Heat Loss Calculator
Heat Loss Results
Walls Heat Loss-
Windows Heat Loss-
Roof Heat Loss-
Floor Heat Loss-
Ventilation Loss (estimated)-
Total Heat Loss-
Boiler/Heat Pump Size Needed-
U-Value Reference Table
| Element | Uninsulated | Part L 2013 | Part L 2021 | Passivhaus |
|---|---|---|---|---|
| External Wall | 2.1 | 0.30 | 0.18 | 0.10-0.15 |
| Roof | 2.3 | 0.20 | 0.11 | 0.10 |
| Floor | 0.7 | 0.25 | 0.13 | 0.10-0.15 |
| Windows | 4.8 | 1.6 | 1.2 | 0.8 |
| Doors | 3.0 | 1.8 | 1.0 | 0.8 |
U-values in W/m²K. Lower values = better insulation. Part L 2021 values are for new builds in England.
Typical Heat Loss Distribution
Walls
30-35%
Roof
20-25%
Windows/Doors
15-20%
Floor
10-15%
Ventilation
15-25%
Draughts
5-15%
How to Use This Calculator
1
Enter wall details
Input your external wall area in m² and select the wall type. This is usually the largest source of heat loss.
2
Enter window and roof details
Input window area and glazing type, plus roof area and insulation level.
3
Enter floor details
Input ground floor area and floor type. Suspended timber floors lose more heat than solid concrete.
4
Set temperatures
Enter your desired internal temperature (typically 21°C) and the external design temperature for your area.
5
Review heat loss
The calculator shows heat loss through each element, total loss in watts, and recommended boiler or heat pump size.
Frequently Asked Questions
What is a U-value?
A U-value (thermal transmittance) measures how effectively a building element conducts heat. It is expressed in W/m²K (watts per square metre per degree Kelvin). Lower U-values indicate better insulation. For example, a wall with U=0.3 loses half the heat of one with U=0.6 for the same area and temperature difference.
How is heat loss calculated?
Heat loss through a building element = U-value x Area x Temperature Difference. Total heat loss is the sum of losses through all elements (walls, windows, roof, floor) plus ventilation losses. The result in watts tells you how much heating power is needed to maintain the internal temperature.
What external temperature should I use?
UK design external temperatures typically range from -1°C in mild areas (South West coast) to -4°C in cold areas (Scottish Highlands). The commonly used design temperature for England is -3°C to -1°C. Your heating system should be sized to cope with this worst-case scenario.
How big a boiler do I need?
As a rule of thumb: well-insulated 3-bed house needs 15-24 kW, average 3-bed needs 24-30 kW, poorly insulated 4-bed needs 30-40 kW. The heat loss calculation gives a more accurate figure. For heat pumps, aim for a system that can deliver the calculated heat loss at the design temperature.
What is the biggest source of heat loss?
In most UK homes, walls account for 30-35% of total heat loss, followed by the roof (20-25%), windows and doors (15-20%), ventilation (15-25%), and the floor (10-15%). Addressing wall insulation therefore typically has the biggest impact on energy bills.
How much can insulation save?
Cavity wall insulation typically saves £200-£400 per year. Loft insulation (0 to 270mm) saves £250-£400. Upgrading from single to double glazing saves £100-£200. Floor insulation saves £40-£70. These figures are based on a typical semi-detached house with gas heating at 2025/26 energy prices.
Official Sources & References
Data verified against official UK government sources. Last checked April 2026.