How much CO₂ does my grow room need?
CO₂ only pays in a sealed, bright room. You need enough gas to raise the room to your target once, then enough each hour to replace what leaks out and what the plants use. With the exhaust running, nearly all of it goes straight outside.
Your result
How it works, in plain words
Reach the target
Gas = room volume × (target − outdoor ppm) ÷ 1,000,000. A 10 × 10 × 8 ft room (22.7 m³) going from 420 to 1,100 ppm needs about 15 L of CO₂ gas, about 28 g.
Hold it
Each hour, replace what leaks out (volume × air changes × the same ppm difference) plus what the plants fix, roughly 4 g per m² of canopy in bright light. In that room: about 45 g an hour, about 1.2 lb (0.56 kg) per 12-hour day.
Exhaust wastes it
Swap the air every minute and you're replacing the whole room's extra CO₂ 60 times an hour. That's why enrichment and a running exhaust don't mix.
Density
CO₂ weighs about 1.8 kg per m³ at room temperature (ideal gas law), so 1 kg of liquid CO₂ is roughly 550 L of gas.
Every number this calculator uses
The same constants feed the Calculate it properly guides, so the tools and the articles agree. Rules of thumb are labelled as such.
| Constant | Value | What it is | Evidence |
|---|---|---|---|
CO2_AMBIENT | 420 ppm | Outdoor CO₂ | Established |
CO2_M | 44.01 g/mol | Molar mass of CO₂ (density from the ideal gas law at room temperature, ≈ 1.83 kg/m³ at 20 °C) | Established |
ACH_TIGHT | 0.1 air changes per hour | Sealed room, well taped | Estimate |
ACH_SEALED | 0.25 air changes per hour | Sealed room, average | Estimate |
ACH_LEAKY | 2 air changes per hour | Zipped tent or leaky room, exhaust off | Estimate |
CO2_UPTAKE | 4 g CO₂ per m² canopy per hour | Net canopy uptake at high light and ~1,000 ppm (≈ 25 µmol/m²/s) | Estimate from leaf data |
PROPANE_CO2 | 3 lb CO₂ per lb fuel | Propane burner: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O (132 ÷ 44.1) | Established (stoichiometry) |
PROPANE_H2O | 1.63 lb water per lb fuel | Propane burner: water vapour made | Established (stoichiometry) |
PROPANE_BTU | 21600 BTU per lb fuel | Propane burner: heat released (higher heating value) | Established |
NG_CO2 | 2.74 lb CO₂ per lb fuel | Natural gas burner (as methane): CH₄ + 2 O₂ → CO₂ + 2 H₂O (44 ÷ 16) | Established (stoichiometry) |
NG_H2O | 2.25 lb water per lb fuel | Natural gas burner: water vapour made | Established (stoichiometry) |
NG_BTU | 23900 BTU per lb fuel | Natural gas burner: heat released (higher heating value) | Established |
CO2_PEL | 5000 ppm | 8-hour workplace limit (OSHA PEL, NIOSH REL, ACGIH TLV) | Established |
CO2_STEL | 30000 ppm | 15-minute short-term limit (ACGIH STEL, NIOSH ST) | Established |
CO2_IDLH | 40000 ppm | Immediately dangerous to life or health | Established |
Sources
- NOAA Global Monitoring Laboratory. Trends in atmospheric carbon dioxide (global mean above 420 ppm since 2023). https://gml.noaa.gov/ccgg/trends/
- GenetixBay Knowledge Base. CO₂ enrichment (targets 800–1,200 ppm at lights-on only; worth it only sealed and at high light). https://genetixbay.com/knowledge/environment/co2-enrichment.html
- NIST. Guide for the Use of the International System of Units (SP 811), Appendix B: conversion factors (1 W = 3.412 142 BTU/h; 1 ft³ = 0.028 316 8 m³; 1 US gal = 3.785 412 L). https://www.nist.gov/pml/special-publication-811
- ASHRAE. 2021. ASHRAE Handbook: Fundamentals. Ch. 1 (psychrometrics: sensible heat of air, q = 1.08 × cfm × ΔT at standard air), Ch. 18 (heat gain from people, lighting and equipment), Ch. 21 (duct design: fitting and friction losses), Ch. 26–27 (thermal resistance of building assemblies). ASHRAE, Atlanta. https://www.ashrae.org/technical-resources/ashrae-handbook
- Chandra S, Lata H, Khan IA, ElSohly MA. 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO₂ conditions. Physiology and Molecular Biology of Plants 14(4):299–306. https://doi.org/10.1007/s12298-008-0027-x
- Combustion stoichiometry and heating values of propane and methane: NIST Chemistry WebBook (enthalpies of combustion) and US EIA, 'Units and calculators: British thermal units' (propane ≈ 91,500 BTU per gallon at 4.24 lb per gallon). https://webbook.nist.gov/chemistry/
- OSHA. Carbon dioxide, chemical data: PEL 5,000 ppm 8-hour TWA (29 CFR 1910.1000, Table Z-1). https://www.osha.gov/chemicaldata/183
- NIOSH Pocket Guide to Chemical Hazards: carbon dioxide (REL 5,000 ppm TWA; short-term 30,000 ppm; IDLH 40,000 ppm). ACGIH lists the same TLV-TWA 5,000 ppm and TLV-STEL 30,000 ppm. https://www.cdc.gov/niosh/npg/npgd0103.html
