Sizu Riatto
Grow by Numbers · Chapter 6

How Many Days Until…

A logarithm counts multiplications. Ask it how many days, how many years or how many times as acid, and it answers in one key press.

Home › Calculators › How Many Days Until…

Opens on the book's Microgreen Shelf. Change any number to your own and press Calculate.

A tray on its S-curve: the day it reaches a weight

Money: how many years until it grows to a multiple

pH: the acid ratio between two readings

Enter a crop and a target, a growth rate, or two pH readings, then press Calculate.

The formula

  • On the S-curve: day = m − ln(K ÷ target − 1) ÷ r (K, r, m fitted as in Chapter 5)
  • At one steady multiplier instead: day = log(target ÷ seed) ÷ log(multiplier)
  • Years to grow to a multiple = log(multiple) ÷ log(1 + rate); the rule of 72: 72 ÷ the rate in percent (for doubling)
  • Acid ratio = 10(difference in pH): each pH step is ten times
The book's worked example, Chapter 6. The radish tray reaches 250 g on about day 7.43 on its S-curve (a steady multiplier would say day 8.56). Savings at 5% double in 14.21 years (the rule of 72 says 14.4). Tap water at pH 7.5 has 101.5 ≈ 31.6 times less acid than pH 6.0. The nutrient-water band is 5.5–6.5; tap water of 6.5–8.5 is fine for a soil or coir shelf.
Check it. Put the answer back in: the S-curve at that day should give the target, and (1 + rate) to the power of the years should give the multiple.

Honest limits. The S-curve is a model fitted to a start and a finish, not a measurement of your tray. A pH strip reads to about half a step; a meter needs calibrating. 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.