What this calculator works out
This calculator multiplies lawn area by a sowing rate to give the weight of seed needed, in kilograms, and adds a 10% spare figure.
The rate is the decision. A new lawn needs roughly half as much again as overseeding an existing one, and sowing too heavily produces a weak, disease-prone sward rather than a thicker one.
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Before you sow
- New lawns are commonly sown at 35 to 50 g per square metre. Overseeding an existing lawn uses 25 to 35 g.
- Check the rate on the seed box — mixtures differ, and fine ornamental mixtures use different rates from hard-wearing ones.
- Sow in spring or early autumn. Autumn is generally better: warm soil, reliable rain and less competition from weeds.
- Seed needs contact with soil and consistent moisture for two to three weeks. Sowing before a dry spell wastes it.
Oversowing produces dense seedlings competing for the same light, water and nutrients, which leaves them thin, weak and prone to damping off. The correct rate gives each seedling room to tiller — to produce side shoots — which is what actually creates a dense sward.
How this calculator works
Area times rate, converted to kilograms:
Lawn area = length × width
Seed in kg = area × rate in g/m² ÷ 1,000
With spare = seed × 1.1The 10% spare covers uneven spreading and patching. Seed keeps reasonably well in a cool dry place for a season or two, so a small surplus is not wasted.
Worked example: a 10 m by 6 m lawn
Using the default figures — 10 m by 6 m at 35 g per square metre:
- Lawn area: 60 m²
- Seed needed: 2.10 kg
- With 10% spare: 2.31 kg
Around 2.3 kg for a modest lawn, which is one large box. At the top of the new-lawn range, 50 g/m², the same area needs 3.3 kg. That difference is worth getting right in both directions: too little leaves bare patches for weeds, too much wastes seed and produces a weaker result.
Common mistakes
- Sowing at double the rate for a thicker lawn. It produces a weaker one.
- Sowing onto unprepared ground. Seed needs a firm, raked, weed-free surface.
- Letting it dry out. Consistent moisture for two to three weeks is essential.
- Sowing in midsummer. Heat and drought make establishment far harder.
- Using an ornamental mixture on a lawn that takes traffic. Match the mixture to the use.
Frequently asked questions
When is the best time to sow?
Early autumn, generally September in the UK — the soil is warm, rainfall is more reliable, and annual weeds are less competitive. Mid-spring is the second-best window. Both are preferable to summer, when keeping seed consistently moist is difficult and expensive.
Seed or turf?
Seed is far cheaper, offers more choice of mixture, and establishes a stronger root system. Turf gives an instant result and can be laid over a longer season, but costs several times as much and still needs weeks of careful watering. For a large area, seed is usually the practical choice.
How do I prepare the ground?
Clear weeds, dig or rotavate, remove stones, then rake level and firm by treading or rolling — repeating the rake and firm cycle until the surface is even. A firm, fine seedbed matters more than almost anything else. Leaving prepared ground for a fortnight lets a flush of weeds germinate for removal before sowing.
Why is my new lawn patchy?
Usually uneven sowing, birds, or the surface drying out in the first fortnight. Patches can be overseeded once the main sward is established, ideally with the same mixture. Sowing half the seed in one direction and half at right angles gives noticeably more even coverage than a single pass.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
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References
- Royal Horticultural Society — lawn establishment, seed rates and mixture selection
- GOV.UK — guidance on water use, hosepipe restrictions and garden practice
Sources are checked at publication and can change — how I choose and check references.