BTU Calculator

Enter the room size, insulation level, climate and window area to get a heating and a cooling load in BTU per hour.

BTU calculator

Room length (ft)

Room width (ft)

Ceiling height (ft)

Window area (sq ft)

Insulation level

Climate

Heating load: —

Cooling load: —

Heating load in kilowatts: —

Cooling load in kilowatts: —

Rule of thumb comparison: —

—

This is a rule of thumb estimate for sizing, based on room volume, insulation quality, climate and window area.

Common questions about the free online btu calculator.

Multiply the room volume in cubic feet by the insulation factor and the climate factor. A 2400 cubic foot room averages about 10800 BTU per hour.

Heating load is volume times the insulation factor times the climate factor, and the cooling load adds a solar gain allowance for the window area.

A common estimate is 45 BTU per hour for every square foot of floor area, which the tool shows beside the detailed result for comparison.

Yes. Poor insulation can raise the calculated load by nearly three times compared with a high performance modern building fabric.

Windows add solar gain in summer, so more glazing increases the cooling load. The calculation adds 30 BTU per hour for each square foot of glass.

Divide the BTU per hour figure by 3412. One kilowatt equals about 3412 BTU per hour, so 12000 BTU per hour is about 3.5 kilowatts.

Match the unit to the cooling load. A 12000 BTU per hour air conditioner is one ton and suits a well insulated room of about 500 square feet.

No. The heating and cooling loads are calculated in your browser, and nothing you type is transmitted to any server.

Yes. It is free, needs no account, and there is no limit on the number of rooms you can size.

Last updated: October 10, 2026