How it works
This calculator follows the ENERGY STAR sizing guide for room air conditioners: it looks up a base capacity from the room's floor area, then adjusts for shade or strong sun, extra occupants and kitchen heat.
Bigger is not better. An oversized unit cools the air quickly and shuts off before it removes humidity, leaving the room cold and clammy. An undersized unit runs constantly and may not keep up on the hottest days.
Ceilings taller than 8 feet add volume, so the base capacity is scaled in proportion. For whole houses, poorly insulated rooms or sunrooms with lots of glass, a professional Manual J load calculation is more reliable.
Formula
BTU = chart value for floor area × (ceiling height ÷ 8, if over 8 ft) × sun adjustment (±10%) + 600 per person over 2 + 4,000 for a kitchen.
Questions people ask
How many BTU do I need per square foot?
Roughly 20 BTU per square foot for typical rooms, but the ENERGY STAR chart steps it: 5,000 BTU for up to 150 sq ft, 8,000 for 300 to 350 sq ft and 12,000 for 450 to 550 sq ft.
What size AC for a 12 × 15 bedroom?
That is 180 sq ft, which falls in the 150 to 250 sq ft band, so about 6,000 BTU with normal sun and two occupants.
How many BTU is a ton of air conditioning?
12,000 BTU per hour. A 2-ton central system delivers 24,000 BTU per hour.
Should I size up for a sunny room?
Add about 10% for a room with strong afternoon sun or large west-facing windows, and reduce by 10% for a heavily shaded room.