Air Changes Per Hour Calculator
ACH from airflow and room size — a number whose whole content is its denominator, so one fan spans a fortyfold range of ventilation.
This is a perfect-mixing figure: it assumes the incoming air distributes evenly, which no real room does. A badly placed inlet can short-circuit straight to the extract and leave a corner barely ventilated at the same nominal rate. Nothing is uploaded.
Air changes per hour is a fact about the room, not the fan
The whole content of the number is its denominator: sixty times the airflow, over the volume. So the same 100 CFM extractor is 18.8 air changes an hour in a small bathroom and 0.44 in a double-height open-plan space — a 42-fold difference from identical equipment.
| Room | Size | Volume | ACH at 100 CFM | One change takes |
|---|---|---|---|---|
| A small bathroom | 8 × 5 × 8 ft | 320 cu ft | 18.75 | 3 min |
| A bedroom | 12 × 11 × 8 ft | 1,056 cu ft | 5.68 | 11 min |
| A living room | 18 × 14 × 9 ft | 2,268 cu ft | 2.65 | 23 min |
| An open-plan floor | 30 × 25 × 9 ft | 6,750 cu ft | 0.89 | 68 min |
| A double-height space | 30 × 25 × 18 ft | 13,500 cu ft | 0.44 | 135 min |
And the ceiling counts every bit as much as the floor. The last two rows have an identical footprint — 30 by 25 feet — and the taller one gets exactly half the rate, because volume is a product of three numbers and sizing by floor area silently drops one of them.
Which is why a rate and an airflow are different instructions
Specify six air changes and the fan you need depends entirely on the room — 32 CFM in the bathroom, 675 in the open plan. Specify 100 CFM and the ventilation you get depends entirely on the room too. Quoting one when the other was meant is a common and expensive mistake.
| Target | Rate | Bathroom | Living room | Open plan |
|---|---|---|---|---|
| A whole-house minimum | 0.35 ACH | 2 CFM | 13 CFM | 39 CFM |
| General ventilation | 3 ACH | 16 CFM | 113 CFM | 338 CFM |
| A busy kitchen or bathroom | 6 ACH | 32 CFM | 227 CFM | 675 CFM |
| A workshop or lab | 12 ACH | 64 CFM | 454 CFM | 1350 CFM |
| Heavy fume extraction | 20 ACH | 107 CFM | 756 CFM | 2250 CFM |
The relation is exactly proportional in both directions, so doubling the target doubles the fan and doubling the room doubles it again. A whole-house minimum is a trickle; heavy fume extraction in a large space is an industrial machine. Same formula, four orders of magnitude between the answers.
How to use
- Enter the airflow in CFM.
- Enter the room length, width and ceiling height.
- Read the air changes per hour and the time per change.
- Check what a target rate would cost in airflow.
Frequently asked questions
How do I calculate air changes per hour?
Sixty times the airflow in CFM, divided by the room volume in cubic feet. A hundred CFM into a 2,268 cubic foot living room is about 2.6 air changes an hour.
Why does the same fan give such different rates?
Because the whole content of the number is its denominator. A hundred CFM is 18.75 air changes an hour in a small bathroom and under half of one in a double-height open-plan space — a fortyfold difference from identical equipment.
Does ceiling height really matter that much?
As much as floor area does. Volume is a product of three numbers, so doubling the ceiling exactly halves the rate with the footprint unchanged. Sizing by floor area silently drops one of the three.
What rate should I aim for?
That depends entirely on the use — a whole-house minimum is well under one air change an hour, general ventilation is a few, a busy kitchen or bathroom is around six, and fume extraction is far higher.
How long does one air change take?
Sixty divided by the rate. Six air changes an hour is one every ten minutes; half an air change an hour is one every two hours. That reciprocal is usually the more intuitive way to feel the number.
Is specifying ACH the same as specifying CFM?
They are different instructions, and confusing them is expensive. Six air changes needs 32 CFM in a small bathroom and over 600 in an open-plan floor, so a spec written in one and read as the other can be out by a factor of twenty.
Does the room really mix perfectly?
No, and that is the main limitation here. This assumes incoming air distributes evenly. A badly placed inlet can short-circuit straight to the extract and leave a corner barely ventilated at the same nominal rate.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.