BTU Calculator
Calculate the required BTUs to cool or heat a room efficiently based on square footage, ceiling height, insulation, sun exposure, and occupancy.
Complete Guide: BTU Calculator
Everything you need to know about using this tool effectively
Enter a value in any energy unit (BTU/h, watts, kilowatts, or tons of refrigeration) to convert to all other units. The calculator also estimates the BTU capacity needed to heat or cool a room based on square footage, ceiling height, insulation quality, sun exposure, and number of occupants. Useful for selecting air conditioners, heaters, and HVAC equipment.
One BTU per hour equals 0.293071 watts, 0.000293071 kilowatts, and 0.0000833 tons of refrigeration. Room sizing uses a base of 20 BTU per square foot, adjusted upward for poor insulation, high sun exposure, extra occupants, and higher ceilings. The ton of refrigeration unit equals 12,000 BTU/h, derived from the heat needed to melt one ton of ice in 24 hours.
Air conditioner sizing
Determine the correct AC unit size for a room to ensure efficient cooling without oversizing.
Heater selection
Calculate the BTU output needed to adequately heat a space based on its dimensions and insulation.
Unit conversion
Convert between BTU, watts, and kilowatts when comparing equipment specifications from different manufacturers.
Energy auditing
Estimate heating and cooling loads for buildings as part of an energy efficiency assessment.
Choose calculation type
Select unit conversion or room sizing based on your needs.
Enter your values
For conversion, enter a number in any unit. For room sizing, input the room dimensions and conditions.
Review the results
See all converted units or the recommended BTU capacity for your space.
Oversizing an AC unit wastes energy and causes short cycling that reduces dehumidification
Add 600 BTU for each person regularly in the room beyond the first two occupants
South-facing rooms with large windows need 10 to 20 percent more cooling capacity
For open floor plans, calculate the total connected area rather than individual rooms
What is a BTU?
BTU stands for British Thermal Unit. It is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, BTU per hour (BTU/h) measures heating or cooling capacity.
How many BTU do I need per square foot?
A general rule is 20 BTU per square foot for a standard room with 8-foot ceilings and average insulation. Adjust upward for poor insulation, high ceilings, or sun exposure, and downward for excellent insulation or shaded rooms.
What is a ton of refrigeration?
One ton of refrigeration equals 12,000 BTU/h. The term comes from the early days of air conditioning when cooling capacity was measured by the equivalent amount of ice needed to provide the same cooling in 24 hours.
Can I use this for heating and cooling?
Yes, the BTU calculator works for both heating and cooling equipment sizing. The room sizing tool includes adjustments relevant to both applications, such as insulation and window area.
Why does ceiling height matter?
Higher ceilings mean more air volume to condition. Rooms with 10-foot ceilings have 25 percent more air volume than standard 8-foot ceilings, requiring more BTU capacity to reach and maintain the target temperature.