👋About

By the BTU Sizer editorial desk

BTU Sizer is a small independent site that answers one question in thirty-one different shapes: how big should the thing in this room be? Air conditioner, furnace, heat pump, dehumidifier, extractor fan, ceiling fan — each is a different machine with a different rule, and each rule is published somewhere by somebody who tested it.

Why it exists

The sizing question is asked by people who are about to spend real money, usually once every ten years, with no way to tell whether the number they have been given is right. The calculators that already exist fall into two camps. Manufacturers and retailers have one, and it exists to land you on their product page. General-purpose calculator sites have the arithmetic, but they stop at a raw figure — 7,340 BTU per hour — without telling you that no such machine is made and the choice is between 6,000 and 8,000.

This site does the last step. Every tool ends on a size that is actually sold, and shows the raw figure next to it so you can see how close the call was.

Where the numbers come from

Nothing here is invented. Cooling starts from the ENERGY STAR room air conditioner rate and its published corrections for ceiling height, sun, insulation, occupancy and kitchens. Heating uses BTU-per-square-foot rates that vary by IECC climate zone. Dehumidifiers use the post-2019 US Department of Energy rating scale, which is the detail that makes older charts wrong by about 30%. Air purifiers use the AHAM two-thirds rule, bathroom fans the HVI recommendation of one CFM per square foot. Each tool lists its own sources under How we calculate, and the shared formulas are set out in full on the methodology page.

What this is not

It is not an ACCA Manual J load calculation. Manual J measures every wall, window, door and duct in a house and is what a contractor should run before installing a whole-house system. A rate per square foot with corrections lands close for a single room and a single machine, which is what most people are buying — and it is a good enough check on a quote to know which questions to ask.

Nothing you type is sent anywhere. Every calculation happens in your own browser, there is no account needed to use any tool, and the site holds no record of what you worked out.

Found a mistake, or a rule we have got wrong? Tell us — corrections are welcome and get made.