Air Conditioner BTU Calculator
Enter your room's floor area in square feet to estimate the cooling capacity in BTU per hour, using the standard 20 BTU per square foot rule of thumb.
Size your AC in seconds
Enter the room area in square feet and the air conditioner BTU calculator returns the recommended cooling capacity in BTU per hour.
A rule of thumb
20 BTU per square foot is a starting point — sunny, crowded, or high-ceiling rooms need more, so treat the result as an estimate, not a spec.
How big an air conditioner do you need?
Cooling capacity by room size
The air conditioner BTU calculator turns one measurement — your room's floor area in square feet — into the cooling capacity you should look for when buying a unit. Cooling capacity is measured in BTU per hour (British Thermal Units of heat removed each hour), and the widely used rule of thumb from the US Department of Energy and ENERGY STAR is roughly 20 BTU per hour for every square foot of living space. Pick a unit that is too small and it will run flat out without ever cooling the room; pick one that is far too large and it short-cycles, leaving the air cold but clammy. This tool gives you a sensible starting number in seconds.
Enter your room's area in square feet to get the recommended cooling capacity in BTU per hour instantly.
The recommended cooling capacity is simply the room's floor area multiplied by 20 BTU per hour per square foot.
BTU/h = area (sq ft) × 20The factor of 20 BTU per hour per square foot reflects a typical room with average ceiling height, insulation, and sun exposure. Multiply your floor area by it and you get the baseline capacity to shop for, which you can then nudge up for the conditions described below.
Suppose you want to cool a 300 square foot bedroom.
Measure the floor area
Multiply length by width — say 20 ft × 15 ft = 300 sq ft.
Apply the rule of thumb
300 sq ft × 20 BTU/h per sq ft = 6,000 BTU/h.
Pick a matching unit
Look for a room air conditioner rated around 6,000 BTU/h — a very common size for a single bedroom.
The number this calculator returns is the baseline cooling capacity to shop for, and it maps neatly onto the sizes air conditioners are actually sold in. The 20 BTU per hour per square foot baseline puts a 150 sq ft room at about 5,000 BTU/h, a 300 sq ft room at 6,000 BTU/h, a 500 sq ft room at 10,000 BTU/h, and a 700 sq ft open-plan space at around 14,000 BTU/h — all standard ratings you will see on the shelf. Because the relationship is linear, doubling the floor area doubles the capacity you need, which makes it easy to sanity-check a quote: if a contractor proposes a 12,000 BTU/h unit for a small 200 sq ft study, that is roughly three times the rule-of-thumb figure and worth questioning. Treat the result as the centre of a sensible range rather than an exact target, and round to the nearest size the manufacturer offers.
The 20 BTU per square foot figure is a rule of thumb, not a precise heat-load calculation.
Adjust for your room's real conditions
Area alone ignores ceiling height, sun, occupancy, and climate. ENERGY STAR suggests reducing capacity by about 10% for a heavily shaded room and increasing it by about 10% for a very sunny one, adding roughly 600 BTU/h for each person beyond two who regularly uses the room, and adding about 4,000 BTU/h if the unit cools a kitchen. Tall ceilings, poor insulation, and hot climates all push the figure up. For a whole house or a precise specification, use a Manual J load calculation rather than this estimate.