Grow tools
What size extraction fan do I need?
Enter your tent size and lighting and this works out the extraction you need in m³/h, the duct size to run it in, and the minimum carbon filter rating. Then it shows you what we stock that actually fits.
Add up every light in the tent. Leave at zero if you have not chosen one yet.
Advanced: duct run
Every metre beyond three, and every bend, costs you airflow. Leave these alone if your fan sits on top of the tent.
You need extraction of
296 m³/h
- Duct size
- 150mm
- Filter rated at least
- 370 m³/h
- Air changes
- 1.7×
Why this number
Extraction fans that fit
Carbon filters that fit
Matching ducting
Keep it quiet
Want the full ventilation plan?
Join the grow list and we will send you a printable setup checklist covering fan, filter, ducting and silencer placement.
Your result is already saved in the link above — copy it and you can come back to it any time without giving us anything.
Extraction is sized on the volume of air in your tent, not its floor area. The starting point is one complete air change every minute.
Worked examples
1.2 × 1.2 × 2.0m tent, 600W LED, carbon filter
296 m³/h — 150mm fan
2.88 m³ of air × 60 = 173 m³/h for one change a minute. A 600W LED adds 15%, the carbon filter costs another 35%, and a 3m run with two bends adds 10%. That lands at 296 m³/h. A 125mm duct carries that, but the smallest fan we stock is 150mm, so 150mm is what you buy.
1.2 × 2.4 × 2.0m tent, 2 × 600W HPS, carbon filter
823 m³/h — 200mm fan
5.76 m³ × 60 = 346 m³/h. 1,200W of HPS is a heavy heat load and adds 60%, the filter adds 35%, and the duct run 10%. That reaches 823 m³/h, so you want 200mm ducting and a fan rated comfortably above 823 — around 1,000 to 1,230 m³/h.
0.8 × 0.8 × 1.8m tent, 200W LED, no filter
80 m³/h — 100mm duct
1.15 m³ × 60 = 69 m³/h, and a 200W LED adds only 5%. Without a carbon filter there is no resistance penalty, so the duct run is the only other factor and the total is 80 m³/h. 100mm ducting is plenty. Note that the smallest fan we stock is far larger than this, so run it on a speed controller.
Common questions
How often should the air in a grow tent be changed?
Once a minute is the working rule, which is where this calculator starts. Anything slower and heat builds up in the canopy; much faster and you are pulling humidity out so hard the plants struggle and your carbon filter wears out early.
Does a carbon filter reduce fan power?
Yes, by roughly a third. Carbon is a dense bed of granules and the fan has to pull air through all of it. That is why this calculator adds 35% when you tick the filter box, and why the filter must always be rated above the fan rather than matched to it.
What size carbon filter do I need for a 600W light?
Wattage alone does not decide it — tent volume does. A 600W light in a 1.2 × 1.2 × 2.0m tent needs about 296 m³/h, so a filter rated 370 m³/h or above. The same 600W in a 2.4 × 1.2m tent needs roughly double. Enter your actual dimensions above.
Should the carbon filter go before or after the fan?
Before it. The filter sits inside the tent, the fan pulls through it, and the ducting carries the scrubbed air out. Pushing air through a filter instead of pulling it leaks unfiltered air at every join before the filter.
What size ducting do I need for a 6 inch fan?
150mm, which is what 6 inch means. Never reduce ducting below the fan's flange: a 150mm fan pushed into 125mm ducting loses a large part of its airflow and gets noticeably louder.
Do I need a silencer?
Only if noise matters where the tent is. An acoustic box fan or a silenced inline fan solves it at source and is usually the better answer. An inline silencer fitted after the fan is the retrofit for a standard fan you already own.
Why is my calculated figure higher than the fan's advertised rating?
Manufacturers quote free-air figures, measured with nothing attached. Once a carbon filter, several metres of ducting and a couple of bends are in the way, real-world output is lower. Sizing with headroom, as this calculator does, is what stops that gap becoming a heat problem in August.
Can one fan run two tents?
It can, but add the volumes of both tents together and size for the total, then run a Y-piece. Bear in mind that any smell, and any pest, is then shared between both tents.