Short answer, in plain words. Filling a room with extra CO₂ takes surprisingly little gas. Keeping it full is what costs: the plants eat it, and every leak and every fan blows it away. A sealed 10 × 10 × 8 ft room needs only about 33 g (0.07 lb) to go from about 420 to 1,200 ppm, then about 1.2 lb a day to hold it under strong light. With an exhaust fan changing the air every minute, it would lose about 50 lb a day. So CO₂ is for sealed, air-conditioned rooms with strong light. And CO₂ can hurt you: buy a monitor with an alarm before you buy the gas. Supported (physics and exposure limits; plant use is an estimate).
Use the calculator: CO₂ calculator. For whether CO₂ pays at all and what ppm to run, read CO₂ enrichment.
Safety first. CO₂ has no smell. OSHA's 8-hour workplace limit is 5,000 ppm; NIOSH's 15-minute limit is 30,000 ppm; 40,000 ppm (4%) is immediately dangerous to life or health [5][6][7]. CO₂ is about 1.5 times heavier than air and pools low. Use a CO₂ monitor with an audible alarm at plant height and near the floor, and never enter a room you know or suspect is above 5,000 ppm without ventilating it first [8].
What are the formulas?
Gas volume to raise the level (ft³ or m³) = room volume × (target ppm − starting ppm) ÷ 1,000,000.
Mass of CO₂: at about 20 °C (68 °F) and sea-level pressure, CO₂ gas weighs about 1.83 g per litre. 1 lb of CO₂ ≈ 8.75 ft³ (248 L) of gas; 1 ft³ ≈ 52 g (ideal gas, molar mass 44.01 g/mol [1]).
Loss to air exchange (per hour) = room volume × air changes per hour × (inside ppm − outside ppm) ÷ 1,000,000.
Plant use (per hour) ≈ canopy photosynthesis × canopy area. We use about 4 g of CO₂ per m² of canopy per hour (about 25 µmol/m²/s; 0.37 g per ft² per hour) under strong light, an estimate from cannabis leaf data [2] and the same value the GenetixBay calculators use. Real canopies vary with light, CO₂, temperature and leaf areabeing reviewed.
Tank duration (days) = tank contents ÷ (plant use + losses) per lights-on day.
How much CO₂ for each size?
Target 1,200 ppm from about 420 ppm outside (a rise of 780 ppm). 12 hours of light. "Sealed" assumes about 0.25 air changes per hour of leakage; "vented" assumes the exhaust fan changes the air once a minute (60 per hour).
| Space | Volume | To raise 420 → 1,200 ppm | Plant use per hour | Sealed: per 12 h day | 20 lb tank, sealed | Vented: per 12 h day |
|---|---|---|---|---|---|---|
| 2 × 4 × 6 ft tent | 48 ft³ (1.36 m³) | ~2 g | ~3.0 g | ~42 g (0.09 lb) | months | ~3.1 lb |
| 4 × 4 × 6.5 ft tent | 104 ft³ (2.94 m³) | ~4 g | ~5.9 g | ~84 g (0.19 lb) | ~108 days | ~6.8 lb |
| 5 × 5 × 6.5 ft tent | 163 ft³ (4.60 m³) | ~7 g | ~9.3 g | ~131 g (0.29 lb) | ~69 days | ~10.7 lb |
| 4 × 8 × 6.5 ft tent | 208 ft³ (5.89 m³) | ~8 g | ~11.9 g | ~168 g (0.37 lb) | ~54 days | ~13.6 lb |
| 10 × 10 × 8 ft room | 800 ft³ (22.65 m³) | ~32 g (0.07 lb) | ~37 g | ~543 g (1.2 lb) | ~17 days | ~52 lb |
Worked example, 10 × 10 × 8 ft room:
- Raise: 800 ft³ × 780 ÷ 1,000,000 = 0.62 ft³ = 0.62 × 52 g ≈ 32 g.
- Plant use: 9.29 m² × 4 g/m²/h ≈ 37 g/h (estimate).
- Leaks at 0.25 ACH: 22.65 m³ × 0.25 × 0.00078 × 1,830 g/m³ ≈ 8 g/h.
- Day: (37 + 8) × 12 ≈ 543 g ≈ 1.2 lb. A 20 lb tank (9,070 g) lasts about 17 days.
- Same room with constant exhaust at 60 ACH: losses ≈ 1,940 g/h, about 52 lb a day.
