What germination rate should I assume for old seed?

As a rough rule, knock 10-20% off the printed rate for each year past the packed-for date, more for short-lived seed like onions and parsnips. A ten-seed paper-towel test gives you a real number in under a week.

Why add a safety margin at all?

Germination tests happen in ideal lab conditions. Real trays suffer damping-off, uneven moisture and weak seedlings you will cull - 20% is a sensible default cushion.

Should I sow extra seeds per cell?

Two seeds per cell, thinned to the strongest, is the classic approach when seed is cheap. With expensive seed, sow singly and rely on the margin instead.

Does the calculator handle direct sowing?

Yes - the math is identical. Direct-sown rows often germinate a little lower than trays, so nudge the margin up.

Seed Calculator - gardencalculator.net

🌱 Grow a Garden Calculator

Enter how many plants you want, the packet’s germination rate and a safety margin to see exactly how many seeds to sow.

Written by Jude Ramsey · Last updated August 2026

The formula

Seeds = plants wanted ÷ germination rate × (1 + safety margin). Germination rate is printed on most seed packets as a percentage from the supplier’s test; the safety margin covers damping-off, patchy trays, and seedlings you cull for being weak.

Worked example

You want 24 tomato plants from a packet tested at 80% germination, with a 20% margin: 24 ÷ 0.80 = 30 seeds minimum, × 1.2 = 36 seeds to sow — under one standard 50-seed packet.

Getting the inputs right

Fresh seed from a reputable supplier usually germinates at or above the printed rate; seed stored for years germinates below it — drop the rate 10–20% per year past the packed-for date for most vegetables. Sow two seeds per cell and thin to the strongest if you would rather trade seed for certainty. The calculator also converts the answer into 50-seed packets so ordering is one glance. Everything runs in your browser.

August 18, 2026