About Free BTU Calculator

Free BTU Calculator answers one question: how big an air conditioner or heater does this room need? Every retailer has a sizing chart, every forum has an opinion, and most of them disagree by a unit size or two. We wanted a single place where the method is written down, the inputs that matter are all on one screen, and the result is explained rather than just asserted.

The calculator uses the standard rule-of-thumb method that HVAC contractors use for quick quotes and that Energy Star's room air conditioner guidance is built on: a base of 20 BTU per square foot for cooling, climate-specific factors of 30 to 55 BTU per square foot for heating, and adjustments for ceiling height, sun exposure, insulation, occupants and kitchens. The exact multipliers are printed on the home pageand inside the calculator's breakdown panel, so you can check our arithmetic.

It is not a Manual J load calculation. For a single room appliance (window unit, portable, space heater, single-zone mini-split), the rule of thumb lands within one unit size of a full calculation almost every time, and the ±10% range we show covers the rest. For a whole-house central system, use our number as a sanity check on your contractor's quote and insist that they run a proper load calculation before you sign.

The site is built and maintained by Prasen, a software engineer who builds calculation-heavy web tools. Every formula lives in a small, unit-tested module; the programmatic room-size pages are generated from the same code the calculator runs, so they can never disagree with it.

Free BTU Calculator is free, needs no account, and runs entirely in your browser; your room measurements are never sent anywhere. The site is supported by advertising, which has no influence on the numbers.

Spotted an error, or have a room type we should add? Get in touch.