Air conditioner BTU calculator
Bigger is not better with air conditioning — an oversized unit cools fast, shuts off, and leaves the room clammy. This aims for the right size.
Why oversizing backfires
An air conditioner does two jobs: it drops the temperature and it removes moisture. Removing moisture takes runtime. A unit that is too powerful hits the target temperature quickly and switches off before it has dehumidified, leaving a room that is cold and damp — the clammy feeling people blame on the unit being weak, when it is actually too strong. Short cycling also wears the compressor and wastes energy.
The base figure of 20 BTU per square foot is the standard starting point. Rooms that take direct afternoon sun need roughly ten percent more; kitchens need a substantial addition because cooking dumps both heat and moisture into the space; and each person beyond two adds a few hundred BTU of body heat.
What this cannot see is your ceiling height, insulation quality, window glazing, or how many walls face outside. A room with nine-foot ceilings or single-glazed windows needs meaningfully more than the same floor area in a well-insulated space. Treat the result as a starting point and step up one size if several of those apply.
Questions
What happens if I buy too big?
The room gets cold but stays humid, because the unit switches off before it has removed moisture. It also short-cycles, which wastes energy and shortens the compressor's life.
Does ceiling height matter?
Yes, and this assumes a standard eight feet. Add roughly ten to twenty percent for high or vaulted ceilings, since you are cooling considerably more air.
What about a whole apartment?
Size each room separately for window or portable units, since they cool the space they sit in. Whole-home systems need a proper load calculation that accounts for insulation and ductwork.