BTU Calculator
Right-size an air conditioner for a room — BTU and tonnage estimate
Frequently Asked Questions
Right-sizing an air conditioner for a room means matching cooling capacity (measured in BTU, British Thermal Units per hour) to the room's actual cooling load — and both an undersized and an oversized unit cause real problems, not just an oversized one wasting money.
Why bigger isn't automatically better
An undersized air conditioner struggles to bring a room to the target temperature, especially during peak heat, and runs continuously trying to keep up — higher electricity cost and reduced comfort than a correctly sized unit. An oversized air conditioner cools the room quickly but cycles on and off frequently rather than running for longer, steadier stretches — this short-cycling reduces the unit's ability to properly dehumidify the room (dehumidification happens more effectively during longer, continuous run cycles), which can leave a room feeling cool but clammy, and short-cycling also causes more wear on the compressor over time than steady operation. Room area, ceiling height, sun exposure, insulation quality, and number of occupants all factor into the actual cooling load — a rough BTU-per-square-foot rule of thumb is a reasonable starting estimate, but a room with poor insulation or heavy sun exposure needs meaningfully more cooling capacity than the same square footage with good insulation and shade.
A worked example
Two rooms of identical floor area — one with large west-facing windows getting direct afternoon sun, one with small, shaded windows — have genuinely different real-world cooling loads despite being the same size, and sizing both using the same flat BTU-per-square-foot figure without adjusting for sun exposure would undersize the sunnier room and potentially oversize the shaded one.
How this connects to your other utility calculations
Once an appropriately-sized AC unit is chosen, its actual running cost depends on wattage, hours of daily use, and your electricity tariff — the Appliance Electricity Calculator and Electricity Bill Calculator handle that ongoing cost calculation, which is separate from the sizing question this calculator answers.
Common mistakes
Sizing an AC purely by room square footage without adjusting for factors like sun exposure, ceiling height, insulation quality, and occupancy is the most common mistake — a flat per-square-foot rule of thumb is a reasonable starting point, but real rooms vary enough in these factors that a straightforward area-only calculation can meaningfully under- or over-size the actual recommendation.