BTU SizerHow many air changes an hour does this room get?
Air changes per hour calculator
3.9 air changes per hour
To hit 5 changes an hour this room needs 128 CFM.
- Floor area
- 192 sq ft · 17.8 mยฒ
- Volume
- 1,536 cu ft
- Cooling at 20 BTU/h per sq ft
- 3,840 BTU/h
Compare fans honestly: 100 CFM is eight changes an hour in a bathroom and barely one in an open-plan floor. The CFM alone tells you nothing.
One comfort check a month
MonthlyOne thing a month worth measuring, cleaning or switching over before the weather turns โ the maintenance side of sizing, in two sentences. No product pitches.
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How do you calculate air changes per hour?
Multiply the airflow in CFM by 60, then divide by the room’s volume in cubic feet. A 100 CFM fan in a 16 ร 12 ft room with an eight-foot ceiling โ 1,536 cubic feet โ gives 100 ร 60 รท 1,536 = 3.9 air changes an hour.
Air changes are the only fair way to compare ventilation between rooms. The same 100 CFM extractor is eight changes an hour in a small bathroom and less than one in an open-plan living space, so the CFM number on the box means nothing on its own.
The targets worth knowing: about 8 for a bathroom, 15 over a cooktop, 4 to 5 for an air purifier in general use and 8 to 10 for allergies or smoke, and roughly 0.35 for whole-house background ventilation โ which is where the “a house needs to breathe” figure comes from. Run the calculation backwards and it tells you the CFM to shop for.
The same fan in different rooms is a completely different fan. This is what 100 CFM actually does, room by room.
| Room | Volume at 8 ft | ACH from 100 CFM | Target |
|---|---|---|---|
| Cloakroom, 4 ร 5 ft | 160 cu ft | 37.5 | 8 |
| Bathroom, 8 ร 6 ft | 384 cu ft | 15.6 | 8 |
| Bedroom, 12 ร 14 ft | 1,344 cu ft | 4.5 | 4โ5 |
| Living room, 16 ร 20 ft | 2,560 cu ft | 2.3 | 4โ5 |
| Open plan, 20 ร 30 ft | 4,800 cu ft | 1.3 | 4โ5 |
Read the last two rows against the target column and the purifier market makes sense: this is why a unit that transforms a bedroom does nothing noticeable in an open-plan floor, and why CADR is quoted against room size rather than on its own.
Frequently asked questions
What is a good number of air changes per hour?
It depends entirely on the room’s job. A bathroom wants around eight so steam clears before it condenses. A kitchen hood works at fifteen or more over the cooktop. An air purifier in a living room is doing well at four or five, and eight to ten during wildfire smoke. Whole-house background ventilation is far lower, around 0.35, because it runs continuously.
Is more ventilation always better?
No. Every air change replaces conditioned air with outdoor air that has to be heated or cooled again, so over-ventilating a house is expensive and, in winter, uncomfortably dry. High rates belong in bathrooms and kitchens during use; the rest of the house wants steady, modest ventilation rather than a lot of it.
How do I convert CFM to air changes per hour?
ACH = CFM ร 60 รท room volume in cubic feet. The 60 converts minutes to hours. Going the other way, CFM = volume ร ACH รท 60 โ which is how you decide what to buy rather than how you judge what you have.
Does this apply to air purifiers as well as fans?
Yes, with one difference: a purifier recirculates and cleans the same air rather than replacing it with outdoor air, so the correct term is clean-air changes per hour. The arithmetic is identical, using the purifier’s CADR in place of CFM. It is the honest way to compare two purifiers in two different rooms.
What is ACH50 and is it the same thing?
No. ACH50 is a blower-door test result โ how leaky a building is when pressurised to 50 pascals โ and it measures the shell rather than a fan. Natural air changes at ordinary conditions are roughly ACH50 รท 20. Do not compare an ACH50 figure with the ventilation rates on this page; they are different measurements with the same-looking unit.
Does ceiling height change the answer much?
Directly. A room with a twelve-foot ceiling has half again the volume of the same footprint at eight, so the same fan produces a third fewer air changes. This is the main reason a bathroom rule written per square foot works well in ordinary houses and less well in converted lofts and old buildings with high ceilings.
How we calculate this
Worked example
With Length (ft) 16, Width (ft) 12, Ceiling height (ft) 8, Fan or unit rating (CFM) 100, Air changes you want 5 per hour, this page works out 3.9 air changes per hour. To hit 5 changes an hour this room needs 128 CFM.
- Floor area192 sq ft ยท 17.8 mยฒ
- Volume1,536 cu ft
- Cooling at 20 BTU/h per sq ft3,840 BTU/h
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