CO₂ Grow Room Calculator (grams of CO₂ to reach and hold a target ppm)

Understanding the grow room CO₂ calculator — concept and principle

This calculator estimates how much bottled CO₂ is needed to raise a sealed grow room or greenhouse to a target concentration and keep it there while the lights are on, and how long one cylinder lasts. Indoor growers use it to size a regulator and budget gas.

Topping up a grow room: with the default inputs the result is 69 days (9.07 kg cylinder, 132 g/day).

How it is calculated

Room volume V (m³) = length × width × height (feet are converted with 1 ft = 0.3048 m).
Rise Δ (ppm) = target − outdoor level.
CO₂ to fill (g) = V × Δ × 1.80 ÷ 1000, using 1 ppm of CO₂ = 1.80 mg/m³ at 25 °C and 1 atm.
CO₂ to hold (g/h) = fill amount × air changes per hour.
CO₂ per lit day (g) = fill + hourly top-up × lit hours.
Days per cylinder = cylinder kg × 1000 ÷ grams per day.

The principle

A ppm is a volume fraction: 1,000 ppm means one litre of CO₂ in every cubic metre of air. Converting that volume to mass with the NIOSH factor gives the grams you must add. Once the room is at the target, leaks and ventilation keep swapping room air for outdoor air, and every exchange carries away the extra CO₂. With n air changes per hour, you lose n room-fills of the extra CO₂ every hour, which is why tight rooms use so much less gas. Plants also take up some CO₂ while photosynthesising, so real use is somewhat higher.

Worked example

Default inputs: 3 × 2.5 × 2.4 m, outdoor 420 ppm, target 1,000 ppm, 0.5 air changes per hour, 12 lit hours, 9.07 kg cylinder.

V = 3 × 2.5 × 2.4 = 18 m³; rise = 1,000 − 420 = 580 ppm.
Gas volume = 18 × 580 ÷ 1000 = 10.4 L; mass = 18 × 580 × 1.80 ÷ 1000 = 18.8 g.
Hold = 18.79 × 0.5 = 9.4 g/h.
Per lit day = 18.79 + 9.40 × 12 = 132 g.
Cylinder = 9,070 ÷ 131.5 = 69 days.

Each point is the calculator re-run with only “Room length (m or ft)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Room length (m or ft)	CO₂ to reach the target (g)
0.75	4.7
1.5	9.4
2.25	14.1
3	18.8
3.75	23.5
4.5	28.2
6	37.6

How to read the result

Growers commonly enrich to about 1,000–1,500 ppm, where most of the growth benefit is reported. If the cylinder empties much faster than predicted, the room leaks more than you think or an exhaust fan is running during enrichment; a fan running alongside the CO₂ wastes most of the gas. Cutting air leakage pays off more than buying larger cylinders.

Common mistakes

Enriching with the exhaust fan on.
Enriching during dark hours, when plants do not use the extra CO₂.
Forgetting that outdoor CO₂ is about 420 ppm, not zero.

Keep in mind: CO₂ is heavier than air and has no smell. Keep a CO₂ monitor in the room, never exceed 5,000 ppm (the 8-hour workplace exposure limit) and ventilate before working inside.

Frequently asked questions

What CO₂ level is best for plants?

Most guidance targets about 1,000 ppm, with the greatest benefit reported at 1,000–1,500 ppm.

Is 1,000 ppm safe for people?

It is well below the 5,000 ppm 8-hour limit, but high levels can cause headaches. Ventilate before spending time inside.

Should I run CO₂ at night?

No. Plants use the extra CO₂ only in light, so enrichment normally runs with the lights.

Sources: NIOSH Pocket Guide to Chemical Hazards, Carbon dioxide (REL TWA 5,000 ppm; 1 ppm = 1.80 mg/m³) · AHDB, "CO₂ best practice guide: how much to use"

Checked 9 Nov 2026. NIOSH Pocket Guide to Chemical Hazards, Carbon dioxide (REL and OSHA PEL TWA 5,000 ppm, short-term 30,000 ppm, IDLH 40,000 ppm; 1 ppm = 1.80 mg/m³); target levels: AHDB, “CO₂ best practice guide: how much to use” (about 1,000 ppm with vents closed) and D. Schwane, Minnesota Flower Growers Bulletin 42(1), 1993 (greatest benefit 1,000–1,500 ppm). Air-change rate and cylinder size are example values.

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