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
| Element | U (W/m²K) |
|---|---|
| Walls – no extra insulation | 2.2 |
| Walls – mediocre insulation | 1.0 |
| Walls – very well insulated | 0.6 |
| Windows | 2.5 |
| External doors (2 m² each) | 2.4 |
| Ground floor | 1.0 |
| Ceiling / roof | 0.7 |
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
| Field | What to enter |
|---|---|
| Room length | Units: m, ft, cm, in, yd |
| Room width | Units: m, ft, cm, in, yd |
| Room height | Units: m, ft, cm, in, yd |
| Level | Choose one: Ground floor (floor loses heat), Intermediate floor, Top floor (ceiling loses heat), Single-storey (floor and ceiling) |
| Wall insulation | Choose one: No extra insulation (U = 2.2), Mediocre insulation (U = 1.0), Very well insulated (U = 0.6), Custom U-value |
| Custom U-value | In W/m²K |
| External walls | Choose one: 1, 2, 3, 4 |
| Windows | A number |
| Area of each window | Units: m², ft², yd² |
| External doors | A number |
| Inside temperature | Units: °C, °F |
| Outside design temperature | Units: °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
Formula
Heat loss coefficient = Σ U × A (walls, windows, doors, floor, ceiling)Power = coefficient × (inside − outside temperature)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.
Frequently asked questions
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).