# Heat Loss Calculator

URL: https://speedycalculator.com/calculators/construction/heat-loss/
Last updated: 2026-10-25

> The heat loss calculator estimates how much heat a room loses through its external walls, windows, doors, floor and ceiling, using U-values for the insulation level, and the heater power needed for the design temperatures.

## Inputs

- Room length (m, ft, cm, in, yd)
- Room width (m, ft, cm, in, yd)
- Room height (m, ft, cm, in, yd)
- Level
- Wall insulation
- Custom U-value (W/m²K)
- External walls
- Windows
- Area of each window (m², ft², yd²)
- External doors
- Inside temperature (°C, °F)
- Outside design temperature (°C, °F)

## Results

- External wall area (net)
- Heat loss coefficient
- Heat loss / heater power
- Energy over 24 h at this ΔT

## Formula

```
Heat loss coefficient = Σ U × A (walls, windows, doors, floor, ceiling)
Power = coefficient × (inside − outside temperature)
```

## Explanation

Every surface between a heated room and the outdoors leaks heat at a rate proportional to its area, its U-value and the temperature difference. Uninsulated solid walls (U ≈ 2.2 W/m²K) lose almost four times as much as well-insulated walls (U ≈ 0.6). Windows and doors are usually the weakest parts of the envelope.

### How to use the Heat Loss Calculator

- **Room length**
- **Room width**
- **Room height**
- **Level**
- **Wall insulation**
- **Custom U-value**
- **External walls**
- **Windows**
- **Area of each window**
- **External doors**
- **Inside temperature**
- **Outside design temperature**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the room dimensions, its level in the building (which decides whether the floor and ceiling lose heat), the insulation, the number of external walls, windows and doors, and the inside and coldest outside temperatures.

### U-values used

ElementU (W/m²K)Walls – no extra insulation2.2Walls – mediocre insulation1.0Walls – very well insulated0.6Windows2.5External doors (2 m² each)2.4Ground floor1.0Ceiling / roof0.7

### Tips for an accurate estimate

- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer's coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.

### Common mistakes to avoid

- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.

### Inputs explained

FieldWhat to enterRoom lengthUnits: m, ft, cm, in, ydRoom widthUnits: m, ft, cm, in, ydRoom heightUnits: m, ft, cm, in, ydLevelChoose one: Ground floor (floor loses heat), Intermediate floor, Top floor (ceiling loses heat), Single-storey (floor and ceiling)Wall insulationChoose one: No extra insulation (U = 2.2), Mediocre insulation (U = 1.0), Very well insulated (U = 0.6), Custom U-valueCustom U-valueIn W/m²KExternal wallsChoose one: 1, 2, 3, 4WindowsA numberArea of each windowUnits: m², ft², yd²External doorsA numberInside temperatureUnits: °C, °FOutside design temperatureUnits: °C, °F

### What each result means

- **Heat loss / heater power** = [Heat loss coefficient]×([Inside temperature] - [Outside design temperature])
- **Energy over 24 h at this ΔT** = [Heat loss ÷ heater power]×24

## Worked example

Inputs — Room length: 5 m; Room width: 4 m; Room height: 2.5 m; Level: Single-storey (floor and ceiling); Wall insulation: Mediocre insulation (U = 1.0); Custom U-value: 0.35 W/m²K; External walls: 2; Windows: 2; Area of each window: 1.5 m²; External doors: 0; Inside temperature: 21 °C; Outside design temperature: -5 °C.

Results — External wall area (net) = 19.5 m²; Heat loss coefficient = 61 W/K; **Heat loss / heater power = 1,586 W**; Energy over 24 h at this ΔT = 38.1 kWh.

## Key terms

- **U-value**: Thermal transmittance, W/(m²·K); the inverse of the total R-value.

## FAQ

**What is a U-value?**
The heat flow through 1 m² of a building element for each degree of temperature difference, in W/m²K. Lower is better.

**How many watts per square metre to heat a room?**
Roughly 50–100 W/m² of floor in an average home at design conditions; well-insulated new homes may need under 40.

**What outside temperature should I use?**
The local design temperature – roughly the coldest temperature exceeded 99% of the year (often −3 to −10 °C in the UK and central Europe).

**What formula does the Heat Loss Calculator use?**
Heat loss coefficient = Σ U × A (walls, windows, doors, floor, ceiling).


## Sources

- [CIBSE Guide A – Environmental design](https://www.cibse.org/)