Air Conditioner BTU Calculator

An air conditioner's job is to remove heat, and its capacity is measured in BTU per hour, with twelve thousand BTU making one "ton" of cooling. Sizing is a balance: too small and the unit runs continuously without reaching the set temperature, too large and it cools in short bursts that leave the room clammy, wear out the compressor and waste electricity. The aim is a unit matched to the room's actual heat gain. The rule of thumb is around twenty BTU per square foot of floor area for a normal room with a standard ceiling, adjusted upward for extra ceiling height because more air means more volume to cool. Rooms in full sun all day need about ten per cent more, shaded rooms a little less. Each additional occupant adds roughly four hundred BTU, since people are heat sources, and a kitchen in the same space adds several thousand more because cooking produces heat directly. These adjustments are why the same floor area can need very different units. A shaded bedroom occupied by one person is a far lighter load than a sunny, west-facing living space with a kitchen at one end and four people in it. Heat also enters through large windows, poorly insulated walls and appliances such as computers and televisions, which the simple rule of thumb does not capture; a professional load calculation accounts for all of them. This calculator estimates capacity from area, height, sun, occupancy and a kitchen allowance, and suggests a typical unit type. It is a planning guide rather than a formal Manual J load calculation, which also considers insulation values, window area and orientation and local design temperatures. Use it to shortlist units, then confirm the size with an installer before buying.

This calculator returns estimates for planning and general information only. It is not engineering, architectural, structural, electrical or code advice, and it cannot replace a qualified professional who has seen the site. Material quantities depend on the actual dimensions, the product you buy and local building codes, so confirm the figures before ordering materials or starting work, and always follow local regulations.