BTU Calculator
Rule-of-thumb cooling load from floor area and adjustment factors. Not a substitute for a Manual J calculation.
Room
Approximate cooling: BTU/h
Nominal tons (÷12,000):
Why this is a rough estimate
This calculator starts from a common rule-of-thumb baseline (roughly 25 BTU/h per square foot) and adjusts it for ceiling height, insulation quality, and sun exposure — factors that meaningfully affect actual cooling load. A true HVAC sizing calculation ("Manual J") also accounts for window count and orientation, local climate data, occupancy, and appliance heat load, which is why professional installers don't rely on a simple per-square-foot rule alone.
Frequently asked questions
What happens if my AC unit is undersized or oversized? An undersized unit struggles to cool the space on hot days and runs constantly; an oversized unit cools too quickly without adequately removing humidity, leading to a clammy feel and less efficient operation from frequent on/off cycling.
Why does ceiling height matter? Cooling load scales with the actual volume of air being cooled, not just floor area — a room with an 8 ft ceiling and the same footprint as a room with a 12 ft ceiling requires noticeably more cooling capacity for the latter.
Should I get a professional assessment before buying an AC unit? For anything beyond a small single room, yes — a proper Manual J load calculation from an HVAC professional accounts for far more variables than this quick estimate and helps avoid an incorrectly sized (and less efficient) system.