How to Use This Tool
Enter the room and the conditions. The output is the load in BTU per hour, the unit size to buy, and how much oversizing the next size up would introduce.
Why oversizing is the more common mistake
Air conditioning does two jobs: it lowers temperature and it removes moisture. Only the first one is fast. Moisture condenses on the evaporator coil, and that requires the coil to stay cold for a sustained period — typically ten minutes or more of continuous running.
An oversized unit satisfies the thermostat in three minutes and stops. Repeat that six times an hour and the temperature is correct while the humidity is barely touched. The room feels cold and damp, and the usual response is to lower the setpoint, which makes it colder and no drier.
base 500 sq ft x 20 BTU = 10,000 BTU ceiling 8 ft, no adjustment = 10,000 BTU sun average, +0% = 10,000 BTU people 2, none over two = 10,000 BTU kitchen no = 10,000 BTU nearest unit size = 10,000 BTU (0% over) next size up = 12,000 BTU (20% over)
The adjustments that actually matter
Sun exposure is the largest single factor after area: a west-facing room with large windows can run 20 per cent above the base figure, while a shaded ground-floor room runs 10 per cent below. Insulation and glazing quality move it a similar amount.
Occupancy is smaller than people expect. Each additional person beyond two adds about 600 BTU, which is roughly a 6 per cent adjustment on a 10,000 BTU room. A kitchen is the exception, at a flat 4,000 BTU, because the appliances are the load rather than the people.
Ceiling height is applied on volume rather than floor area, so a 10 ft ceiling is a 25 per cent increase over the standard 8 ft. This is the adjustment most calculators omit entirely.
Rounding to real unit sizes
Units come in fixed sizes: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000 and 24,000 BTU are the common ones. A calculated load of 10,400 BTU therefore has two candidates, and the smaller one running slightly longer is usually the better choice.
The rule of thumb among installers is to stay within about 15 per cent above the calculated load, and never exceed 25 per cent. Beyond that, short cycling starts to dominate.
Underrating is a real fault too
A unit that is genuinely too small runs continuously and never reaches the setpoint on the hottest days. The comfort penalty is real, but it is a gradual one: the room is a few degrees warm. Oversizing produces a room that is simultaneously too cold and too humid, which is harder to diagnose and cannot be fixed by adjusting anything.
What this does not do
This is a room-level rule-of-thumb estimate suitable for a window or portable unit. Sizing a whole central system needs a Manual J load calculation that accounts for duct losses, infiltration, orientation and local design temperatures. For that, get a professional calculation rather than any web page.
