BTU SizerAir and moisture calculators
Temperature is only half of comfort. These five size the machines that handle the other half: moisture, smell, dust and moving air.
Humidifier size calculator
Output in gallons a day — and why tank size is the number you feel.
Air purifier CADR calculator
Two thirds of the floor area — the rule the industry actually publishes.
Bathroom fan CFM calculator
One CFM per square foot, fifty minimum — and the duct that undoes it.
Ceiling fan size calculator
Blade span by room area — plus the direction switch nobody uses.
Dehumidifier size calculator
Pints per day on today's rating scale, not the one from before 2019.
Range hood CFM calculator
Sized by the cooktop, not the room — and the make-up air rule above 400.
Ask somebody what makes a room comfortable and they will say temperature. Ask them which room in the house is unpleasant and they will describe a basement that smells of damp, a bathroom whose mirror stays fogged, or a kitchen where last night’s dinner is still in the air. None of those is a temperature problem, and no amount of cooling capacity fixes them.
These five size the machines that do. Each one runs on a published rule rather than a rule of thumb: two thirds of the floor area for a purifier, one CFM per square foot for a bathroom fan, 100 CFM per foot of cooktop for a hood. Where the rule is a table rather than a formula — which is the case for dehumidifiers — the whole table is on the page, because the number you are given is worth less than the row it came from.
Frequently asked questions
Why is a dehumidifier sized in pints rather than square feet?
Because the job is removing water, and how much water there is depends on the room being damp rather than on the room being big. A pint rating is litres of water pulled out of the air in 24 hours under a standard test. Sizing charts therefore cross floor area with how damp the space is, and the dampness column moves the answer as much as doubling the area does.
Did dehumidifier pint ratings change?
Yes, and it catches people out constantly. The US Department of Energy changed the test temperature from 80 °F to 65 °F in 2019. Less moisture condenses out of cooler air, so the same machine measures roughly 30% lower now than it did before. A 70-pint unit from 2016 and a 50-pint unit from today are close to the same machine — never compare a chart to a rating without checking which scale it is on.
What CADR does an air purifier need?
At least two thirds of the room’s floor area in square feet, which is the rule the Association of Home Appliance Manufacturers publishes. A 300 sq ft room wants a CADR of at least 200. That two-thirds ratio delivers roughly five air changes an hour in a real room, which is the point where the purifier keeps up with what keeps coming in.
Is more CFM always better for a bathroom fan?
No. A fan far larger than the room pulls conditioned air out of the house faster than it needs to, is louder, and in a tight modern home can pull combustion gases back down a flue. One CFM per square foot with a 50 CFM floor is the Home Ventilating Institute recommendation, and the fix for a slow-clearing bathroom is usually duct size and run length rather than a bigger fan.
How we calculate this
Sources: