Room Heat Loss Calculator
The Room Heat Loss Calculator works out how many watts of heating each room in your home actually needs. It accounts for wall, floor, ceiling, window and ventilation losses using standard CIBSE / Building Regulations Part L heat-loss methodology. The result tells you how to size radiators and feeds into whole-house heat pump or boiler sizing.
Room Heat Loss Calculator
Calculate heat loss for individual rooms to size radiators and heating systems.
See 20 real-world examples
Each link below runs this calculator with real UK city and property inputs and publishes the exact computed savings, payback time, install cost and CO2 figures.
- How Much Heat Does Your Liverpool Room Lose?
- Room Heat Loss Calculator for a 20m² Room in Leicester
- Room Heat Loss Calculator for a Belfast Home
- Room Heat Loss Calculator for a Cardiff Home
- Room Heat Loss Calculator for a Coventry Home
- Room Heat Loss Calculator for a Plymouth Home
- Room Heat Loss Calculator for a Typical London Home
- Room Heat Loss Calculator for a Typical Manchester Home
- Room Heat Loss Calculator for a Typical Nottingham Home
- Room Heat Loss Calculator for Sheffield Homes
- Room Heat Loss Calculator for Your Birmingham Home
- Room Heat Loss Calculator for Your Brighton Home
- Room Heat Loss Calculator for Your Bristol Home
- Room Heat Loss Calculator for Your Edinburgh Home
- Room Heat Loss Calculator for Your Home
- Room Heat Loss Calculator for Your Leeds Home
- Room Heat Loss Calculator for Your Newcastle Home
- Room Heat Loss Calculator for Your Southampton Home
- Room Heat Loss Calculator for Your Swansea Home
- Room Heat Loss Calculator Glasgow Example
How this calculator works
Total heat loss = fabric loss + ventilation loss. Fabric loss = Σ (U-value × area × ΔT) for each surface (walls, windows, floor, ceiling). Ventilation loss = 0.33 × volume × air-change-rate × ΔT. We use ΔT = 24K (21°C internal, -3°C external — standard UK design condition). Air change rates assume 1.5 ACH for living rooms (industry standard for naturally-ventilated UK homes). Radiator output of 1,500W per standard 600×1200mm panel is the BS EN 442 reference.
Sources & references
Frequently asked questions
Why is my room loss higher than my radiator output?
You probably need more radiator capacity than you currently have. Most UK homes are heated with gas boilers running at 70-80°C flow, so radiators are sized at ΔT 50K. If you're moving to a heat pump (45-50°C flow), the same radiator only delivers about half the heat — which is why heat-pump homes often need radiator upgrades.
Where do the U-values come from?
Building Regulations Part L 1A and Part L 1B publish the U-values used in modelling. Solid uninsulated brick: 2.1 W/m²K. Cavity insulated: 0.5 W/m²K. Modern double glazing: 1.6 W/m²K. Insulated loft: 0.16 W/m²K. The calculator uses these.
Why 1.5 air changes per hour?
1.5 ACH is the CIBSE design air change rate for a typical naturally-ventilated UK living room. Bedrooms often use 1.0 ACH, kitchens and bathrooms 2.0 ACH due to extract fans. Highly airtight modern homes with mechanical ventilation can be 0.5 ACH or lower.
What is ΔT 50?
Radiator manufacturers quote outputs at a Delta T of 50K — the average temperature difference between the radiator surface and the room. Real-world heat-pump radiators run at ΔT 25-30K, which delivers about 35-45% of the stated output. Always size with the right ΔT.
Is this enough for an MCS heat pump install?
No — MCS-certified heat pump installers must do a room-by-room MCS-compliant heat-loss survey using software like HeatPunk, MCS Manual, or LoopCAD. This calculator gives a useful sense-check but isn't accepted as the formal calculation for grant or install purposes.
Tool: Room Heat Loss Calculator. Last reviewed: . Figures based on Ofgem price cap, gov.uk grant guidance, and Energy Saving Trust advice. Verify scheme eligibility with your installer before commissioning work.