Fruiting Chamber Size Calculator

A properly sized fruiting chamber provides the right balance of fresh air exchange (FAE), humidity, and growing space for mushroom cultivation. The two most common types are the Shotgun Fruiting Chamber (SGFC) and the Monotub.

Chamber Volume Rule

A good rule of thumb:

Chamber Volume = Substrate Volume × 4 to 6

This ratio ensures adequate airspace above the substrate for proper air exchange and humidity maintenance. Too small and CO2 builds up (causing leggy, thin mushrooms). Too large and humidity is harder to maintain.

Note that this rule and the tub table below can point at different tubs, and when they do the reason is worth knowing: the rule sizes a chamber around the substrate you actually have, while the table assumes you fill the tub to a proper 3 to 5 inch depth. If your substrate volume suggests a smaller tub than the table does, you have less substrate than that tub wants. Make more, or use the smaller tub.

Monotub Sizing Guide

Two things about that yield column. It is the whole run, not one flush, and the first flush is usually about half of it. And it is quoted for oyster, which is the most productive of the species here at roughly 0.75 lbs fresh per pound of dry substrate. Shiitake and King Oyster return about two thirds of that, Lion’s Mane a little more, and Reishi under half. The calculator scales the figure for whichever species you pick.

SGFC Sizing

The classic SGFC uses a clear tote with 1/4" holes drilled on a 2" grid on all six sides. Substrate sits on perlite for humidity:

Perlite Depth = 4–5 inches Air gap above substrate = at least 6 inches

Substrate Depth

For monotubs, the ideal substrate depth is 3–5 inches (7.5–12.5 cm). Thinner beds dry out too fast. Thicker beds risk anaerobic conditions and contamination in the center.

Worked Example

Growing oyster mushrooms with 5 lbs dry substrate:

• Hydrated weight ≈ 5 × 3 = 15 lbs
• Wet substrate is about as dense as water, so a quart weighs roughly 2 lbs: 15 / 2 = 7.5 quarts
• Read the table above at 5 lbs dry: 58 qt tub, floor 23" × 17" = 391 in²
• Substrate depth: 7.5 qt × 57.75 in³ ÷ 391 in² = 1.1 inches
• Expected yield: 5 × 0.75 = about 3.8 lbs fresh in total, across all flushes

That depth figure is the interesting one, and it is why the answer is not simply “buy the tub the table says”. At 1.1 inches this bed is far too shallow and it will dry out before it finishes. Either make more substrate, around 20 quarts to reach 3 inches in that tub, or put the 7.5 quarts you have into a 32 qt tub instead. The tub table assumes you fill it properly.

A note on the arithmetic: a quart is 57.75 cubic inches. A gallon is 231. Using 231 by mistake makes every depth come out four times too deep, which is exactly the sort of error that has you confidently filling a tub with a third of the substrate it needs.

Environmental Parameters

Humidity Maintenance

For a monotub, drill holes and cover with micropore tape to control FAE. Misting 2–3 times daily maintains surface humidity. For SGFCs, the perlite layer plus fanning provides passive humidity and air exchange.