Seed germination calculator and seed viability chart

Enter how many seeds sprouted in a germination test and the tool gives your germination rate, the seeds to sow per hole and the seeds to sow for the plants you want. A chart shows how long common vegetable seeds usually keep.

How it works

The calculator does four jobs.

Germination rate. Divide the seeds that sprouted by the seeds you tested.

Seeds per hole. If each seed comes up with a chance equal to your germination rate, independently of the others, the tool finds the smallest number of seeds that lifts the chance of at least one seedling to your target:

n = ceil( ln(1 - target) / ln(1 - rate) )

"Ceil" means round up to the next whole number. The answer is never less than one, and at a 100% rate one seed per hole is enough.

Seeds to sow. For a number of plants, divide by the germination rate and by the share of seedlings you expect to survive after they come up (slugs, damping off, transplant losses), then round up.

seeds = ceil( plants / (rate × survival) )

Viability chart. The chart gives the typical years that seed of 26 vegetables stays usable when kept cool and dry, from common extension-service ranges. Pick a crop, enter the age of your seed (years since it was packed or saved), and you get one of three verdicts:

fresh: no older than the low end of the usual range;
test first: past that, but no more than one year beyond the high end;
likely weak: older still.

Onion, leek, parsley and sweet corn are short-lived at 1 to 2 years, and parsnip lasts about 1. Pepper keeps 2 to 3 years, carrot and pea 3, tomato 4 to 5, lettuce 3 to 6, and cucumber and melon about 5. So a 1-year-old packet of onion seed is fresh, a 3-year-old one needs testing, and at 4 years it is likely weak. Tomato seed is still fresh at 4.

Running a paper-towel test.

Count out 10 or 20 seeds. Twenty gives a steadier answer.
Wet a paper towel so it is damp but not dripping, spread the seeds over half of it and fold the other half on top.
Slip it into a plastic bag or lidded box and keep it at warm room temperature, out of direct sun.
Check every day or two and mist the towel if it starts to dry out.
Count a seed as sprouted once a root has clearly emerged. Radish and lettuce often show roots within a few days; parsley, carrot and pepper can take two weeks or more, so wait out the days to germination on the packet.

Worked example

The tool opens with a test of 20 seeds, of which 16 sprouted. 16 / 20 = 0.8, a germination rate of 80%.

Seeds per hole, aiming for a 95% chance of at least one seedling: ln(0.05) / ln(0.2) = 1.86, which rounds up to 2. With two seeds the chance that both fail is 0.2 × 0.2 = 0.04, so 96% of holes produce a seedling. One seed per hole would give only 80%. Thin to the strongest seedling once they are up.

For 40 plants, with 90% of seedlings expected to survive: 40 / (0.8 × 0.9) = 55.6, rounded up to 56 seeds.

What it can't tell you

A paper towel is kinder than a garden bed. Field emergence is usually lower, because soil temperature, sowing depth, crusting, birds and slugs all take their share. The survival setting is the place to allow for that.

A small test is a rough estimate. Sixteen out of 20 fits a true rate anywhere from about 60% to 90%, so the two-seed answer is a sensible bet, not a guarantee. Testing more seeds narrows the range.

The math treats every seed as independent. In a real hole, seeds share the same soil and the same slug, so failures come in clumps and the odds are a little worse than the formula says.

Sprouting is not the same as vigor. Old seed that germinates can still grow slowly or unevenly.

Sources differ by a year or so on viability, and all assume seed kept cool and dry; seed in a warm, humid shed ages much faster. An old storage rule of thumb says the temperature in degrees Fahrenheit plus the relative humidity should add up to less than 100. A germination test of your own beats the chart.

Saved seed has two more catches. Seed from an open-pollinated variety comes true if it was not crossed with a neighboring variety, but seed from an F1 hybrid segregates in the next generation, the pattern the Punnett square calculator shows for an Aa × Aa cross. And depending on the country and the variety, the law may limit what you can save and replant, which our explainer on UPOV 91 and seed laws covers.

Sources

Seed storage tables from US university extension services, giving typical life spans for vegetable seed kept cool and dry. The chart follows their common ranges.
The seed testing rules of the Association of Official Seed Analysts and the International Seed Testing Association, which commercial labs follow.
Basic binomial probability, from any introductory statistics text, for the seeds-per-hole formula.

Questions

How many seeds per hole should I plant?

It depends on your germination rate. At 80%, two seeds per hole give a 96% chance of at least one seedling. At 50%, you need five for a 95% chance, a sign the lot may be worth replacing. Thin to the strongest seedling once they are up.

How do you calculate germination rate?

Divide the number of seeds that sprouted by the number you tested and multiply by 100. Sixteen sprouted out of 20 is 80%.

How do you do a germination test with a paper towel?

Lay 10 or 20 seeds on a damp paper towel, fold it over, keep it moist in a bag at warm room temperature, and count the seeds with a visible root after the packet's days to germination.

How long do vegetable seeds last?

Kept cool and dry, onion, leek, parsley and sweet corn seed usually lasts 1 to 2 years, tomato 4 to 5 and cucumber about 5. Past the usual range, run a germination test before you rely on a packet.

Will old seeds still grow?

Often some will. Germination tends to fall off gradually, so a test tells you whether to sow more thickly or start over with fresh seed.