Free grow-room tool
CO2 Grow Room Calculator
How much CO2 your sealed grow room needs: one-time dose, flow rate (CFH), time to target ppm, and how long your tank lasts.
Scope: this tool is CO2 enrichment only
(dose, flow, tank math). For lighting math — PPFD, DLI, hanging distance, wattage sizing —
use Grow Light Lab (lighting only, no CO2 mode).
1 — Room & target
Outdoor air is ~400–450 ppm.
Common enrichment band: 1200–1500 ppm.
Room volume—
2 — One-time dose to reach target
CO2 needed
—
pounds of CO2
Volume of CO2—
Also equals—
ppm lift (Δ)—
Formula: ft³ CO2 = Δppm × room ft³ ÷ 1,000,000, then lb = ft³ ÷ 8.74.
3 — Sealed-room flow & tank
Grower rule of thumb (0.2–1.0). Covers plant uptake plus small leaks in a sealed room.
Maintenance flow
—
cubic feet per hour (CFH)
Time to reach target at this flow—
Tank duration
Tank lasts
—
days of continuous enrichment
In hours—
CO2 used per day—
One-time doses per tank—
A 20 lb tank holds about 174.8 ft³ of CO2 (20 × 8.74).
4 — Vented room (exhaust running)
Typical exhaust ventilation: 1–4 ACH.
Continuous injection needed
—
CFH to hold target
20 lb tank at this flow—
Sealed beats vented: exhaust throws enriched air outside, so a vented room burns
CO2 nonstop. Seal the room (plus AC & dehumidifier) for enrichment to pay off.
Physics: CFH = ACH × room ft³ × Δppm ÷ 1,000,000.
Methodology
Volume: V = L × W × H (m converted at 3.28084 ft/m).
Dose: ft³ CO2 = Δppm × V ÷ 10⁶;
lb = ft³ ÷ 8.74 (STP); grams = lb × 453.59237.
Sealed flow: grower rule of thumb 0.5 CFH per 100 ft³ (adjustable 0.2–1.0),
covering plant uptake and minor leaks. Not a physics constant — treat as a starting point.
Tank: hours = tank lb × 8.74 ÷ CFH.
Safety: ≤1500 ppm optimal band · 1500–2000 diminishing returns · >2000 ppm unsafe for extended exposure · >5000 ppm dangerous (OSHA 8-hour PEL).
Estimates only. CO2 is an asphyxiant in enclosed spaces — ventilate before entering a sealed room and use a CO2 safety monitor. Not professional cultivation advice.
Frequently asked questions
How much CO2 does my grow room need?
A 4×4×8 ft grow room holds 128 ft³ of air. Raising it from 400 to 1200 ppm (Δ800 ppm) takes 0.1024 ft³ of CO2 — about 0.0117 lb (0.1875 oz, 5.3 g). Formula: ft³ CO2 = Δppm × room ft³ ÷ 1,000,000, then lb = ft³ ÷ 8.74.
What CO2 ppm should I target in a grow room?
Most growers target 1200–1500 ppm with lights on (ambient outdoor air is ~400–450 ppm). Above 1500 ppm gains flatten for most crops; above 2000 ppm is unsafe for extended worker exposure; above 5000 ppm is dangerous (the OSHA 8-hour exposure limit). Only enrich while lights are on — plants do not take up CO2 in the dark.
How do I convert a ppm target into a CO2 flow rate (CFH)?
Two cases. Sealed room (rule of thumb): about 0.5 CFH per 100 ft³, so a 128 ft³ room needs ~0.64 CFH. Vented room with exhaust: CFH = ACH × volume × Δppm ÷ 1,000,000 — e.g. 128 ft³ at 2 air changes/hour, 400→1200 ppm needs 0.2048 CFH continuously.
How long will a 20 lb CO2 tank last in my grow room?
At a sealed-room maintenance flow of 0.64 CFH, a 20 lb tank (174.8 ft³ of CO2) lasts 273.1 hours — about 11.4 days, using 1.76 lb per day. Formula: hours = tank lb × 8.74 ÷ CFH.
How long does it take to bring the room up to target ppm?
Time = one-time dose ÷ flow rate. For the 128 ft³ room, a 0.1024 ft³ dose at 0.64 CFH takes 9.6 minutes to go from 400 to 1200 ppm.
Sealed room or vented room for CO2 enrichment?
Sealed wins: a vented 128 ft³ room at 2 ACH bleeds 0.2048 CFH continuously just to hold 1200 ppm, and a 20 lb tank then lasts 35.6 days at that flow. In a sealed room you only replace what plants use plus small leaks, and you must add air conditioning and dehumidification since no fresh air enters.
Is CO2 above 5000 ppm dangerous?
Yes. 5000 ppm is the OSHA 8-hour permissible exposure limit; levels above it can cause headaches, dizziness and asphyxiation risk in an enclosed space. Never set an enrichment target above 2000 ppm, ventilate the room before entering, and use a CO2 safety monitor/alarm in any sealed grow room.
Tank, burner, or CO2 bags — which is cheapest?
By physics, not brand: a 20 lb tank at typical sealed-room flows (e.g. ~0.64 CFH for 128 ft³) lasts roughly 11–12 days of continuous enrichment; propane/natural-gas burners add heat and humidity and need ventilation for combustion byproducts; CO2 bags/buckets release far less gas and suit only small tents. Compare cost per lb of CO2 delivered, and remember burners fight the 'sealed room' concept.
Should I run CO2 at night?
No. Plants only photosynthesize (and take up CO2) with lights on. Run injection on the same timer as your lights, shut it off 30–60 minutes before lights-out, and let the room air out. Night injection is pure waste.
How many cubic feet of CO2 are in a pound?
About 8.74 ft³ per lb at STP. So a 20 lb tank holds about 174.8 ft³, a 50 lb tank about 437 ft³. By mass, ppm converts as: mg of CO2 = ppm × room volume in m³ × 1.842.
For a bigger room, say 10×10×8 ft?
That is 800 ft³. Raising 400→1500 ppm (Δ1100 ppm) needs 0.88 ft³ = 0.101 lb of CO2 per dose, and sealed maintenance runs about 4 CFH.
Does this tool cover grow lights too?
No — this calculator is CO2 enrichment only (dose, flow rate, tank duration, sealed vs vented). For lighting math (PPFD, DLI, hanging distance, wattage sizing) use Grow Light Lab. The two are companions: light sets the ceiling on photosynthesis, CO2 only helps once light is already strong.
Free tool by NRTechStudio