Size a fan from a target rate, or take the fan you already have and find out what it is actually achieving. The second question is the one that catches people out.
Rates are typical practice, not code. Spray booths, commercial kitchens and anything handling hazardous fumes are governed by specific requirements.
Enter a fan you already own and the ticket shows what it actually achieves in this space. Rated CFM is free-air; ducting and filters take a real bite.
Air changes per hour is airflow expressed relative to room size. The formula runs both ways:
CFM = volume × ACH ÷ 60 and ACH = CFM × 60 ÷ volume
The first sizes a fan. The second is the more revealing one — take the extractor already on the wall, put the room volume in, and find out what it is actually doing. A 600 CFM fan is twelve air changes in a bathroom and under two in a three-car shop.
| Space | ACH | Driven by |
|---|---|---|
| Living space | 2 | comfort, odour |
| General workshop | 4–6 | dust, solvent vapour |
| Garage | 4 | exhaust fumes |
| Bathroom | 8 | moisture |
| Welding bay | 15–20 | metal fume |
| Grow room | 30–60 | heat and CO₂ |
| Spray booth | 60+ | code, not comfort |
These are working figures for general ventilation. Anything involving hazardous fumes, combustion or a permit is governed by specific requirements rather than a table like this one.
Fan ratings are measured in free air. Put the fan on the end of a duct run with elbows, a backdraft damper and a filter, and delivered airflow drops substantially — twenty percent is a modest allowance and long or badly routed runs lose more.
Short, straight, smooth duct at the rated diameter is worth more than a bigger fan.
Exhaust is only half a ventilation system. A fan can only remove the air that something replaces, so in a tight space a large exhaust fan simply cannot achieve its rating — it depressurises the room until the flow drops to whatever leaks in.
In a house that matters for more than performance. Depressurisation can pull combustion gases back down the flue of an atmospherically vented water heater or furnace. Where exhaust is significant, makeup air is part of the design.
No. Over-ventilating a conditioned space throws away heating and cooling you paid for. In a shop it is cheap; in a house it is not.
Intermittent for process ventilation — run it while welding, not all day. Continuous where the source is continuous, such as moisture or CO₂.
Considerably. Intake and exhaust at opposite ends sweeps the room. Both at the same end short-circuits the airflow, and the far corner never sees a change no matter what the arithmetic says.