Heater BTU Sizing
Size a space heater by BTU for the room you want to heat.
About Heater BTU Sizing
Heaters are sized in BTUs using the room volume, a local insulation factor and the temperature rise desired. Watts convert to electric heater ratings. An undersized heater runs forever without warming the room, while an oversized one cycles and wastes energy, so matching BTU to volume is key.
Formula
Heater BTU Sizing
BTU/hr = volume x (insulation/10) x temp rise x 1.1
- insulation
- Lower number = better insulated home
Worked example
With the default values below, Heater BTU Sizing returns Room Volume of 3,600. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Room areaminimum 0 sq ft
- 400 sq ft
- Ceiling heightminimum 0 ft
- 9 ft
- Insulation factorminimum 1 BTU/vol
- 7 BTU/vol
- Desired temp riseminimum 0 °F
- 30 °F
Results
- Room Volume
- 3,600 cu ft
- Required BTU/hr
- 83,160 BTU
- Watts Equivalent
- 24,366 W
- kW Equivalent
- 24.37 kW
Step by step
- 1Values used
- 2Heater BTU Sizing
- 3Room Volume= 3,600 cu ft
- 4Required BTU/hr= 83,160 BTU
- 5Watts Equivalent= 24,366 W
- 6kW Equivalent= 24.37 kW
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Heater BTU Sizing calculated?
BTU/hr = volume x (insulation/10) x temp rise x 1.1 Heaters are sized in BTUs using the room volume, a local insulation factor and the temperature rise desired. Watts convert to electric heater ratings.
Why does Heater BTU Sizing matter?
An undersized heater runs forever without warming the room, while an oversized one cycles and wastes energy, so matching BTU to volume is key.
What values do I need to enter?
This calculator takes 4 inputs: Room area, Ceiling height, Insulation factor, Desired temp rise. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.