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What a germination test tells you
Seeds look identical from the outside, but their ability to sprout varies hugely. Some packets sprout at 95 percent, while an old packet of the same seed may manage only 30 percent. If you sow without knowing, you may end up with half-empty rows and a late, thin crop. A germination test is a small experiment that answers one question: out of a known number of seeds, how many actually sprout? This calculator turns that test into a percent and into the number of seeds you should sow to get the plants you want.
The test itself is simple. Count out a set number of seeds, often 20 or 50 or 100, and sow them in damp soil or on damp paper in a warm spot. Keep them moist but not soaking, and count how many sprout within the normal sprouting time for that seed. Ten sprouts out of 20 seeds is a 50 percent rate. A hundred seeds makes the math easy, but even 20 seeds gives a useful guide if you accept that the true rate may be a little higher or lower.
Why does the percent matter so much? Because it decides how thickly you sow. If you want 50 lettuce plants and your test shows 80 percent germination, you need to sow about 63 seeds, not 50. If the rate is only 40 percent, you need 125 seeds for the same 50 plants. Without the test, gardeners either sow far too thickly and waste seed, or sow too thinly and lose weeks waiting for gaps to fill. The calculator does this division for you, including the extra seeds beyond your target.
A worked example
Small test sizes need care. If you test only 10 seeds and 7 sprout, the calculator says 70 percent, but the true rate could easily be 50 or 90 percent, because one seed either way swings the result by 10 points. Test at least 20 seeds when you can, and 50 or 100 for seeds you depend on. A bigger test costs a few seeds and saves a season of guessing.
How to run a fair test
Warmth matters most. Most vegetable seeds sprout best between about 18 and 24 degrees Celsius, and cold soil can make good seed look bad. If you test indoors on a warm windowsill but sow outdoors in cold soil, the outdoor result will be worse than the test. Match the test conditions to your real sowing conditions as closely as you can, or treat the test as a best-case number.
Moisture is the second factor. Seeds need steady dampness to sprout, not floods and not dry spells. Damp paper in a covered tray works well because you can see the sprouts and keep the moisture even. Soil tests are closer to real sowing but hide the seeds, so you must wait for shoots to appear above the surface. Either way, do not let the test dry out halfway, or you will blame the seed for a watering failure.
Time limits matter too. Each seed type has a normal sprouting window, often 7 to 21 days. Count sprouts at the end of that window, not after a single day, and do not keep counting stragglers for a month: a seed that takes six weeks to sprout will not make a useful plant in a normal season. Check the packet or a seed guide for the expected window for your seed.
What to do with a low rate
Remember that these garden results are general guidance estimates only. Your local climate, soil, weather, and seed storage govern the real result, so a test done in a warm kitchen will not perfectly predict a cold outdoor bed. Use the test to plan quantities, then watch the real bed and sow a second batch if gaps appear. Succession sowing, a small second sowing two weeks later, covers most surprises.
Frequently asked questions
Do I need to test fresh packets? Fresh packets from a good supplier usually germinate well, so a test is optional. Test old packets, saved seed, and any packet stored in a warm or damp place, because those are the ones that surprise you.
Can I test seeds on paper? Yes. Damp kitchen paper in a sealed bag or covered tray, kept warm, is a reliable test for most seeds. Move sprouted seeds gently into soil, or use the test only as a count and sow fresh seed in the bed.
Why did my test beat my garden result? Cold soil, dry soil, slugs, birds, and sowing too deep all cut real germination below the test. Protect the bed with netting or fleece, sow at the right depth, and keep it moist until shoots appear.
Use the calculator above with your test counts and your target plant number, sow the extra seeds it suggests, and thin to the right spacing. A ten-minute test at the start of the season is the cheapest insurance a gardener can buy.
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