Seed Germination Calculator: Test Rate & Sowing Adjustment
Old or poorly stored seed loses vigour. A quick paper-towel test tells you whether it is still worth sowing and how thickly to sow it.
Reviewed November 2026Metric & imperialFormula shownPrintable plan
Seed Germination Calculator
Calculating…
Runs in your browser. Nothing you enter is stored.
What this calculator does
Enter how many seeds you tested and how many sprouted. The calculator gives the germination rate and the seeds to sow for your target number of plants.
How to use the seed germination calculator
Place 10–20 seeds on a damp paper towel, fold it and seal it in a bag.
Keep it warm and check daily for the time on the packet.
Count the seeds that sprouted.
Enter the numbers and your target plants.
Sow the number shown.
How the calculation works
germination = sprouted ÷ tested
seeds per plant = 1 ÷ germination
seeds to sow = ceil(plants ÷ germination)
A test of 20 seeds gives a useful estimate; larger samples are more precise.
Worked example
If 17 of 20 seeds sprout, germination is 85%: sow 59 seeds for 50 plants.
Assumptions and limitations
Outdoor germination is usually lower than an indoor test.
Some seeds need light, cold or soaking first; follow packet advice.
Slow germinators such as parsley may take three weeks.
Frequently asked questions
How do I test seed germination?
Put 10–20 seeds on a damp paper towel, seal in a bag, keep warm and count how many sprout.
What is a good germination rate?
85% or more is excellent; most packets guarantee 65–80% depending on the crop.
Can I still use seeds with low germination?
Yes, sow more thickly. Below about 50%, fresh seed is usually better value.
How long does a germination test take?
From 3 days for radishes and brassicas to 2–3 weeks for peppers and parsley.