BTU SizerRoom and unit calculators
Three small tools that everything else on this site is built on — the unit conversion, the room measurement and the air change rate.
CFM to m³/h converter
Four units, one amount of air — and the ratings to match them to.
Duct size calculator
Diameter from CFM — and the velocity that makes a run whistle.
SEER to EER converter
Five efficiency ratings, one machine — and why SEER2 looks worse.
Air changes per hour calculator
CFM means nothing until you know the room. This is the conversion.
BTU to kW and tons converter
The same machine, three units — depending on whose catalogue you are reading.
Room size and volume calculator
Area, volume and what they imply — in both unit systems at once.
Every answer on this site is one of three small sums wearing a costume: a unit conversion, an area, or a rate of air movement. They are gathered here on their own because sooner or later you need one of them without the rest — reading a spec sheet in a shop, checking a quote, or working out whether the extractor already on the ceiling is doing anything at all.
The conversion tool is the one people come back to. Equipment is advertised in BTU per hour in the United States, kilowatts almost everywhere else, and tons when it is a central system — and a contractor will happily quote you all three in the same conversation.
Frequently asked questions
Why do catalogues use three different units for the same thing?
History, mostly. Tons of refrigeration date from the ice trade — one ton is the cooling given off by a ton of ice melting over a day, which is 12,000 BTU per hour. BTU per hour is the US convention for equipment ratings. Kilowatts are the SI unit and what almost every other country prints. All three describe the same rate of heat movement, and every listing here shows all three.
What is an air change per hour?
How many times an hour a fan or a system replaces all the air in a room. Multiply the fan’s CFM by 60 and divide by the room’s volume in cubic feet. A bathroom aims for about eight changes an hour, a kitchen for fifteen over the cooktop, and a purified living room for four or five. It is the honest way to compare a small fan in a small room with a big one in a big room.
How do I measure an L-shaped room?
Split it into rectangles, work out each one, and add them. Do not measure the longest dimension in each direction and multiply — that counts the missing corner as floor you have to condition. For anything with a sloped ceiling, use the average height rather than the highest point.
Should I include cupboards, hallways and stairwells?
Include anything the conditioned air moves freely into and out of, which usually means an open hallway or a stairwell but not a closed cupboard. A stairwell in particular deserves counting twice over: it connects two floors, and warm air will use it.
How we calculate this
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