Heat Loss Calculator

Your details

Internal length of the room.
m
Internal width of the room.
m
Floor-to-ceiling height.
m
Walls that face the outdoors. Internal walls do not lose significant heat.
Ground-floor rooms lose heat through the floor; top-floor rooms lose heat through the ceiling; middle-floor rooms lose neither.
Combined area of all windows in the external walls.
m²
Combined area of any external doors (not internal doors). A standard door is about 1.9 m².
m²
Target temperature inside the room, typically 18-21 °C (64-70 °F).
°C
The coldest outdoor temperature you design for - often the 1st percentile winter design temperature for your location.
°C
How many times the room air volume is replaced by outside air per hour due to draughts and gaps.
Extra buffer added on top of the calculated heat loss to account for real-world variation when sizing a heater.
Heat loss (watts)Good insulation
972W

Total heat loss including safety factor

Heat loss (kW)0.97kW
Heat loss (BTU/hr)3,316BTU/hr
Heat loss per floor area48.6W/m²
Wall heat loss242W
Window heat loss218W
Door heat loss0W
Ceiling heat loss0W
Floor heat loss114W
Infiltration (ventilation) loss309W
Recommended heater size0.97kW
Temperature difference26Δ°C
Walls242
Windows218
Doors0
Ceiling0
Floor114
Infiltration309

This room needs 0.97 kW (3,316 BTU/hr) of heating capacity.

  • The room loses 0.97 kW (3316 BTU/hr) at a 26.0 °C temperature difference. Your heater should output at least this much.
  • Infiltration accounts for about 35% of total losses. Draught-proofing windows, doors, and floorboards is one of the cheapest improvements available.
  • Windows account for roughly 25% of losses. Upgrading from single to double glazing typically cuts window heat loss by more than 50%.
  • A design temperature difference above 25 °C is severe. Make sure your outdoor design temperature reflects a realistic worst-case for your climate zone.
  • The result includes your chosen safety factor. For final system sizing, also factor in hot water and recovery loads.

Next stepUse the recommended heater size (kW) to choose a radiator, electric panel heater, or heat pump output rating from the manufacturer data sheet.

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