About the Germination Rate Calculator
This germination rate calculator turns a simple seed test into a germination percentage, then uses it to tell you how many seeds to sow to finish with the number of plants you want. Enter how many seeds you tested and how many sprouted, your target plant count and an allowance for seedlings lost after germination (damping off, transplant shock, pests).
It is useful for gardeners checking old seed packets before spring, market growers planning trays and anyone saving their own seed. If you sow more than one seed per cell or hole, it also shows the chance that each cell ends up with at least one seedling, so you can decide between sowing singles and thinning doubles.
A germination test is easy: fold 10–20 seeds in a damp paper towel inside a plastic bag, keep it warm, and count sprouts after the typical germination time for the crop. The larger the sample, the more reliable the percentage.
With the default inputs, the germination rate is 85%. Change any value above to recalculate instantly.
How to use the germination rate calculator
- 1Do a germination test: sprout 10–20 seeds in a damp paper towel and count them.
- 2Enter the number tested and the number that germinated.
- 3Enter how many plants you want and an allowance for seedling losses.
- 4Choose seeds per cell to see the chance of a filled cell.
- 5Sow the number of seeds shown, plus a few spares.
Formula and method
The germination rate is the share of tested seeds that sprouted. Each seed you sow then has a chance of rate × survival of becoming a finished plant, so dividing the plants you want by that probability and rounding up gives the expected number of seeds to sow.
When you sow k seeds per cell, a cell fails only if every seed fails, which happens with probability (1 − rate)^k, so the chance of at least one seedling is 1 − (1 − rate)^k. The number of cells needed divides the target by that chance times survival. These are expected values — sow a few spares for bad luck.
- Rate
- Germination rate as a fraction
- Survival
- Share of seedlings that survive to planting out
- k
- Seeds sown per cell or planting hole
Worked examples
17 of 20 seeds sprouted, aiming for 50 plants
The germination rate is 17 ÷ 20 = 85%. With 90% seedling survival each seed has a 76.5% chance of becoming a plant, so 50 ÷ 0.765 ≈ 65.4 → sow 66 seeds. Sowing two per cell, only 0.15² = 2.25% of cells stay empty, so about 57 cells are needed.
Old tomato seed: 6 of 10 germinated, 24 plants wanted
At 60% germination, 24 plants need 24 ÷ 0.6 = 40 seeds sown singly. Sowing three per cell leaves 0.4³ = 6.4% of cells empty, so 24 ÷ 0.936 ≈ 25.6 → 26 cells.
Market grower: 72 of 100 onion seeds sprouted, 200 transplants wanted
A 72% germination rate and 85% seedling survival give each seed a 0.72 × 0.85 = 61.2% chance of becoming a transplant. 200 ÷ 0.612 ≈ 326.8, so sow 327 seeds — 127 more than the plants you need.
Frequently asked questions
How do I test seed germination?+
Place 10 or 20 seeds on a damp paper towel, fold it, seal it in a plastic bag and keep it at the crop’s germination temperature. Count sprouted seeds after the usual germination time — often 5–10 days — and divide by the number tested.
What is a good germination rate?+
Fresh commercial vegetable seed is usually tested at 80–95% germination. Rates above 85% are excellent, 60–80% are usable if you sow more thickly, and below about 50% it is usually worth buying new seed.
How long do seeds stay viable?+
It varies by crop. Onion, parsnip and leek seed often lose viability within one to two years, while tomato, cucumber and squash seed can stay viable for four to six years or more if kept cool, dry and dark.
Should I plant one or two seeds per cell?+
With germination above about 90%, one seed per cell is efficient. Below that, two seeds per cell greatly cut empty cells — at 80% germination only 4% of double-sown cells stay empty — but you will need to thin the extras.
Why did my seeds not germinate?+
The most common causes are old seed, soil that is too cold or too wet, sowing too deep, and letting the surface dry out. Some seeds also need light, cold stratification or soaking before they sprout.