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Lawn Overseeding Calculator
Calculate seed quantity, preparation steps, and timing for lawn overseeding.
The whole lawn for a full top-up, or just the bare patches if spot-seeding
Drives the seed rate: light 15, patchy 25, heavily worn 35 g/m²
Fine/ornamental fescue uses less seed per m² (small seed); hardwearing rye mixes a little more
Sets expected germination time; autumn is the most reliable UK window
Grass seed bag you plan to buy — used to round up to whole bags
Optional seed price; cost is charged on the actual weight needed, not rounded-up bags
Force a specific g/m² rate (e.g. 50 post-scarify); 0 keeps the condition default
Extra seed for uneven spreading (default 10%)
How We Calculate This
Seed rate depends on lawn condition: lightly thin areas need about 15 g/m², patchy lawns 25 g/m², and heavily worn or scarified lawns 35 g/m². These sit inside the standard UK overseeding band of roughly 15-25 g/m² for a top-up and 35-50 g/m² for heavy renovation (RHS lawn repair; Lawnsmith seeding rates).
The base rate is fine-tuned for grass type — fine fescue and bent mixes have smaller, lighter seed so cover the same area with fewer grams (×0.8), while ryegrass-heavy hardwearing mixes use a little more (×1.1). A waste factor (default 10%) is added for uneven spreading, then the total is divided by your chosen bag size to give whole bags. Estimated cost is based on the actual seed weight required, not the rounded-up bag weight.
Germination timing follows season and soil temperature: perennial ryegrass germinates in about 7-10 days once soil is above ~8-10°C, with fescues a little slower. Autumn (warm soil, reliable moisture) is the most dependable window in the UK.
Frequently Asked Questions
Autumn (September-October) is ideal as the soil is warm and moist. Spring (March-April) is the second-best option. Avoid overseeding in summer unless you can water daily, and avoid winter entirely.
Yes, scarifying removes thatch and opens the soil surface, giving seeds direct contact with the soil. This dramatically improves germination rates compared to simply scattering seed on top of existing grass.
Wait until the new grass is at least 50mm (2 inches) tall before the first mow, and set the mower high. This usually takes 3-4 weeks. Mowing too early can pull out seedlings.
Use a mix that matches your existing lawn type. General purpose mixes work well for most gardens. Fine fescue blends are better for ornamental lawns, and rye-heavy mixes for high-traffic family lawns.
Related Calculators
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Overseeding Calculator
Calculate seed rates for patching and thickening existing lawns.
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Lawn Sand Calculator
Calculate lawn sand quantities for moss control and greening.
Scarification Calculator
Plan your lawn scarification schedule and calculate waste volume.
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Last updated: March 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.