Garden Air conditioner BTU calculator
An air conditioner that is too large is a worse outcome than one slightly too small. It cools the air fast, hits the thermostat, and shuts off before it has removed any meaningful humidity — leaving a room that is cold and clammy, with a compressor short-cycling itself toward an early failure.
How this is calculated
Base = room area × 20 BTU per sq ft. Then ×1.1 if very sunny or ×0.9 if shaded, +600 BTU per person beyond two, +4,000 BTU for a kitchen, scaled for ceilings above 8 ft.
Worth knowing: This sizes a single room for a window or portable unit. Whole-house systems need a proper Manual J load calculation that accounts for insulation, air sealing, window type and local climate — a rule of thumb is not adequate for a system that expensive. Bigger is not safer: oversized units short-cycle, leave humidity behind and wear out sooner.
Common questions
Why is an oversized air conditioner bad?
Cooling and dehumidifying are separate jobs, and dehumidifying takes time. An oversized unit satisfies the thermostat quickly and shuts off before much moisture has condensed on the coil, so the room feels cold and damp. The frequent starts also wear the compressor faster than steady running would.
What does "tons" mean?
A holdover from ice: one ton of cooling is 12,000 BTU per hour, historically the rate at which a ton of ice melts over a day. A 24,000 BTU unit is a 2-ton unit.
Does ceiling height matter?
Yes — you are cooling a volume, not a floor. The standard 20 BTU per square foot assumes an 8 foot ceiling, so a room with a 12 foot ceiling holds half again as much air and needs scaling up accordingly.
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