatch, and rake to expose soil. Seed needs soil contact to germinate. Top dress with a thin layer of compost after seeding.
Wait until new grass is at least 5 cm tall (usually 3-4 weeks). Set mower to a high cut for the first few mows.
A thin layer (5mm) of fine compost or lawn top dressing helps seed-to-soil contact and retains moisture. Do not bury the seed more than 5mm deep.
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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.