Mushroom Growing CO2 Exchange Rate Calculator
Calculate fresh air exchange rate for optimal CO2 in a mushroom fruiting chamber.
Covers oyster, shiitake, and gourmet species with ppm targets by growth stage.
How Mushroom CO2 Exchange Rate Is Calculated
Mushrooms are aerobic organisms that consume oxygen and produce CO2 during growth. Unlike plants, they do not photosynthesize. During fruiting, elevated CO2 causes long, thin stems and small caps — a clear sign of insufficient fresh air exchange (FAE).
Target CO2 Levels
| Growth Stage | Target CO2 (ppm) | Notes |
|---|---|---|
| Colonization | 5,000 - 20,000 | High CO2 is acceptable, minimal FAE |
| Primordia formation | 500 - 800 | Drop in CO2 triggers pinning |
| Fruiting | 500 - 800 | As close to ambient as the fan can get you |
| Oyster mushrooms | 500 - 700 | Oysters are very CO2-sensitive |
| Shiitake | 500 - 1,000 | Moderate tolerance |
| Lions mane | 500 - 700 | Sensitive, needs good airflow |
Outdoor air is the floor, and it is not zero. The atmosphere sits around 420 ppm and rising, and indoor air with people in it runs 600 to 1,200. You cannot ventilate below the air you are ventilating with, so a 400 ppm target is not achievable with any fan at any speed, and the closer you aim to ambient the more air you have to move for each ppm you gain. That is why the curve below climbs so steeply at the left-hand end. If your room genuinely sits at 800 ppm, your chamber cannot do better than 800 either, and the answer is ducting from outside rather than a bigger fan.
Air Exchange Formula
ACH = CO2_Production / ((Target_CO2 - Ambient_CO2) × Chamber_Volume)
Where:
- ACH = Air Changes per Hour
- CO2_Production = rate of CO2 generated by the substrate (mL/hour)
- Target_CO2 and Ambient_CO2 in ppm (parts per million)
- Chamber_Volume in liters
CO2 production rate depends on substrate weight and colonization:
CO2_Rate_mL_per_hr = Substrate_Weight_kg × Species_Factor
| Species | CO2 Factor (mL/hr per kg substrate) |
|---|---|
| Oyster (Pleurotus) | 15 |
| Shiitake (Lentinula) | 10 |
| Lions Mane (Hericium) | 12 |
| King Oyster | 13 |
| Button/Portobello | 8 |
Worked Example
Oyster mushrooms, 10 kg substrate, 500-liter fruiting chamber, target 600 ppm, ambient 420 ppm:
- CO2 production: 10 × 15 = 150 mL/hr
- Air to exchange: 150 / ((600 - 420) / 1,000,000 × 500,000) = 150 / 90 = 1.67 ACH
- This means replacing the chamber air roughly 1.7 times per hour
Fan Sizing
Fan_CFM = (ACH × Chamber_Volume_Liters) / (28.3 × 60)
Converting liters to cubic feet and hours to minutes. For our example: Fan_CFM = (1.67 × 500) / (28.3 × 60) = 0.49 CFM
Even a small 50mm computer fan (typically 5-10 CFM) provides more than enough airflow. The challenge is distributing the air evenly without drying out the substrate. Use an intermittent timer: a few minutes on, then off, cycling throughout the day.
Humidity Balance
Fresh air exchange lowers humidity. Maintain 85-95% RH during fruiting by using a humidifier or misting system that compensates for the dry incoming air.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.