BTU Calculator
Find the right air conditioner size for a room — the BTU rating and tonnage, adjusted for sun and insulation.
Results are a planning estimate, not a Manual J load calculation. For whole-home systems or unusual rooms, have an HVAC professional size the equipment.
How to Size a Room for Cooling
Cooling capacity starts from the room's floor area and is adjusted for the things that add or remove heat.
Room dimensions
Measure the length, width, and ceiling height in feet. Tall ceilings hold more air to cool, so height matters as well as floor area.
Sun exposure
A room with large or south-facing windows gains heat through the day; a heavily shaded room needs less. Pick the option that best matches the space.
Insulation
Older homes with thin walls and drafty windows lose conditioned air quickly. A well-sealed, modern room holds temperature far better.
Occupants
Each person adds body heat. Count the people typically in the room — the calculator adds capacity beyond the first two.
Common Mistakes to Avoid
- Going bigger to be safe. An oversized AC short-cycles and leaves the room cold and humid. Match the capacity to the room.
- Ignoring sun and insulation. Two rooms of the same size can need very different units depending on exposure.
- Forgetting ceiling height. A 10 ft ceiling needs noticeably more cooling than a standard 8 ft one.
- Treating a kitchen like a bedroom. Appliances add heat — kitchens generally need extra capacity beyond this estimate.
Frequently Asked Questions
Size is based on room area, about 20 BTU per square foot. A 200 sq ft room needs roughly 4,000-5,000 BTU, then adjust up for sun, poor insulation, or extra occupants.
About 20 BTU per square foot for cooling, assuming an 8 ft ceiling. Tall ceilings, strong sun, and weak insulation push it higher.
BTU stands for British Thermal Unit, a measure of heat energy. For air conditioners it describes how much heat the unit removes per hour.
One ton equals 12,000 BTU per hour, so a 24,000 BTU system is a 2-ton unit. The calculator shows both figures.
Yes. An oversized unit short-cycles and never runs long enough to remove humidity, leaving the room cold and clammy and wearing the compressor faster.