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Air Conditioner Sizing Calculator

Enter the room area, ceiling height, sun exposure and number of people to size your air conditioner.

How it is calculated

Area is entered in square feet, ceiling height in feet, and occupants as a whole number. The base load is BTU/h = area × 20 × (height / 8), so an 8 ft ceiling is the reference and taller rooms scale up in proportion. The sun setting then multiplies the total: 0.9 for heavily shaded, 1.0 for normal and 1.1 for very sunny. Finally 600 BTU/h is added for every occupant beyond the first two. The result is also shown in tons (BTU/h ÷ 12,000) and kilowatts (BTU/h × 0.000293). A fractional head count, a zero or negative area or height, and a negative occupant count produce no result. Climate, insulation, window area, kitchens and the floor above are not modelled, so treat the figure as a starting point.

Worked example

A 350 ft² room with a 9 ft ceiling, very sunny, with 4 people. Base = 350 × 20 × (9 / 8) = 7,875 BTU/h. The sun factor 1.1 gives 8,662.5. The two people beyond the first two add 2 × 600 = 1,200, so the total is 9,862.5, shown as 9,863 BTU/h after rounding. That equals 0.82 tons of cooling or 2.89 kW. In practice you would pick the nearest standard unit at or just above this figure.

Try this example →

Common mistakes

  • Entering square metres in the area field: it expects square feet, so a 30 m² room typed as 30 gives a capacity about 11 times too small. Multiply square metres by 10.76 first.
  • Entering the ceiling height in metres or centimetres: the field is in feet, and typing 2.7 makes the load far too low.
  • Counting every person: only occupants beyond the first two add load, and visitors who stay briefly do not belong in the count.
  • Rounding down to a smaller unit to save money: an undersized unit may never reach the set temperature on hot days.
  • Ignoring heat sources the formula does not know about, such as a kitchen, large west-facing windows or a hot attic above, which call for more capacity.

About this calculator

Air conditioner sizing calculates the cooling capacity, measured in BTU/hr (British Thermal Units per hour), that a room needs to reach and hold a comfortable temperature. The baseline is roughly 20 BTU per square foot of floor area, then scaled in proportion to ceiling height against a standard 8-foot ceiling (more air volume to cool), adjusted for sun exposure (you choose shaded, average or sunny: a very sunny room, such as one facing south or west, gets 10% more capacity and a heavily shaded one 10% less), and occupancy, since HVAC guidelines add roughly 600 BTU for each additional person regularly in the room beyond the first two, because bodies generate heat.

Homeowners and HVAC installers use this sizing before buying a window unit, split system or portable AC, because an undersized unit runs constantly without ever reaching the target temperature, while an oversized unit cycles on and off too quickly (short-cycling), which wastes energy and fails to properly dehumidify the room — bigger is not automatically better with air conditioning.

Floor area in square feet, ceiling height in feet, how sunny the room is and how many people use it are combined into the BTU/hr capacity to shop for, also given in tons and kW.

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