Sizu Home › Calculators › When to Harvest
Opens on the book's Microgreen Shelf. Change any number to your own and press Calculate.
Enter the seed weight, the harvest weight and the day, then press Calculate.
The formula
- The S-curve: weight(t) = K ÷ (1 + e−r(t − m)): ceiling K, midpoint m, rate r
- Fitted to two points: K = harvest ÷ share; r = [ln(share ÷ (1 − share)) + ln(K ÷ seed − 1)] ÷ harvest day; m = ln(K ÷ seed − 1) ÷ r
- Gain a day = r × weight × (1 − weight ÷ K): fastest at half the ceiling, r × K ÷ 4
- One more day adds weight(day + 1) − harvest
The book's worked example, Chapter 5. Radish, fitted to 28 g at day 0 and 280 g at day 9 (90% of the ceiling): ceiling 311.1 g, fastest gain 39 g a day at day 4.6, 262.9 g the day before, and one more day adds only 11.5 g. Ceilings: broccoli 333.3, sunflower 444.4, pea 250 g.
Check it. Put day 0 into the formula: it should give the seed weight. Put the harvest day in: it should give the harvest weight.
Honest limits. This is a model fitted to a start and a finish, not measurements of a tray: the 90% share is an assumption. Weigh one tray a day and you will see your own curve. The Microgreen Shelf was worked out on paper from published figures, not grown by us: weigh and measure your own. Prices are illustrative US prices from October 2026, shown without a currency sign; put in your own. Everything stays in your browser: no sign-up, nothing stored.
These calculators go with Grow by Numbers (Make It Add Up, Book 1), where every number on this page is worked by hand.