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Opens on the book's Microgreen Shelf. Change any number to your own and press Calculate.
What each shelf asks for (to see why the ladder must stop)
Enter the weekly contribution and the kit price, then press Calculate.
The formula
- Weeks to the next kit = kit ÷ saved a week, rounded up
- Growth a week, if every week's money bought more shelf = saved a week ÷ kit
- Shelves after n weeks = (1 + growth)ⁿ: a geometric sequence
- Doubling time = ln 2 ÷ ln(1 + growth); the rule of 72 says 72 ÷ the growth in percent
- What n weeks would ask: wall = shelves × width; power = shelves × kWh; hours = shelves × trays × minutes ÷ 60; packs = (shelves × grams − household) ÷ pack
The book's worked example, Chapter 3. The shelf's surplus contributes about 48.05 a week. Saved toward a second 324 kit: 6.74 weeks, so 7 weeks. If every week bought more shelf, shelves would grow 14.83% a week, doubling every 5.01 weeks (the rule of 72 says 4.86), and reach about 36.43 shelves in 26 weeks: 145.7 ft of wall, 93.26 kWh a day, 42.5 hours a week and about 890 packs a week to sell. The plan stops at a second shelf.
Check it. Multiply the weekly factor by itself as many times as the doubling time says: you should land close to 2.
Honest limits. The ladder is a model of compounding, not a plan: space, light, your hours and your buyers stop it long before the numbers do (Chapter 5). 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.