This germination rate calculator computes germination percentage from a seed-viability test — and works backwards from your target plant count to how many seeds to sow, losses included.
Paper-towel test protocol: 10–100 seeds on a moist paper towel in a sealed bag, warm spot, count sprouts at the species’ normal window (and use ≥ 25 seeds for a meaningful percentage — 9/10 could be 70% in disguise). Seeds-to-sow = target ÷ (germination × survival): losses compound multiplicatively, which is why old seed needs such heavy oversowing.
Rate = Sprouted ÷ Sown — then Plan Backwards Through Losses
Seed packets print a tested germination floor (often 85%+ for fresh commercial seed); storage and age erode it species by species. Where the surviving plants will stand is our Plant Spacing Calculator. Part of our Ecology Calculators collection.
Worked Example (Verified)
- 85 sprouted of 100 tested (defaults): 85% germination; for 50 target plants at 90% survival → sow ⌈50 ÷ (0.85 × 0.90)⌉ = 66 seeds
- The old-packet scenario: 45% germination → 50 ÷ (0.45 × 0.90) = 124 seeds for the same 50 plants — and thinning labor arrives as the hidden invoice
- Precision warning: a 10-seed test reading “80%” could truly be 55–95% — commercial labs test 400 seeds for the percentages printed on packets; home testers should test 25–100
- Field emergence runs below lab germination (cold soil, crusting, pests): outdoor direct-sowing commonly plans an extra 10–30% over this calculator’s indoor-based figure
Seed Longevity Cheat Sheet
- Short-lived (1–2 yrs): onion, leek, parsnip, parsley, sweet corn — buy fresh yearly.
- Medium (3–4 yrs): beans, peas, peppers, carrots.
- Long-lived (5+ yrs): tomatoes, brassicas, cucurbits, lettuce — cool, dry, dark storage stretches every category.
- The freezer trick: thoroughly dried seed in sealed jars freezes well for years for most vegetables.
Related Tools
- Plant Spacing Calculator — plants per bed by spacing
- Seed Starting Calculator — when to sow indoors
- Ecology Calculators — the full collection
Frequently Asked Questions
How do you calculate germination rate?
Sprouted ÷ sown × 100, counted at the end of the species’ normal germination window on a moist paper-towel test (or in soil). 85 of 100 = 85%. Use at least 25 seeds — small samples swing wildly (each seed in a 10-seed test is worth 10 percentage points).
How many seeds should I sow per plant wanted?
Target ÷ (germination × survival): 50 plants at 85% germination and 90% transplant survival needs 66 seeds. Gardeners compress this into rules of thumb (sow 2–3 seeds per cell for old seed) — the formula here is the same wisdom with your actual numbers.
What germination rate is acceptable?
Fresh commercial seed typically tests 85–99% ( packets guarantee minimums by species and law). 60–84%: usable with oversowing. Below ~50%: replacement is usually cheaper than the space, time and thinning a weak lot consumes — unless the variety is irreplaceable, in which case sow everything and coddle.
Why did my seeds not germinate?
The usual quartet: dead seed (age/storage heat and humidity), wrong temperature (lettuce balks above ~25 °C; peppers sulk below 20 °C), depth errors (light-needing seeds buried; big seeds too shallow), and moisture failure (dried mid-germination = death, guaranteed). The paper-towel test isolates viability from all the environmental suspects.
Does germination rate fall with age?
Yes, species-specifically: parsnip and onion seed fade in 1–2 years; tomato and brassica seed stays strong 5+ years under cool, dry, dark storage. The packet’s packed-for year plus this longevity sense tells you whether to test, oversow, or replace — testing costs ten days and settles it.
Is this germination rate calculator free?
Yes — free, instant, no sign-up.