Find the right AC/BTU capacity for a room based on area, sun and occupancy.
Air conditioner sizing starts from a well-established rule of thumb: about 20 BTU per hour for every square foot of floor, assuming a standard eight-foot ceiling. Taller ceilings scale it proportionally because you are cooling a volume, not a floor. Sun exposure then adjusts it by ten to twenty percent either way, since a south-facing room with large glazing gains solar heat all afternoon while a shaded room does not. Occupants add roughly 600 BTU/h each beyond the first two, covering both body heat and the moisture they exhale. Oversizing is a real fault, not a safety margin: an oversized unit cools the air fast, satisfies the thermostat and switches off before it has dehumidified anything, leaving a room that is cold and clammy while short-cycling wears out the compressor.
Cooling load from area, height, sun and occupancy
Base BTU/h = area x 20 x (ceiling height / 8); required = base x sun factor + 600 x (occupants - 2); tons = required / 12,000
Because dehumidification takes runtime. An oversized unit hits the set temperature quickly and shuts off before much moisture has condensed on the coil, so the room feels cold and damp — and the frequent starts shorten compressor life.
Yes, add roughly 4,000 BTU/h for a kitchen because appliances and cooking are substantial heat sources. Server rooms, conservatories and rooms with west-facing glass also need more than this rule of thumb gives.
No. For ducted or whole-house systems, a proper Manual J load calculation accounts for insulation, air changes, window specification and orientation. Use this to sanity-check a quote or size a room unit.