How to Choose the Right Air Conditioner Size for a Room
Why bigger isn't automatically better when sizing an AC unit
The starting point: area and a BTU estimate
Room air conditioners are sized in BTUs (British Thermal Units) of cooling capacity per hour. A common rough starting point is a general BTU-per-square-foot range for typical rooms, then adjusted upward or downward for specific conditions — this gives a reasonable estimate but isn't a substitute for a proper room-by-room load calculation on larger or more complex jobs.
What pushes the estimate up or down
- Ceiling height — taller ceilings mean more air volume to cool for the same floor area
- Sun exposure — a room with large west-facing windows generally needs more cooling capacity than a similarly-sized shaded room
- Occupancy — more people in a room adds body heat load, relevant for rooms that are regularly crowded
- Heat-generating equipment — a kitchen or a room with significant electronics adds load beyond the base estimate
Why oversizing causes its own problems
It's tempting to assume a bigger unit is simply 'safer,' but an oversized air conditioner cools the room to the thermostat setpoint quickly and then shuts off, without running long enough to properly dehumidify the air. This can leave a room feeling cold but clammy, and the frequent on/off cycling is also harder on the equipment over time than a properly-sized unit running longer, steadier cycles.