BTU Calculator (AC & Heater Sizing)
Estimate the BTU rating needed to cool or heat a room based on size, insulation, climate, and sun exposure.
What this calculates
BTU (British Thermal Units per hour) measures heating and cooling capacity. Sizing matters — an undersized AC runs constantly without cooling well, while an oversized unit short-cycles, fails to dehumidify, and wastes energy. This calculator gives a sensible BTU target based on room size, climate, insulation, and sun exposure.
Formula & how it works
Base BTU = area_sq_ft × 20 (rough rule for moderate climate, average insulation, 8 ft ceiling). Adjust by climate (hot/sunny +20 %), ceiling height (>9 ft, +10 % per extra foot), occupancy (+600 BTU per regular occupant beyond 2), kitchens (+4,000), and heavy sun exposure (+10 %). Round to the nearest commercial size: 5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 14,000 / 18,000 / 24,000 BTU. 12,000 BTU = 1 ton.
Worked example
300 sq ft living room, 9 ft ceilings, moderate climate, 2 occupants, normal sun. Base = 300 × 20 = 6,000 BTU. Ceiling adjustment +10 % = 6,600. Round to nearest size: 8,000 BTU. If west-facing with heavy afternoon sun, bump to 10,000 BTU instead.
Frequently asked questions
What's the difference between BTU and tons?
12,000 BTU/hour = 1 ton of refrigeration capacity. A 'three-ton AC' is 36,000 BTU. Used mostly for central AC systems; window units are sold in raw BTUs.
Why is oversized AC bad?
AC cools and dehumidifies. An oversized unit hits the temperature target fast and shuts off before removing enough moisture — your room ends up cold and clammy. Right-sizing is more comfortable and more efficient.
Do mini-splits use BTUs?
Yes — same rating system. Mini-splits run at variable speed so they handle minor sizing mistakes better than fixed-speed window units, but you still want to be in the right zone.
How much does climate matter?
A lot. Phoenix needs 30–40 % more BTUs than Seattle for the same square footage because the cooling load is dictated by the temperature gradient and humidity. Adjust the climate factor honestly.