The arithmetic, and why it is worth doing once
An air change is the room's own volume of air, replaced. Air changes per hour is how many times that happens in an hour. Fans are sold in cubic feet per minute. So the conversion is a division by sixty and nothing else:
CFM = room volume in cubic feet × ACH ÷ 60
A 10 by 12 bathroom with an 8 foot ceiling is 960 cubic feet. At 8 ACH that is 960 × 8 ÷ 60, which is 128 CFM. Run it the other way and a 110 CFM fan in that room gives 110 × 60 ÷ 960, or about 6.9 ACH — one complete change every 8.7 minutes.
The reason to do it explicitly is that ACH and CFM get quoted interchangeably in the same conversation. A supplier quotes a fan in CFM, a specification asks for air changes, and a forum answer gives a flat number of CFM for a room size without saying what ceiling height it assumed. Volume is what actually matters, and two rooms with the same floor area and different ceilings need different fans.
Where the ACH figures come from
The rates in the selector are the ones in general circulation for each type of space. They are conventions built up from practice, and different sources will give you different numbers for the same room. They are not code requirements, and the code that applies where you are may specify the rate a completely different way — as CFM per square foot, as CFM per occupant, as a flat minimum per fixture, or as a whole-house rate rather than a room rate.
| Space | ACH commonly used | What is driving it |
|---|---|---|
| Bedroom, living space | 4 | Occupant CO2 and general staleness |
| Basement, crawlspace | 5 | Moisture and radon dilution |
| Garage | 6 | Vehicle exhaust, fuel vapour |
| Bathroom | 8 | Moisture, removed fast enough to beat condensation |
| Workshop | 8 | Dilution of what the machines throw off |
| Kitchen, residential | 15 | Grease, moisture, combustion products from cooking |
| Welding bay | 20 | Metal fume, which is a health exposure not a smell |
| Finishing, spray | 25 and up | Solvent vapour, both toxic and flammable |
Treat these as a starting point that gets you into the right size bracket. If a rate matters for a health exposure — welding fume, solvent vapour, silica — the ACH figure is the wrong tool entirely, and the exposure limit and the capture method are what govern.
The rated CFM problem
Nearly every fan you can buy has its CFM measured at zero static pressure. That means the fan running with nothing attached to it. It is a repeatable test and it makes comparisons between fans fair, but it is not the condition your fan will ever operate in.
Attach a duct and the fan has to push against the friction of the duct wall, the losses at every elbow, the resistance of the backdraft damper and the wall or roof cap on the outside. The flow drops. How far it drops depends on the whole path. A short, smooth, straight run of rigid duct with a generously sized cap costs you a little. Twenty-five feet of undersized flex duct with two sharp elbows and a sagging middle costs you a great deal — enough that people install a 110 CFM bathroom fan and measure 40 CFM at the grille.
The derate field on this page is a crude allowance for that. The better move, when the manufacturer supports it, is to use the published flow at a stated static pressure — 0.1 or 0.25 inches of water gauge are the common test points — and take that number at face value. Some manufacturers publish a whole curve. Certified ratings from an independent body are more trustworthy than a number printed on a carton.
Continuous, intermittent, and the thing about run time
A ventilation rate assumes the fan is running. A bathroom fan that runs for the four minutes someone is in the room does not achieve any ACH figure at all, because it has barely started moving the moist air before it shuts off. The moisture load from a shower does not leave when the water stops; it is sitting in the towels, on the walls and in the air, and it takes the fan a good while to get it outside. A timer switch, a humidity-sensing control or simply a fan left running is worth more than the difference between a 110 and a 150 CFM unit. If you fix one thing in a bathroom with a mould problem, fix the run time before you buy a bigger fan.
Questions people ask
How do I convert CFM to air changes per hour?
Multiply the CFM by 60 to get cubic feet per hour, then divide by the room volume in cubic feet. A 150 CFM fan in a 1,200 cubic foot room is 150 x 60 / 1,200, which is 7.5 air changes an hour. Going the other way, multiply the volume by the ACH you want and divide by 60. The only place people trip is the volume: floor area times ceiling height, using an average height if the ceiling is sloped, and it is the volume rather than the floor area that sets the answer.
What CFM bathroom fan do I need?
For a bathroom up to about 100 square feet, the rule in widest use is 1 CFM per square foot with a floor of around 50 CFM, so an 8 by 10 bathroom lands at 80 CFM. Above that size, or where the toilet is in a separate compartment, the common approach is to add an allowance per fixture instead. The 8 ACH figure used on this page gives a similar answer for a typical 8 foot ceiling and diverges for tall ones, which is the honest outcome since a tall bathroom does hold more moist air. What the calculation cannot tell you is whether the fan will actually move that much once it is ducted, or whether it will run long enough to matter. Both of those change the result more than the size on the box.
Can an exhaust fan be too big?
Yes, for two reasons. The obvious one is that a fan moving a lot of air is a fan making a lot of noise, and a fan that annoys people gets switched off, which is worse than a slightly small fan that runs. The serious one is depressurisation: air pulled out of a tight house has to come back in, and if the easiest path back in runs down the flue of an atmospherically vented gas water heater or furnace, the fan will pull combustion products into the house. Large exhaust — a big range hood in particular — needs a deliberate makeup air path designed with the appliance in mind, and it is one of the things the adopted code in most places has specific rules about. A CO alarm belongs in any house with a fuel-burning appliance regardless.
Is general ventilation enough for a woodshop?
No, and treating it as though it were is the common mistake. Room air changes dilute what has already escaped into the room. They do nothing about what is at the machine, and by the time the fine fraction has dispersed enough for the room fan to work on it, you have been breathing it. Capture at the source with a dust collector on the machine, an air cleaner to handle what escapes, and general ventilation to keep the room from becoming a box are three different jobs. The general ventilation figure here is the least important of the three, and it is the only one that gets sized from room volume.