Plain words: CO₂ and an always-on exhaust fan fight each other. The fan wins.
- ~32 gTo raise it from 420 ppm
- ~1.2 lb/dayTo hold it (plants + leaks)
- ~17 daysFrom a 20 lb tank
- ~52 lb/dayWith the exhaust changing the air every minuteWasted
Source: Worked example above; CO₂ density [1]; uptake estimate from [2].
Plant use about 37 g/h (4 g/m²/h, an estimate); each air change per hour loses about 32 g/h at 1,200 ppm.
Source: Computed with the formulas above; CO₂ density [1]; canopy rate estimate from [2].
Show the numbers
| Air changes per hour (leakage or exhaust) (ACH) | Tank life (days) |
|---|---|
| 0 | 20.30 |
| 0.25 | 16.70 |
| 0.5 | 14.20 |
| 1 | 10.90 |
| 2 | 7.40 |
| 5 | 3.80 |
| 10 | 2.10 |
When is CO₂ worth it?
CO₂ is worth it when three things are true: the room is sealed (cooled by an AC or mini-split, little or no exhaust during lights-on), the light is already high (about 900 µmol/m²/s or more), and climate and feeding keep up. Below that, the plants can't use the extra CO₂ much, and with exhaust the gas leaves before they can. Cannabis photosynthesis rises with CO₂ most strongly at high light [2]; see CO₂ enrichment for targets and the evidence. Supported.
Only enrich during lights-on; plants don't photosynthesise in the dark.
Tank or burner?
| Compressed CO₂ tank + regulator | Propane or natural gas burner | |
|---|---|---|
| Adds heat | No (slight cooling as gas expands) | Yes: ~21,600 BTU per lb of propane; ~23,900 BTU per lb of natural gas [4] |
| Adds water | No | Yes: ~1.6 lb (0.74 L) per lb of propane; ~2.25 lb (1.0 L) per lb of natural gas |
| CO₂ made | Exactly what you meter | ~3.0 lb per lb of propane; ~2.7 lb per lb of natural gas (~11.7 lb per therm) [3] |
| Combustion risk | None | Carbon monoxide if combustion is poor; needs a CO alarm and a flame-failure shutoff [9] |
| Best for | Tents, small rooms, any sealed space | Larger sealed rooms with spare cooling and dehumidification |
The burner figures come from the chemistry of combustion (propane: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O) and the published heat content and emission factors [3][4].
Worked example, burner in the 10 × 10 room: 1.2 lb of CO₂ a day needs about 0.4 lb of propane, adding about 8,600 BTU and 0.3 L of water over the day, about 720 BTU/h across 12 hours. Small here, but a burner sized for a bigger room cycling hard adds thousands of BTU/h. Add it to your AC and dehumidifier sums.
What are the safety rules?
| Limit | CO₂ level |
|---|---|
| Outdoor air today | ~420 ppm |
| Typical enrichment target | 1,000–1,500 ppm |
| OSHA PEL and ACGIH TLV, 8-hour average | 5,000 ppm |
| NIOSH and ACGIH 15-minute limit | 30,000 ppm |
| Immediately dangerous to life or health (IDLH) | 40,000 ppm (4%) |
Rules:
- Use a CO₂ monitor with an alarm that reads up to at least 5,000 ppm, and a controller that shuts the gas at your setpoint. Place one sensor at canopy height and check low spots: CO₂ is about 1.5 times as dense as air and can pool [8].
- Burners: add a carbon monoxide alarm and keep combustion air supplied [9].
- Shut off the CO₂ before you enter, and ventilate if the monitor reads high.
- Keep CO₂ out of living spaces, and keep children and pets out of enriched rooms.
Sealed-room CO₂ treatment for pests
Very high CO₂ in a sealed chamber has killed spider mites in research, at concentrations many times above the human danger level [10][11]. GenetixBay's rule matches the spider mite house position: sealed space, no entry during treatment, full ventilation before anyone goes back in, a CO₂ monitor, and the product's or method's directions followed. These levels can kill people and pets. We don't give a cannabis concentration or duration because none has been verified.
Common mistakes
- Running CO₂ with the exhaust fan on. You're venting money outdoors.
- Enriching under low light, where the plants can't use it [2].
- Enriching at night.
- No monitor, or a cheap sensor that reads only to 2,000 ppm.
- Forgetting a burner's heat and water in the AC and dehumidifier sizing.
- Treating pests with CO₂ in a room someone can walk into.
