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CO₂ Calculator

This CO₂ calculator for grow rooms shows how much gas it takes to reach your target ppm, what a day of air leaks uses, and how long a tank lasts. It also gives the regulator timer settings.

Units
Common tent sizes

Fills in a typical height too; change it if your tent is taller or shorter.

m

m

m

ppm

About 1,000 ppm by day is the usual greenhouse recommendation.

per hour

0 for a sealed room; leaks still count.

h/day

CO₂ from
SCFH

The number on the flowmeter: standard cubic feet per hour. Most grow regulators go from about 0.5 to 15.

Tank size
More options
ppm

The 2025 global average (NOAA). Cities and closed rooms can be higher.

kg

The CO₂ weight on the tank, not the empty tank. Optional: 0 skips it.

CO₂ per day

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—

To fill the room once
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Top-up per hour
—
Room volume
—
A tank lasts
—

Regulator and timer

    Safety limits for people

    • 5,000 ppm: the most for an 8-hour working day (OSHA, NIOSH).
    • 30,000 ppm: the most for 15 minutes (NIOSH).
    • 40,000 ppm: immediately dangerous to life (NIOSH).

    Use a CO₂ monitor with an alarm in any enriched room.

    Next: Check your light is strong enough for CO₂ to pay off

    How it works

    One part per million of one cubic metre is one millilitre. So raising a room from the outdoor level to your target takes the room volume × the difference in ppm, in millilitres. Every air change loses that amount again. A litre of CO₂ weighs 1.84 g at 20 °C.

    Formula
    Fill (L) = volume (m³) × (target − outdoor ppm) ÷ 1,000
Fill (g) = fill (L) × 1.84
Per day  = fill + fill × air changes/h × lights-on hours

    What a CO₂ calculator for a grow room works out

    Enriching a grow room means lifting the air from the outdoor level, about 426 ppm, to a target. Ontario’s greenhouse guidance gives about 1,000 ppm by day. The first fill is small. What empties a tank is the air that leaks or is pushed out, because each air change takes your enriched air with it.

    So the calculator splits the job in two: one fill at lights-on, then a top-up for every air change while the lights are on. It turns both into grams, litres, regulator minutes and tank life.

    If the exhaust fan runs while you enrich, its airflow sets the air changes. A fan that swaps all the air every 2 minutes makes 30 changes an hour, and the top-ups then dwarf the fill. The exhaust fan calculator shows the air changes per hour for your fan and room.

    How long will a 20 lb CO₂ tank last?

    Take the starting room: 4 × 4 × 2.5 m (40 m³, about 1,413 ft³), a target of 1,000 ppm, half an air change per hour and 12 hours of light.

    • One fill: 23 litres, or 42 g.
    • Top-up: 21 g an hour.
    • Per day: about 0.30 kg (0.65 lb).

    A 20 lb tank holds 9.1 kg of CO₂, so it lasts about 31 days. At one air change per hour the same room uses 0.55 kg a day, and the tank lasts about 17 days. Sealing the room does more for tank life than any other setting.

    CO₂ regulator and timer settings (SCFH)

    A flowmeter is set in standard cubic feet per hour (SCFH). One cubic foot of CO₂ weighs about 51.9 g. With the regulator at 10 SCFH, the example room needs:

    • 4.9 minutes of gas at lights-on (0.81 cubic feet) to fill it.
    • Then 2.4 minutes every hour, or a steady 0.41 SCFH, to replace what leaks out.

    Set the solenoid’s timer to run only during the lights-on hours. A low steady flow suits a room that leaks evenly; a timed burst suits one that holds its air between bursts. A controller with a sensor does either and stops at the target.

    Tank or burner: what a CO₂ generator adds

    A burner makes CO₂ by burning gas, which also makes heat and water. For every kilogram of CO₂, propane gives about 16.8 MJ of heat and 0.55 kg of water; natural gas about 20.2 MJ and 0.82 kg.

    In the example room, a propane burner adds about 392 BTU/h while it runs and about 0.2 L of water a day. Add the heat to your grow room air conditioner size and the water to your dehumidifier size. In a room that already runs warm, check the cooling has room for it before choosing a burner over a tank.

    Common mistakes, and CO₂ ppm limits for people

    • Entering the empty tank’s weight. Tank contents means the CO₂ weight stamped on it: 9.1 kg for a 20 lb tank.
    • Counting the dark hours. The figures cover lights-on hours only. Gas released at night is extra use the calculator does not show.
    • Aiming too high. Most crops gain little above about 1,000–1,300 ppm, so the calculator flags any target over 1,300.
    • Forgetting the light. CO₂ and light work together. An LED maker’s rule for cannabis is at least 800 ppm, or 1 ppm per µmol/m²/s of PPFD. Check your light with the LED grow light calculator.

    For people, 5,000 ppm is the limit over an 8-hour working day and 40,000 ppm is immediately dangerous. A grow room at 1,000 ppm is far below both, but a stuck solenoid is not. Fit an alarm that reads ppm.

    FAQ

    Questions growers ask

    How do you calculate CO₂ for a grow room?

