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AC BTU Calculator

Find the BTU rating for your room size.

How it is calculated

This is a rough air-conditioner sizing rule: about 20 BTU per hour of cooling for each square foot of floor. The room area in square metres is converted to square feet (× 10.7639104) and multiplied by 20: BTU/h = m² × 10.7639104 × 20. The kilowatt equivalent is kW = BTU/h × 0.000293071. The area must be above zero. The rule ignores ceiling height, sun exposure, number of people, windows, insulation and heat from appliances, so rooms with large south-facing windows or a kitchen need a larger unit, and the 20 BTU per sq ft figure is only a starting rule of thumb.

Worked example

A room of 20 m². Area in square feet: 20 × 10.7639104 = 215.28 sq ft. Cooling load: 215.28 × 20 = 4,305.6, shown as 4,306 BTU. In kilowatts: 4,305.6 × 0.000293071 = 1.26 kW. Many split units are sold in sizes such as 9,000 BTU, so you would typically round up to the next available size, especially for a sunny room.

Try this example →

Common mistakes

  • Entering the area in square feet. The field is in square metres, so 215 would give ten times too large a result.
  • Choosing the unit exactly at the result without allowing for sun, a top-floor ceiling, or many occupants.
  • Confusing cooling capacity (BTU/h or kW of cooling) with the electrical power the unit consumes, which is lower.
  • Adding the areas of connected rooms and cooling them with one small unit.

About this calculator

This calculator estimates the cooling capacity, in BTU (British Thermal Units) per hour, an air conditioner needs to handle a room, based primarily on floor area with adjustments for factors like ceiling height, sun exposure, and number of occupants — the rule-of-thumb baseline is roughly 20 BTU per square foot before adjustments.

Homeowners shopping for a window or split-unit AC use it to avoid the two common mistakes: undersizing a unit that runs constantly without ever cooling the room properly, or oversizing one that cycles on and off too fast and leaves the air clammy. HVAC installers use the same logic as a quick sanity check before a full Manual J load calculation.

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