Lawn Seed Calculator

The calculator multiplies your lawn length by width to get the area in square metres, then applies a seed rate based on the job type you choose. A new lawn on bare soil uses 35g/m², while overseeding an existing lawn uses 25g/m². The total grams are converted to kilograms, and the bag suggestion function works out the most efficient combination of standard UK bag sizes (10kg, 5kg and 1kg) to buy.

The bag suggestion logic tries to use the largest bags first to minimise cost, then tops up with smaller bags for any remainder. For example, if you need 7.2kg, it suggests 1 x 5kg + 1 x 1kg + 1 x 1kg rather than 7 x 1kg bags. This isn't a perfect optimisation — it's a practical guide that balances cost, storage and the fact that you'll likely use leftover seed for future top-ups.

A lawn 10m long and 5m wide has an area of 50m². For a new lawn at 35g/m², the total seed needed is 50 × 35 = 1,750g = 1.75kg. The bag suggestion would be 1 x 1kg bag + 1 x 1kg bag (2kg total), leaving 250g for touch-ups. If you chose to overseed instead, the calculation would be 50 × 25 = 1,250g = 1.25kg, suggesting 1 x 1kg bag + 1 x 1kg bag (2kg) or a single 5kg bag if you have other lawns to do.

For a larger 20m x 10m lawn (200m²), a new lawn would need 200 × 35 = 7,000g = 7kg. The bag suggestion would be 1 x 5kg + 2 x 1kg = 7kg exactly. Overseeding the same area would need 200 × 25 = 5,000g = 5kg, suggesting a single 5kg bag. These are starting figures — odd-shaped lawns with lots of edges, borders and awkward corners typically need 10-15% extra.

The two best windows for sowing grass seed in the UK are early autumn (September to early October) and mid-spring (April to May). Autumn is often considered ideal because soil is still warm from summer, there's less weed competition than spring, and autumn rainfall reduces the need for watering. Spring sowing works well too, but new lawns are more vulnerable to summer drought and summer annual weeds.

Soil temperature matters more than air temperature. Grass seed germinates best when soil is consistently between 8-10°C. In most of the UK, this is reliably true from late March through to early October. If you sow in late autumn or winter, the soil is too cold for reliable germination, and the seed may rot or be eaten by birds before it establishes. Check a soil thermometer if you're unsure — a cheap digital probe from a garden centre is accurate enough.

UK-specific seed mixes are formulated for our climate and soil conditions. A general-purpose lawn mix typically contains perennial ryegrass (fast-germinating, hard-wearing), strong creeping red fescue (fine texture, shade-tolerant) and sometimes smooth-stalked meadow grass. For shady areas, look for a shade-tolerant mix with more fescue and less ryegrass. For ornamental or fine lawns, choose a fescue-dominated mix for a denser, more luxurious finish.

Check the germination rate on the bag. UK regulations require grass seed to have a minimum germination rate of 70% for ryegrass and 60% for fescues, but good commercial mixes exceed 85-90%. Higher germination means fewer seeds per kilogram, but more of what you pay for actually grows. Avoid budget mixes with vague labelling — if it doesn't state the species and germination rate, it's probably poor quality.

Finally, think about how much to buy. Grass seed stays viable for 2-3 years if stored in a cool, dry place, so buying a 5kg bag when you only need 3kg isn't wasteful if you have other lawns to sow or want to keep some for patch repairs. Keep the bag sealed and store it in a shed or garage over winter.