Grow Tent Ventilation Calculator
Size your exhaust fan so the air in your tent turns over fast enough.
Result
Estimates assume reasonable ducting. Long runs, bends, silencers and dirty filters all reduce real airflow — size up if in doubt.
History
What it is
This calculator sizes the exhaust fan your grow tent or room needs by working out how much air must move to replace the full tent volume often enough to control heat and humidity. It outputs the required airflow in CFM (US) or m³/h (metric) and accounts for the resistance added by a carbon filter and the extra load from hot climates or HID lamps.
Who should use it
Tent and grow-room growers choosing or checking an inline exhaust fan, and anyone adding a carbon filter who needs to confirm their current fan still has enough headroom.
How to use it
- Enter the length, width and height of your tent or room.
- Set how often the air should be fully exchanged — once per minute is a safe default.
- Select whether a carbon filter is fitted (filters add roughly 25% resistance).
- Select whether you run hot ambient temperatures or HID lamps, which adds heat load.
- Read the required fan size and choose a fan rated at or above that figure.
Example calculation
Worked example
A 4 ft × 4 ft × 6.7 ft tent (48 × 48 × 80 in) holds about 107 ft³. Exchanging that volume once per minute needs ~107 CFM, and adding a carbon filter (×1.25) raises it to about 133 CFM (≈226 m³/h). Pick a fan rated a little above this so you can run it below max and keep noise down.
Formulas used
Fan = (L · W · H) · (60 / minutesPerExchange) · filterFactor · heatFactor → CFM (US) or m³/h (metric)
How to read your result
Buy a fan with headroom above the result — running a larger fan at a lower speed is quieter and lasts longer than running a small fan flat out. If you add ducting bends, long runs or a silencer, step up another size.
Recommended ranges
| Tent size | Approx. CFM | Approx. m³/h |
|---|---|---|
| 2×2 ft (0.6×0.6 m) | ~40 | ~70 |
| 3×3 ft (0.9×0.9 m) | ~80 | ~135 |
| 4×4 ft (1.2×1.2 m) | ~135 | ~225 |
| 5×5 ft (1.5×1.5 m) | ~210 | ~355 |
Common mistakes to avoid
- Forgetting the carbon filter, which can cut real-world airflow by 20–25%.
- Sizing the fan to its maximum rating instead of leaving headroom for filters and ducting.
- Ignoring intake — passive or active intake must roughly match exhaust or the fan starves.
- Measuring the tent footprint but using the wrong height (tents are taller than they look).
Frequently asked questions
How do I calculate CFM for a grow tent?
Multiply length × width × height to get the volume in cubic feet, then divide by how many minutes you want per air exchange (usually 1). Add about 25% if you run a carbon filter.
How often should grow tent air be exchanged?
Roughly once every 1–3 minutes. Once per minute is a robust default; hot rooms or CO₂-free setups benefit from faster exchange.
Does a carbon filter reduce fan airflow?
Yes, typically by 20–25%. Always size the fan above the bare requirement so it still hits the target with the filter attached.
Should intake be as big as exhaust?
Intake airflow should roughly match or exceed exhaust. With passive intake, the intake opening area should be larger than the exhaust duct to avoid choking the fan.
What size fan for a 4x4 tent?
About 130–200 CFM with a carbon filter, depending on lamp heat. A 6-inch inline fan is a common, flexible choice for a 4×4.