GardenCalcs.uk
Seed Starting Calculator — How Many Seeds to Sow
Calculate how many seeds to sow from your germination rate, seedling survival and module-tray size.
Final number of healthy seedlings you want to plant out
Check seed packet; typically 70-90%
Number of cells in your module trays (default 24)
Extra multiplier for handling losses (default 1.0)
Percentage of germinated seedlings that survive to planting (default 95%)
How We Calculate This
Formula: Seeds to sow = Plants wanted ÷ (Germination rate ÷ 100) × Waste factor ÷ (Survival rate ÷ 100), rounded up.
The germination divisor compensates for seeds that never sprout; the survival divisor allows for seedlings lost to damping-off, transplant shock and handling; the waste factor adds an optional safety margin for spilled or mis-sown seed.
For example, to raise 50 plants at 80% germination, 95% seedling survival and a 1.0 waste factor: 50 ÷ 0.80 = 62.5, ÷ 0.95 = 65.8, rounded up to 66 seeds.
Trays = seeds ÷ cells per tray, rounded up (default 24-cell modules, a common UK size — 40-cell and 6/12-cell trays are also standard).
Frequently Asked Questions
Not all seeds germinate. If germination rate is 80%, you need to sow 25% extra to account for failures. This calculator does that maths for you.
Most common vegetable seeds germinate at 70-90%. Tomatoes, peppers, and lettuce are usually 80%+. Carrots and parsnips can be as low as 60-70%. Check the seed packet.
Standard module trays come in 6, 12, 24, and 40 cells. This calculator uses 24-cell trays as the standard. Adjust if you use different sizes.
In the UK, most tender vegetables (tomatoes, peppers, aubergines) are started in February-March. Hardy crops (brassicas, lettuce) can be started from January.
Yes. Most seeds last 2-4 years if stored cool and dry. Parsnip and onion seed is best used fresh. Old seed may have much lower germination rates.
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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.