βοΈ AC Size Calculator
The right BTU for your room, no more guessing.
Rule of thumb: 20 BTU per square foot, adjusted for your conditions.
How AC Sizing Works
Air conditioners are sized in BTU (British Thermal Units), the cooling power needed per hour. The baseline rule is 20 BTU per square foot of living space: a 180-square-foot bedroom needs about 3,600 BTU at baseline. This calculator starts there and adjusts for the factors that actually change the load: ceiling height above the standard 8 feet, sunny versus shaded rooms, hot climates, extra occupants beyond two (each adds about 600 BTU), and kitchens, which add a flat 4,000 BTU for cooking heat.
Getting the size right matters more than getting a big number. An oversized unit cools the room so fast it shuts off before dehumidifying, leaving you cold and clammy while it short-cycles itself to an early death. An undersized unit runs constantly on the hottest days and never quite catches up. The sweet spot is a unit that runs in long, steady cycles.
BTU Quick Reference
For standard 8-foot ceilings in a moderate climate: 100 to 150 sq ft needs 5,000 BTU, 150 to 250 needs 6,000, 250 to 350 needs 8,000, 350 to 450 needs 10,000, 450 to 550 needs 12,000, and 550 to 700 needs 14,000. Open floor plans should be sized on the combined space, and rooms with large west-facing windows go one size up. Portable units need about 20 percent more BTU than window units for the same room because their efficiency is lower.
Efficiency Matters Too
Check the CEER rating (Combined Energy Efficiency Ratio): higher means cheaper to run. An 8,000 BTU unit at CEER 11 costs noticeably less per summer than the same size at CEER 9.7. ENERGY STAR models run about 10 percent more efficient than the federal minimum. Size right first, then buy the most efficient model in that size.
When to Size Up or Down
The base math assumes average insulation and sun exposure. Size up about 10 percent for kitchens (appliances add heat), rooms with heavy afternoon sun, or ceilings over 8 feet. Size down for heavily shaded rooms, excellent insulation, or mild climates where oversizing causes short cycling: the unit cools the air so fast it shuts off before dehumidifying, leaving you cold and clammy. For whole-house systems, contractors use a Manual J load calculation that accounts for windows, orientation, and ductwork: worth requesting for any central install. And remember BTUs describe cooling power, not efficiency: compare SEER or CEER ratings when choosing between two right-sized units.
Frequently Asked Questions
How many BTU per square foot do I need?
The standard rule is 20 BTU per square foot for 8-foot ceilings in moderate climates. Adjust up for high ceilings, sunny rooms, hot climates, extra people, and kitchens.
Is a bigger AC better?
No. Oversized units short-cycle: they cool the air quickly but shut off before removing humidity, leaving the room cold and clammy while wearing out the compressor faster.
What size AC for a 12x15 bedroom?
180 square feet x 20 BTU = 3,600 BTU baseline. A standard 5,000 BTU window unit covers it; go 6,000 if it is sunny or in a hot climate.
Window vs portable AC: which size?
Portable units are less efficient, so add about 20 percent to the BTU need versus a window unit for the same room. Window units also dehumidify better.
What is CEER on an air conditioner?
Combined Energy Efficiency Ratio: BTU of cooling per watt of electricity. Higher is cheaper to run. ENERGY STAR models are about 10 percent more efficient than the federal minimum.