Mushroom Recipe Scaling Calculator: Spawn Rate on a Wet or Dry Basis, the Recipe Card per kg, Bags to Buy

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

Start from fills the boxes below

Dry straw a tub kg; blank = from the straw as bought

Or straw as bought kg a tub

Its moisture % (Chapter 2's oven test)

Tubs

Target moisture % wet basis

Spawn rate %

Measured against

Pasteurizing water L per kg of dry straw

Buying and mixing

Spawn bag

Bag weighed in

Layers in the tub for layering the spawn

Enter the dry straw and a spawn rate, then press Calculate.

The formula

The recipe card is written per kg of dry straw: wet straw = 1 ÷ (1 − target), spawn = rate × the straw it is measured against
Scale it: every line × the kg of dry straw × the number of tubs
Change basis: rate on dry = spawn ÷ dry straw × 100; rate on wet = spawn ÷ wet straw × 100. 10% of the wet straw is 33% of the dry
Bags to buy = the spawn needed ÷ one bag, rounded up; the leftover = bags × bag − spawn needed
Spawn a layer = spawn ÷ layers

The book's worked example, Chapter 3. Per kg of dry straw the book's card is 3.33 kg of wet straw, 333.3 g of spawn and 18 L of pasteurizing water. The tub has 1.80 kg of dry straw, so 600 g of spawn: 10% of the wet straw is 33.33% of the dry. A 5 lb bag (2.268 kg) lasts 3.78 tubs; in 4 layers that is 150 g a layer. Three tubs still need only 1 bag, with 0.47 kg left over.

Check it. Weigh what you actually mix in against the plan: within 5% scores 3 points.

Honest limits. Published spawn rates are written on different bases (grower guides use 5 to 15% of the dry weight; one farm uses 10% of the wet): read which before you copy one. More spawn colonizes faster and leaves molds less room, at more cost. The book's tub, its straw and its harvests were worked out on paper from published figures, not grown by us: weigh your own. Prices are illustrative, as seen in November 2026 (US mid prices, with UK beside where they differ), shown without a currency sign; put in your own. No yield or profit is promised. Everything stays in your browser: no sign-up, nothing stored.