The BTU Calculator estimates the cooling and heating capacity, in BTU per hour, needed for a room or home.
About 4,219 BTU/hr of cooling
| Cooling needed (air conditioner) | 4,219 BTU/hr |
| In tons of cooling | 0.35 |
| Heating needed (estimate) | 4,219 BTU/hr |
| In kilowatts | 1.24 kW |
A British Thermal Unit (BTU) is the energy needed to raise one pound of water by 1°F, about 1,055 joules. Air conditioners and heaters are rated in BTU per hour; 12,000 BTU/hr equals one "ton" of cooling.
The calculator starts from about 25 BTU/hr per square foot for an 8-foot ceiling, scales for ceiling height, adjusts for insulation and sun, adds 600 BTU for each regular occupant beyond two and 4,000 BTU for kitchens.
Worked example (defaults): a 150 sq ft bedroom with 9 ft ceilings, average insulation and sun, two people: about 4,200 BTU/hr, so a 5,000 BTU window unit is enough.
An undersized unit runs constantly without cooling enough. An oversized one cools quickly but switches off before removing humidity, leaving a clammy room and wasting energy. For whole-house systems, a professional Manual J load calculation is best.
How many BTU per square foot? Roughly 20–30 for cooling.
Does climate matter for cooling? Hot, humid climates may need 10–20% more.
What about heating? Colder climates need more heating capacity; the result scales by climate.