# Grow room CO₂ calculator

> Free grow room CO₂ calculator: gas to reach and hold a target ppm, litres, kg or lb per hour, tank duration, why exhaust wastes it, and the exposure limits that keep people safe.

Interactive: https://genetixbay.com/knowledge/tools/co2.html

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.

## How it works

### 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.

## CO₂ safety: read this first

- CO₂ has no smell. Fit a CO₂ monitor with an alarm at head height, outside the canopy and away from the emitter, plus a separate alarm by the door.
- Exposure limits: 5,000 ppm averaged over 8 hours (OSHA PEL, NIOSH REL, ACGIH TLV); 30,000 ppm for 15 minutes (ACGIH STEL, NIOSH short-term); 40,000 ppm is immediately dangerous to life or health (NIOSH IDLH).
- Never enter a room above 5,000 ppm. Ventilate first and wait for the monitor to fall.
- Never enter during a sealed-room CO₂ pest treatment. Those concentrations are far above the lethal range for people and pets. Lock the door, post a sign, ventilate fully and check the monitor before anyone goes in.
- Keep cylinders upright and strapped, use a regulator rated for CO₂, and lock enrichment out whenever the exhaust runs.
- Burners add heat, water and a carbon monoxide risk: fit a CO alarm too, and use only units with flame-failure shut-off.

## Constants

| Constant | Value | Unit | 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

1. NOAA Global Monitoring Laboratory. Trends in atmospheric carbon dioxide (global mean above 420 ppm since 2023). https://gml.noaa.gov/ccgg/trends/
2. 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
3. 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
4. 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
5. 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
6. 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/
7. 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
8. 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