    Multiply the room volume in cubic metres by the ppm you need to add, then divide by 1,000 to get litres. A 40 m³ room going from 426 to 1,000 ppm needs 23 litres, or 42 g. Each air change while enriching loses that much again, so add one fill per air change.

    What CO₂ ppm should I run in flower?

    About 1,000 ppm by day; none of our sources gives a separate figure for flowering. Ontario’s greenhouse guidance says most crops stop gaining at about 1,000–1,300 ppm. Under strong light, an LED maker’s cannabis rule is at least 800 ppm, or 1 ppm per µmol/m²/s of PPFD, so 900 µmol calls for 900 ppm.

    How long does a 20 lb CO₂ tank last?

    About 31 days in a 4 × 4 × 2.5 m room held at 1,000 ppm for 12 hours a day with half an air change per hour. A 20 lb tank holds 9.1 kg of CO₂. At one air change per hour it lasts about 17 days; in a sealed room, months.

    How much is 1 kg of CO₂?

    About 544 litres of gas at 20 °C, since a litre weighs 1.84 g. That would raise a 40 m³ room by about 13,600 ppm, or fill it from outdoor level to 1,000 ppm about 23 times. How fast you use it depends on leaks, not on the first fill.

    How many ppm is 5% CO₂?

    50,000 ppm, because 1% is 10,000 ppm. That is above the 40,000 ppm level NIOSH calls immediately dangerous to life, and ten times the 5,000 ppm limit for an 8-hour working day. A grow room held at 1,000 ppm is 0.1% CO₂.

    Is CO₂ worth it in a grow tent?

    Only if the tent can stay closed while you enrich. A 120 × 120 × 200 cm (4×4 ft) tent needs just 3 g to reach 1,000 ppm. With the exhaust changing the air every 2 minutes, it uses about 1.1 kg a day and a 20 lb tank lasts about 8 days.

    Does a CO₂ burner heat the room?

    Yes. Propane gives about 16.8 MJ of heat and 0.55 kg of water for every kilogram of CO₂; natural gas about 20.2 MJ and 0.82 kg. In a 40 m³ room held at 1,000 ppm, that is about 392 BTU/h from propane. Add it to your cooling and dehumidifier sizing.

    What CO₂ level should I aim for?

    Ontario’s greenhouse guidance recommends about 1,000 ppm during the day when the vents are closed, 800–1,000 ppm for seedlings, and says most crops stop gaining at about 1,000–1,300 ppm. Only enrich while the lights are on.

    Sources

    The formulas and target ranges on this page come from these references.

    1. Ontario Ministry of Agriculture, Food and Agribusiness: Supplemental carbon dioxide in greenhouses (1,000 ppm by day when vents are closed; 800–1,000 ppm for seedlings; most crops saturate at about 1,000–1,300 ppm). www.ontario.ca/page/supplemental-carbon-dioxide-greenhouses
    2. NOAA Global Monitoring Laboratory: global annual mean CO₂ (425.62 ppm in 2025; the latest year may still be revised). gml.noaa.gov/webdata/ccgg/trends/co2/co2_annmean_gl.txt
    3. NIST Chemistry WebBook: thermophysical properties of carbon dioxide at 1.01325 bar (density 1.8393 kg/m³ at 20 °C, 1.8080 at 25 °C). webbook.nist.gov/cgi/fluid.cgi?Action=Data&Wide=on&ID=C124389&Type=IsoBar&Digits=5&P=1.01325&THigh=25&TLow=20&TInc=5&RefState=DEF&TUnit=C&PUnit=bar&DUnit=kg%2Fm3&HUnit=kJ%2Fmol&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm
    4. NIST Chemistry WebBook: carbon dioxide at 70 °F and 14.696 psia (density 0.11439 lb/ft³), the standard conditions of SCFH flowmeters. webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C124389&Type=IsoTherm&Digits=5&PLow=14.696&PHigh=14.696&PInc=1&T=70&RefState=DEF&TUnit=F&PUnit=psia&DUnit=lbm%2Fft3&HUnit=Btu%2Flbm&WUnit=ft%2Fs&VisUnit=lbm%2Fft*s&STUnit=lb%2Fin
    5. AC Infinity: CO₂ regulator with solenoid valve and flowmeter (0.5–15 SCFH). acinfinity.com/co2-regulator-carbon-dioxide-monitor-with-solenoid-valve-and-gas-flow-meter/
    6. NIST Chemistry WebBook: propane, enthalpy of combustion of the gas −2,219.2 kJ/mol (Pittam & Pilcher, 1972). webbook.nist.gov/cgi/cbook.cgi?ID=C74986&Mask=1
    7. NIST Chemistry WebBook: methane, enthalpy of combustion of the gas −890.7 kJ/mol (Pittam & Pilcher, 1972). webbook.nist.gov/cgi/cbook.cgi?ID=C74828&Mask=1
    8. NIOSH Pocket Guide to Chemical Hazards: carbon dioxide (OSHA PEL and NIOSH REL 5,000 ppm over 8 hours; NIOSH short-term 30,000 ppm; IDLH 40,000 ppm). www.cdc.gov/niosh/npg/npgd0103.html

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