Seed Germination Calculator

How to Use the Seed Germination Calculator

• Run a quick test: ten to twenty seeds on a damp paper towel in a bag, at the crop's favorite soil temperature.
• Count sprouts at the end of the typical window shown in the results, then wait half again and count finally.
• Enter seeds sown and seeds sprouted to get the measured germination percentage.
• Set the seedling count you actually want to grow this season.
• Read the sowing quantity the percentage demands — and the spare seedlings it predicts.
• Anything under 50% is a flag to reorder, though 70% seed sown thicker still gardens fine.

The Germination Formula, Explained

Worked Example: The Garage-Shelf Onion Seed

• Twenty-five onion seeds from last season go into the paper-towel bag by the radiator.
• At day 10, eight have sprouted; at day 18, the final count is 14.
• Germination: 14 ÷ 25 = 56% — the packet is alive but tired.
• You want 40 transplants for two 4x8 beds, so sowing at 56% means ceil(40 × 100 ÷ 56) = 72 seeds.
• That is nearly double the packet-count instinct — and the difference between two full beds and forty gaps to fill in May.
• Spares come out at 32 seedlings, plenty for neighbors, the salad border, or a second-round resow if the flat germinates unevenly.
• If you had guessed 80% instead — fresh-seed territory — you would have sown 50 seeds and gotten only about 28 sprouts against the 40 you needed.

Typical Germination Windows by Crop

Key Terms

Germination rate

Share of seeds that produce normal seedlings under test conditions — the packet's real currency.

Viability

Whether the embryo is alive at all; old seed can sprout weakly and die at the first true-leaf stage.

Paper-towel test

The ten-minute home viability assay: damp towel, sealed bag, warm spot, final count at window-plus-half.

Guaranteed analysis

The germination percentage the seller legally promises on the seed tag, tested before sale.

Succession sow

Splitting a low-rate planting into two dates so the first batch's gaps inform the second's depth.

What This Means in a Real Seed Tray

• Temperature is the input people forget: parsley at a cool 60 °F looks like a 30% failure that is really just February — match the test to the crop's ideal, not the spare bedroom.
• Pre-sprouting (soaking overnight, incubating in a bag until the radicle pops) turns slow germinators into even ones, and the sow-count math then assumes nearly the full percentage.
• Weak sprouts do not count: the test measures normal seedlings, and a spindly white hook that never greens up is a zero with enthusiasm.
• Keep the math per packet, not per variety — two-year-old and five-year-old beans in the same bin germinate 30% apart, and only the test sorts them.
• Low percentages are a thinning bill, not a death sentence: sowing 56% seed two seeds per cell and thinning later gardens exactly like 80% seed sown one per cell.

Common Mistakes & Fixes

• Counting at the first green hook instead of the final tally, which reads slow germinators as dead — always use the window plus half.
• Testing on a dry paper towel or soggy one — damp, not swimming, or the seeds rot and blame falls on the packet.
• Sowing fresh-seed quantities from an old packet's plan and then mourning the gaps — the spares output exists precisely to right-size the tray.
• Confusing germination with establishment: field loss to damping-off and cold is another 10–30%, so direct-sown beds still want the rate plus a cushion.
• Tossing 50%-germinating heirloom seed — at that rate it is worth double-sowing and half the price of a new packet, especially when it is the only source of the variety.

When to Use This Test (and When Not To)

Seed Germination Calculator: Frequently Asked Questions