Heating BTU Calculator

About Heating BTU Calculator

Heating a room takes more capacity than cooling the same room, and sizing a heater from a cooling figure leaves it short. This calculator switches to heating rates, starting at 35 BTU/h per square foot for a standard insulated room and rising to 45 for a garage and 55 for a conservatory. The gap between heating and cooling exists because the temperature difference the equipment works against is usually larger in winter. Holding 21 degrees inside when it is 0 outside is a 21 degree difference, whereas holding 21 inside when it is 30 outside is only 9. Two inputs that matter for cooling are deliberately ignored in heating mode. Sunlight helps rather than hurts in winter, so there is no penalty for a sunny room, and occupants add heat that reduces rather than increases the requirement, so the occupancy adjustment is dropped instead of being applied backwards. Insulation is the dominant variable in heating, more so than in cooling, which is why the space type selector changes the result so much. For cooling, switch the mode toggle or see BTU Calculator.

How to use Heating BTU Calculator

  1. Enter room area in m² or ft²
  2. Set ceiling height, sunlight exposure, and number of occupants
  3. See recommended BTU, kW, and AC unit type

Frequently Asked Questions

How many BTU do I need to heat a room?
For a standard insulated room, around 35 BTU/h per square foot, so a 200 square foot room needs roughly 7,000 BTU/h. Poorly insulated spaces need considerably more: a garage is 45 and a conservatory 55 per square foot.
Why is the heating figure higher than the cooling figure?
Because the temperature difference is usually larger. Holding 21 degrees against a 0 degree outside is a 21 degree gap, while holding 21 against 30 outside is only 9. The bigger the gap, the more capacity is needed to maintain it.
Why does sunlight not increase the heating requirement?
In winter solar gain helps rather than hinders, so a sunny room is easier to heat, not harder. The sunlight adjustment only applies to cooling, where the sun works against you.
How do I convert BTU to kilowatts for an electric heater?
Divide by 3,412. A 7,000 BTU/h requirement is about 2 kW, which matches a typical large panel heater or a small heat pump. The calculator shows the kilowatt figure alongside the BTU result.
Does insulation matter more for heating than cooling?
Yes, generally. The temperature differences are larger in winter, so heat escapes faster through a poorly insulated surface than it enters in summer. That is why the gap between a standard room and a garage is wider in heating mode.

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