Mushroom Spawn and Straw Weigh-In Calculator: Tare, Spawn Rate, Ratio and Price per kg

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

The straw: weigh it, then take off the container (tare)

Scale reading kg, straw and container

Container kg; 0 if you pressed tare

The spawn: a percentage of the wet straw

Wet straw kg, soaked and drained (Chapter 2)

Spawn rate % of the wet straw

Spawn you weighed g; blank = what the rate asks for

Price per kilogram

Spawn bag price

Bag weight

Weighed in

Any percent question: fill in two, get the third

Part blank = work it out

Percent %

Whole

Enter what the scale reads, then press Calculate.

The formula

Net weight = the scale reading − the container (or press tare first)
Spawn = wet straw × the spawn rate ÷ 100
A ratio a : b compares two amounts; 600 g to 6,000 g is 1 : 10
One number as a percentage of another = part ÷ whole × 100: the whole you divide by changes the answer
Price a kg = bag price ÷ bag weight in kg (1 lb = 453.59237 g); tubs a bag = bag kg ÷ spawn a tub
The three percent questions: part = percent × whole ÷ 100; percent = part ÷ whole × 100; whole = part ÷ percent × 100

The book's worked example, Chapter 1. The book's straw weighs 2.0 kg as bought (4.41 lb). Soaked and drained it is 6.0 kg, so the spawn at 10% of the wet straw is 600 g, a ratio of 1 : 10. The mix is 6.6 kg, and the spawn is 9.09% of that: a different whole, a different percentage. A 5 lb bag at 30 is 13.23 a kg and lasts 3.78 tubs.

Check it. Weigh a level 1 L jug of your dry chopped straw before you look at the answer, then call the whole tub's straw from it: Chapter 4 shows how.

Honest limits. A kitchen scale that reads in 1 g steps is good to about 1 g; weigh straw in a bag or a bucket and tare it first. Spawn bags are sold by weight, not by tubs. 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.