Seed Germination Calculator
Test old seeds before you sow a whole bed. Enter your paper-towel test results and how many plants you want: the calculator tells you how many seeds to sow.
Old seeds are a gamble. A packet left over from last year can still be perfectly good. Or nearly dead. And you cannot tell by looking at it. A five-minute paper-towel test tells you the truth, and this calculator turns that test into a sowing number: how many seeds to sow to end up with the plants you actually want.
It works for any seed you can count: vegetables, herbs, and flowers. Test first, sow second, and use the Plant Spacing Calculator afterwards to work out how many of those plants fit in your beds.
Your result
- –seeds to sow
- –extra seeds vs. plants wanted
- –overplanting rate
- –packets needed
How this is worked out
Germination rate (%) = (seeds germinated ÷ seeds tested) × 100. Seeds to sow = plants wanted ÷ (germination rate ÷ 100), rounded up. Seeds are whole units, so the result always rounds up. Rounding down would leave you short of your target.
Assumptions:
- The germination rate from your test applies to the whole packet, because the tested seeds were a random sample of it.
- Germinated seeds cannot exceed the seeds tested. If they do, the test was counted wrong.
- The target rate must be above 0%; a packet with 0% germination cannot produce plants at any sowing rate.
- Field emergence is usually 5 to 15% below a paper-towel test, because soil is colder, drier, and less even than a damp towel. Sow a little extra if you are direct-sowing outdoors.
- Packet counts are estimates: if you also want a packet count, divide seeds to sow by seeds per packet and round up.
Three worked examples
1. Three-year-old heirloom tomato seed. You test 10 seeds and 7 sprout in 8 days: 70% germination. For 50 transplants, sow 50 ÷ 0.70 = 72 seeds (2 seeds in each of 36 tray cells, then thin to the stronger seedling).
2. Fresh commercial seed, 95% germination. You want 200 cabbage seedlings: 200 ÷ 0.95 = 211 seeds. Single-seeding plug trays saves thinning work and leaves almost no blank cells.
3. Low-viability parsnip seed, 40% germination. You test 10 seeds and only 4 sprout. For 100 plants you would need 100 ÷ 0.40 = 250 seeds. And parsnip seedlings emerge weakly, so even that often fails. Discard the packet and buy fresh current-season seed.
What your rate means
| Germination rate | What it means | What to do |
|---|---|---|
| 90 to 100% | Very high | Sow at the normal rate. |
| 80 to 89% | High: standard for commercial packets | Sow at the normal rate. |
| 70 to 79% | Moderate | Sow thicker; divide your target by the rate. |
| 50 to 69% | Low | Sow double or triple, and only for seed you cannot replace. |
| Below 50% | Too weak for a reliable stand | Buy a fresh packet instead of gambling a whole bed. |
These bands follow Utah State University Extension and University of Arkansas Extension guidance on home germination tests: test 25 to 50 seeds on moist paper towels in an unsealed bag at 70 to 80°F (21 to 27°C), count every 2 to 3 days, and treat anything above 80% as high quality.
Common germination mistakes
- Soaking seeds under water for more than 24 hours. A 6 to 12 hour presoak softens coats, but longer soaking suffocates the embryo. Seeds need dissolved oxygen to germinate.
- Overheating a heat mat above 95°F (35°C). An unregulated mat in direct sun can cook seeds. Use a thermostat probe in the soil, set to 75 to 80°F.
- Letting cool-season seeds get too hot. Lettuce, spinach, and celery enter thermo-dormancy above 80°F (27°C) and will not sprout. Germinate them indoors in a cool spot, around 65°F.
- Testing too few seeds. Testing 5 seeds gives a rate in 20% jumps. Too coarse to plan with. Test 20 to 25 seeds so each sprout is 4 to 5%.
- Counting swollen seeds as germinated. Only count seeds with a visible root tip. A seed that only swelled has not germinated.
What to do next
Once you know how many seeds to sow, plan the sowing date with the Plant Spacing Calculator for bed space, and keep the test habit: test every packet older than one season before you commit a bed to it. A test costs five minutes; a failed sowing costs weeks of the growing season.
Putting the result to work
A germination rate is only useful if it changes what you sow. Use it three ways. First, adjust the sowing rate: the calculator’s seeds-to-sow number already includes the rate, so sow that number and thin later rather than sowing the target number and hoping. Second, decide what to keep: a packet above 80% is fine for another season if stored cool and dry; a packet at 50 to 70% should be used up this season, because it will only get worse. Third, compare packets: if two packets of the same variety test at 90% and 60%, sow the strong one first and keep the weak one as backup.
Timing matters too. Test in late winter, before the sowing season starts, so a failed test still leaves time to order fresh seed. Test warm-season seed (tomato, pepper, cucumber) at 75 to 80°F and cool-season seed (lettuce, spinach, pea) at 65 to 70°F. A cool-season seed tested hot can look dead when it is only dormant. And keep the test itself honest: use seeds from the middle of the packet, not the ones on top, and count on the same day each time so the rate is comparable from year to year.
How to run the paper-towel test (5 minutes to set up)
- Moisten 2 to 3 layers of paper towel. It should feel like a wrung-out sponge, not dripping wet.
- Place 25 to 50 seeds in rows on the top towel, then roll or fold the towel over them.
- Put the roll in a plastic bag. Do not seal the bag completely. Seeds still need air.
- Keep it warm, about 70 to 80°F (21 to 27°C), for example on a kitchen counter.
- Count germinated seeds every 2 to 3 days and remove them as you count. Radishes may sprout in 2 to 3 days; peppers can take 10 to 14 days. When no more seeds germinate, your test is done.
Formula: germination % = (seeds germinated ÷ seeds tested) × 100. Seeds to sow = plants wanted ÷ (germination % ÷ 100), rounded up.
What is a good germination rate?
85% or higher: sow at the normal rate. 70 to 84% is workable if you sow thicker. 50 to 69% is low. Sow double or triple and only if you cannot replace the seed. Below 50% the seed is too weak for a reliable stand, so buy a fresh packet instead of gambling a whole bed on it.
How do I perform a seed germination test at home?
Place 10 to 25 seeds on a damp paper towel, fold it over, and put it inside a plastic bag left partly open so air can reach the seeds. Keep it warm, 70 to 80°F (21 to 27°C), and count the seeds with a visible root tip every 2 to 3 days for 7 to 14 days. Divide germinated by tested and multiply by 100.
How many seeds should I plant to get 50 healthy seedlings?
Divide 50 by your germination rate as a decimal. At 80% germination that is 50 ÷ 0.80 = 62.5, which rounds up to 63 seeds. With a 10% safety margin for thinning and damping-off, sow about 69 seeds. Seeds are whole units, so always round up, never down.
Why should I not round down when calculating seeds to sow?
Because seeds are discrete physical units. Rounding down means sowing fewer seeds than the maths requires, so even with perfect germination you end up short of your target number of plants. Ceiling rounding is the only rounding that guarantees the target.
Can I save seed that tested low?
No. If germination is already low, it will only fall further in storage, even in a cool, dry jar. Use what is left this season at a thicker sowing rate, or replace the packet with fresh seed and test the new lot the same way.
Method and thresholds follow Utah State University Extension and University of Arkansas Extension guidance on home germination testing: USU. Collecting and Storing Seeds from Your Garden · Arkansas Extension. How to Conduct a Seed Germination Test at Home. Results are estimates for planning, not a lab test. Field emergence is typically 5 to 15% below a paper-towel test.
