BTU Calculator

BTU stands for British Thermal Unit, the amount of energy needed to raise one pound of water by 1°F. In HVAC, the number that matters is BTU/hr: how fast a system moves heat.

Too small and the equipment runs continuously without reaching your target temperature. Too large and it short-cycles (blasts cold air, hits the setpoint, shuts off) and never runs long enough to pull humidity out of the air. The room feels 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. Poor insulation adds 20% to the base load; good insulation cuts it by 15%, a 41% swing between extremes on the same room.

The setting also stands in for air leakage (infiltration), since a drafty room and a poorly insulated one lose heat the same way. If the room is well insulated but noticeably drafty, choose the next setting down.

In older homes, topping up thin attic insulation is often the cheapest way to shrink both loads before you size any equipment.

Choose Sunny and count each large window individually. West-facing rooms with afternoon exposure can run 10°F or more warmer than a shaded room the same size. If you have cellular shades or reflective film, splitting between Normal and Sunny is reasonable.

A licensed contractor with a full Manual J load calculation will always be more precise: they measure the actual building, account for ductwork, and use local design temperatures.

This tool uses rule-of-thumb formulas appropriate for planning and cross-checking quotes. A half-ton difference is within normal tolerance. A full ton or more is worth pushing back on: oversizing is the most common and expensive mistake contractors make.

Yes. Heat pump sizing uses the same BTU/tonnage framework. The cooling result here is your target. One caveat for cold climates: heat pump heating capacity drops below 35–40°F, so verify the unit's rated capacity at your area's 99% winter design temperature still covers the heating load shown, or plan for supplemental strips.