BTU Calculator
EnergyStarAC SizingHeating/Cooling
EnergyStarAC Sizing
BTU Calculator
Enter your room details to calculate the recommended BTU capacity
Frequently Asked Questions
Common questions about this calculator
Measure your room's square footage (length × width) and use the EnergyStar baseline: 20 BTU per square foot. Then apply adjustments for ceiling height (add 10% per foot above 8ft), occupants (add 600 BTU per person beyond 2), sun exposure (add 10% for sunny rooms, subtract 10% for shaded), and insulation quality.
BTU (British Thermal Unit) measures cooling/heating capacity - the energy needed to change the temperature of one pound of water by 1°F. For AC, higher BTU means more cooling power. Correct sizing prevents short-cycling (oversized) or insufficient cooling (undersized), both of which waste energy and reduce equipment life.
Oversized AC short-cycles, failing to dehumidify properly and wasting energy on frequent starts. Undersized AC runs constantly, can't maintain temperature on hot days, and wears out faster. Proper sizing (within 10% of calculated BTU) ensures comfort, efficiency, and equipment longevity.
Standard calculations assume 8-foot ceilings. Higher ceilings mean more air volume to cool. Add approximately 10% to BTU requirements for each foot above 8 feet. A room with 10-foot ceilings needs about 25% more BTU than the same floor area with 8-foot ceilings.
Kitchens generate significant heat from cooking appliances. Add 4,000 BTU to the baseline calculation for kitchens. If you have a commercial-style range or multiple appliances running simultaneously, consider adding more. Good ventilation can offset some of this heat load.
One ton of cooling equals 12,000 BTU/hour. Divide your BTU requirement by 12,000 to get tonnage. For example, 36,000 BTU = 3 tons. Central AC systems are typically rated in tons (1.5, 2, 2.5, 3, etc.), while window units use BTU ratings directly.