Heat Loss Calculator — Room-by-Room Heating Requirements
Calculate fabric and ventilation heat losses for any room to correctly size radiators and heating systems. Uses UK CIBSE design conditions (21°C inside, -1°C outside).
Room Dimensions
Longest dimension of the room
Shortest dimension of the room
Standard UK ceiling: 2.4m
External Exposure
Walls facing outside
Average window area on each external wall
Doors to outside
Construction U-Values
How We Calculate This
This calculator uses the standard CIBSE (Chartered Institution of Building Services Engineers) method for calculating room heat loss, which underpins heating system design across the UK.
Fabric heat loss formula
Fabric heat loss (W) = Σ(Area × U-value × ΔT)
Each building element (walls, windows, doors, roof, floor) contributes heat loss based on its area, U-value, and the temperature difference between inside and outside (ΔT = 22K for standard UK conditions: 21°C inside, -1°C outside).
Ventilation heat loss formula
Ventilation heat loss (W) = 0.33 × Volume × Air changes per hour × ΔT
The 0.33 factor comes from the volumetric heat capacity of air (1,200 J/m³K divided by 3,600 seconds). Air changes per hour (ACH) represents how many times the room's air volume is replaced each hour through draughts and ventilation.
Total heat loss
Total = Fabric heat loss + Ventilation heat loss
The total heat loss in Watts is the heating output required to maintain the design temperature. This is used to select the correct radiator size or heating system output for the room.
Frequently Asked Questions
Related Calculators
U-Value Calculator
Calculate the U-value of wall, floor and roof constructions.
Insulation Thickness Calculator
Determine the insulation thickness needed to meet UK Building Regulations.
Cavity Wall Insulation Calculator
Estimate cavity wall insulation quantities and potential savings.
Loft Insulation Calculator
Calculate loft insulation rolls needed for your loft area.
Boiler Size Calculator
Determine the right boiler kW output for your UK property.
Air Source Heat Pump Sizing Calculator
Size air source heat pumps from whole house heat loss with defrost derating.
Last updated: February 2026
All calculations are estimates. Verify with your supplier.