Find how many BTUs you need to cool or heat a room. Enter room size, ceiling, sun, and occupants for the right AC or heater size and the unit to buy.
Pick a room, then adjust the size, ceiling, and conditions to match yours.
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Buying an AC or heater that's the wrong size is a common, costly mistake — too small and it never keeps up, too big and it short-cycles, wasting energy and leaving the room humid. The fix is matching the BTUs (British Thermal Units) to your room. This calculator uses the standard rule of about 20 BTUs per square foot for cooling, then adjusts for ceiling height, sun exposure, the number of people, and whether it's a kitchen; for heating it uses 30–60 BTUs per square foot based on your climate. Enter your room and it returns the recommended BTUs, the standard unit size to buy, and the cooling tonnage — no signup required.
A BTU (British Thermal Unit) measures heating or cooling energy — the more BTUs, the more a unit can heat or cool. To size a room air conditioner, start with about 20 BTUs per square foot, so a 300-square-foot room needs roughly 6,000 BTUs. Then adjust: add about 10% for a very sunny room (or subtract 10% if heavily shaded), add 600 BTUs for each person beyond two, add 4,000 BTUs for a kitchen, and scale up for ceilings over 8 feet. Heating needs more — typically 30 to 60 BTUs per square foot depending on your climate and insulation. Air conditioners are also rated in 'tons,' where one ton equals 12,000 BTUs. Common sizes: a 5,000-BTU unit suits up to ~150 sq ft, 8,000 BTUs up to ~350 sq ft, and 12,000 BTUs up to ~550 sq ft.
BTU Sizing Formula
Size a room air conditioner so it cools efficiently without short-cycling.
Find the BTUs and tonnage for a ductless mini-split zone.
Match a heater's BTUs to a room and your climate.
Adjust for west-facing sun or a shaded north room.
Add capacity for cooking heat and extra people.
Scale up the BTUs for vaulted or 9–10 ft ceilings.
Avoid an AC or heater that's too small (never cools) or too big (short-cycles and wastes energy).
Sizes both air conditioners and heaters, with climate-based rates for heating.
Accounts for ceiling height, sun, occupants, and kitchens — not just a flat per-square-foot number.
Recommends the standard unit size and the cooling tonnage, so you know what to shop for.
Enter your room and get the answer immediately — no signup or sales pitch.
Bedrooms, living rooms, kitchens, studios, and whole open-plan spaces.
As a rule of thumb, you need about 20 BTUs per square foot for cooling, so a 300-square-foot room needs roughly 6,000 BTUs. Adjust up for high ceilings, sun, extra people, or a kitchen. Heating typically needs 30 to 60 BTUs per square foot depending on climate. Enter your room above for a tailored number.
For cooling, about 20 BTUs per square foot is the standard starting point. For heating, it's higher — roughly 30 BTUs per square foot in mild climates up to 60 in very cold ones, since holding a warm indoor temperature against cold outdoor air takes more energy.
A 20×20 room is 400 square feet, so at 20 BTUs per square foot you'd start around 8,000 BTUs — but a sunny room, high ceilings, or several people push it toward 10,000–12,000. This calculator applies those adjustments and suggests a standard unit size.
A 12,000-BTU air conditioner (1 ton) cools roughly 450–550 square feet under average conditions. Sun, high ceilings, kitchens, and extra occupants reduce that coverage, so size up if those apply.
Cooling removes heat and is sized at about 20 BTUs per square foot. Heating adds heat and usually needs more — 30 to 60 BTUs per square foot — because it must overcome heat loss to the colder outdoors, which depends heavily on your climate and insulation.
Yes. A room with 10-foot ceilings has 25% more air to condition than one with 8-foot ceilings, so the BTUs scale up. A sunny, west-facing room needs about 10% more cooling, while a shaded room needs about 10% less. This calculator factors both in.
In air conditioning, one ton equals 12,000 BTUs per hour, so a 2-ton AC is 24,000 BTUs. The term dates back to the cooling power of a ton of ice; this calculator shows your need in both BTUs and tons.
Too few BTUs and the unit runs constantly but never reaches your target temperature. Too many and it short-cycles — cooling fast then shutting off — which wastes energy and leaves the room humid and uncomfortable. Matching the BTUs to the room avoids both.