Estimate building heat loss from fabric performance, air changes and the indoor-to-outdoor temperature difference.
Planning estimate only. A room-by-room assessment should account for each element, thermal bridges, ventilation system, intermittent heating and local design conditions.
How to use the Heat Loss Calculator
Enter the exposed building-element area, its area-weighted average U-value, heated volume, estimated air changes per hour, target indoor temperature, outdoor design temperature and any design margin. The tool estimates the heating output required to offset heat loss at those conditions.
How heat loss is calculated
| Component | Calculation |
|---|---|
| Temperature difference | Indoor temperature − outdoor temperature |
| Fabric heat loss | Area × average U-value × temperature difference |
| Ventilation heat loss | 0.33 × volume × air changes per hour × temperature difference |
| Design heat loss | (Fabric + ventilation loss) × (1 + margin) |
Understanding U-values
A U-value describes heat transfer through a building element in watts per square metre for each degree of temperature difference. Lower U-values indicate better insulation. For accuracy, calculate walls, roof, floor, windows and doors separately, then sum each area × U-value result.
Ventilation and infiltration loss
Air changes replace warm indoor air with colder outdoor air. The 0.33 factor is a commonly used approximation for the heat capacity of air in W·h/m³K. Actual infiltration depends on construction, airtightness, exposure, wind and ventilation operation.
Choosing temperatures and margin
The temperature difference should reflect the indoor setpoint and a suitable winter design temperature for the location. Excessive margins can oversize equipment, while an unrealistically warm outdoor input can underestimate the load. Use professional design data for equipment selection.
Building guidance
UK government guidance defines U-value as a measure of a building element’s ability to conduct heat and addresses limiting heat loss from building fabric and services. Review the current Approved Document L guidance where Building Regulations apply.
Related calculators
Compare heating upgrade cost with the Boiler Cost Calculator UK. Explore electricity generation with the Solar Panel Calculator UK, examine fabric performance using the U-Value Calculator, or estimate cooling capacity with the Air Conditioning Size Calculator. Browse all tools on the United Kingdom Calculator homepage.
Frequently asked questions
What does the heat-loss result mean?
It is the estimated continuous heating output needed to balance fabric and ventilation losses at the entered design temperatures.
Should I calculate each room separately?
Yes for radiator, emitter or detailed system sizing. Whole-building averages are useful only for an early estimate.
What U-value should I enter?
Use an area-weighted average based on the construction of the included walls, roof, floor, windows and doors.
What are air changes per hour?
They estimate how many times the heated air volume is replaced in one hour through ventilation and infiltration.
Can I size a boiler or heat pump from this result?
Use it as an initial estimate only. Final equipment sizing should use a competent room-by-room design assessment.