What does this seed germination calculator calculate?
This calculator converts a simple germination test into a decision. Divide the number of sprouted seeds by the number tested and you get the germination rate. The calculator then compares that rate against common thresholds and shows how many seeds to sow for the number of plants you actually want.
How to run a germination test
The standard home test takes about a week and needs only a paper towel, a plastic bag, and a warm spot.
- Moisten a paper towel and lay ten seeds along one edge. Ten makes the percentage easy to read.
- Fold or roll the towel, slip it into a plastic bag, and seal it so the towel stays damp.
- Label the bag with the crop and the date, then keep it somewhere warm, such as the top of a refrigerator.
- Check after the number of days listed on the packet, or after 7 to 10 days if you do not know the timing, and count the seeds that have sprouted.
Use seeds pulled at random from the packet rather than the largest ones, or the test will flatter the lot.
What you will need
Seeds tested
The number of seeds you placed in the towel. Ten is the usual sample size because the count becomes a percentage with no math.
Seeds that sprouted
Count a seed as sprouted when the root emerges. Some seeds swell without sprouting, and those do not count.
Plants you want
The number of plants you hope to end up with. The calculator divides this by the germination rate to show how many seeds to sow. Sow extra anyway, because germination is only the first hurdle between a seed and a harvest.
How to read the result
90 percent or more
The seed lot is in good shape. Sow at the rate and spacing on the packet.
70 to 89 percent
Viability has started to decline. The seed is still usable, but sow roughly a third more than normal and plan to thin.
50 to 69 percent
About half the seed is dead. You can double the seeding rate and thin hard, but a packet of fresh seed is often the better use of bed space and time.
Below 50 percent
Buy fresh seed. Sowing a mostly dead lot wastes weeks of the season on gaps and weak, uneven rows.
Germination is not the same as vigor
A seed that sprouts in a warm paper towel can still be too weak to push through real soil. Viability often fades before the germination rate does, so a marginal test result understates how the seed will perform outdoors. When the rate lands near a threshold, treat the seed as older than the number suggests.
Worked examples
Eight out of ten
A gardener tests ten lettuce seeds and eight sprout. The 80 percent rate lands in the "sow a little thicker" band, so for a row of 24 plants the calculator suggests sowing 30 seeds.
Four out of ten
Ten onion seeds produce four sprouts after ten days. At 40 percent, the recommendation is to buy fresh seed, since onion seed is already short lived and cheap to replace.
A large sample
A gardener testing saved bean seed uses fifty seeds and counts 41 sprouts. The 82 percent rate supports normal sowing with a modest increase, and the larger sample gives more confidence than a ten seed test.
Common mistakes
Testing too few seeds
Three sprouted seeds out of five is 60 percent, but the sample is so small the number means little. Ten seeds is the practical minimum.
Counting too early
Seeds that need two weeks are scored as dead at one week. Check the packet for the expected germination time before you count.
Letting the towel dry out
A dried out towel pauses the test. Check every few days and mist the towel if the bag loses humidity.
When this calculator may not be the right choice
The test measures germination under near ideal conditions. It cannot predict performance in cold, wet, or crusted soil, and it does not test for seedborne disease. For crops that need soil contact to germinate, such as some carrot and lettuce seed sown on the surface, a small direct sowing trial tells you more than a towel test.
Frequently asked questions
How many seeds should I test? Ten is the practical minimum for a readable percentage, and twenty gives a steadier answer. Testing a whole packet wastes good seed, and testing three seeds tells you almost nothing.
My old seed came back at 60 percent. Sow or buy? You can sow roughly double and cover the shortfall, but low germination often arrives with weak vigor, meaning slower, spindlier seedlings. For a crop that matters, fresh seed is cheap insurance.
Does a high germination rate mean strong seedlings? Not exactly. The test shows seeds can sprout, not that they will grow well. Old seed that germinates slowly or unevenly tends to produce weaker plants than fresh seed at the same rate.
