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.
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.
Who builds it
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. The method and every guide were last reviewed in September 2026.
Where the formulas come from
The calculator uses the standard rule-of-thumb method that HVAC contractors use for quick quotes. None of it is invented here; each factor traces back to a published source or to the way equipment is actually sold:
- 20 BTU per square foot for cooling is the industry baseline behind ENERGY STAR's room air conditioner sizing chart and the sizing tables printed by window and portable AC manufacturers. That chart also supplies the sun and shade adjustments (+10% for a very sunny room, −10% for heavy shade), the +600 BTU per extra person, and the +4,000 BTU kitchen allowance.
- 30, 40 and 55 BTU per square foot for heating in hot, moderate and cold climates come from the heater sizing charts published by electric and gas heater manufacturers, which in turn approximate the design-temperature differences in the ASHRAE climate zone map for the United States.
- Ceiling height scaling (multiply by height ÷ 8) follows from the fact that load tracks air volume; 8 ft is the assumption baked into every square-footage chart.
- Insulation factors (poor +15% to +20%, good −10% to −15%) are the typical spread between pre-1980 uninsulated construction and modern sealed construction in residential load calculations.
- Unit classes (5,000, 6,000, 8,000 BTU and so on) are simply the capacities that window, portable, mini-split and central equipment is sold in.
Results are estimates. A licensed HVAC contractor's ACCA Manual J load calculation, which accounts for window area and orientation, air leakage, duct losses and your local design temperatures, is the gold standard for sizing whole-home systems and should be used for any central air or furnace purchase.
The exact multipliers are printed on the home page and inside the calculator's breakdown panel, so you can check our arithmetic.
Free, private, ad-supported
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.