BTU Calculator

BTU stands for British Thermal Unit — the amount of energy needed to raise one pound of water by 1°F. In HVAC, BTU/hr measures how much heat an air conditioner removes (or a furnace produces) each hour.

Getting the BTU count right matters on both ends. Undersized equipment runs continuously and can't reach your target temperature. Oversized equipment short-cycles — it blasts cool air, shuts off, and never runs long enough to dehumidify the room, leaving it clammy even when it's "cool."

Tonnage is shorthand for capacity: 1 ton of cooling equals 12,000 BTU/hr. The term comes from the era before mechanical refrigeration, when cooling was done with ice — one ton of ice melting over 24 hours removed exactly 12,000 BTU/hr.

A 2-ton AC removes 24,000 BTU/hr of heat. Contractors typically quote system size in tons; equipment spec sheets list both. This calculator gives you both so you can match whichever unit your quote uses.

This is a room-level calculator, best suited for sizing a window AC, portable unit, or single-zone mini-split for one space. For a whole-home central air system — which also accounts for duct losses, building orientation, and total envelope area — use the HVAC Load Calculator instead.

Insulation controls the rate of heat transfer through your walls, ceiling, and floor. A well-insulated room in Good condition retains conditioned air far longer — needing only 85% of the BTUs a comparable room with Poor insulation requires. Adding or upgrading insulation is often the highest-ROI HVAC improvement: it permanently reduces the system size you need and the energy it consumes year-round.

Choose Sunny and count each large window individually. West-facing rooms with afternoon exposure can run 10–20°F warmer than a shaded room the same size. The Sunny multiplier (+15%) combined with the per-window BTU addition captures this load accurately. If you have cellular shades or reflective film, you might split the difference between Normal and Sunny.

A licensed contractor performing a full Manual J load calculation will always be the most accurate source — they measure the actual building, account for ductwork, and use local design temperatures.

This tool uses industry rule-of-thumb formulas, appropriate for planning and cross-checking quotes. A half-ton difference is within normal tolerance. A full ton or more suggests asking your contractor for their methodology — oversizing is a common and costly mistake in the industry.

Yes — heat pump sizing uses the same BTU/tonnage framework as conventional AC. The cooling result here is your target. One important note for cold climates: heat pump heating capacity drops as outdoor temperatures fall below 35–40°F. The heating BTU shown here reflects peak design load, so if you're in a cold climate, verify that the heat pump's rated capacity at your area's 99% winter design temperature still meets that number — or plan for supplemental heat strips.