Heat Loss Calculator
Determine the heat loss of a room or whole house through walls, windows, floor, ceiling and ventilation — using the EN 12831 simplified method. Result: total losses in watts, specific W/m² and recommended heating system power.
Why calculate heat loss
Heat loss is the basis of any heating calculation. Without it, you cannot correctly size an electric boiler, radiators, underfloor heating or a heat pump. An error of 20–30% gives either a cold house in winter or constant excess electricity and fuel consumption.
The calculator implements the simplified envelope method: losses through each surface are Q = k × A × ΔT, where k is the surface heat transfer coefficient (W/m²·K), A is the area (m²), ΔT is the difference between indoor and design outdoor temperature. Ventilation losses are then added, plus a 10% margin.
What to enter
- Room floor area and ceiling height
- Climate region — design winter temperature (values for European countries are offered)
- Wall type and their area (or facade area)
- Window area and type (double glazing, timber frames, etc.)
- What is above and below the room (heated floor, attic, ground)
- Air change rate (0.5–0.7 by default for living rooms)
Regulatory framework
The calculation follows BS EN 12831-1 «Energy performance of buildings — Method for calculation of the design heat load», ISO 52016 for seasonal energy demand, and national building regulations (Germany: DIN V 4108 / EnEV; Poland: PN-EN 12831; UK: CIBSE Guide A and Part L of Building Regulations).
Design outdoor temperatures are taken per national climate standards (e.g., -15…-25 °C for Central Europe, -10…-15 °C for the UK and Ireland, -20…-30 °C for Scandinavia, Eastern Europe and Central Asia).
Normalised indoor temperatures: living rooms 20–22 °C, bathroom 24–25 °C, kitchen 19–21 °C. For electric heating, a design margin of 10–15% is recommended.
Heat transfer coefficients table
| Construction | k, W/m²·K |
|---|---|
| Brick wall 640 mm, uninsulated | 1.1 |
| Brick wall 380 mm, uninsulated | 1.6 |
| Concrete panel 300 mm | 1.8 |
| Aerated concrete 300 mm, D400 | 0.55 |
| Aerated concrete 375 mm, D400 | 0.45 |
| Timber 150 mm | 0.8 |
| Timber frame with 150 mm insulation | 0.35 |
| SIP panel 174 mm | 0.32 |
| Wall with 100 mm insulation + facade | 0.35 |
| Wall with 150 mm insulation + facade | 0.27 |
| Window: timber frame, 2 panes | 2.5 |
| Window: PVC double glazing | 1.8 |
| Window: PVC triple glazing | 1.2 |
| Window: triple glazing argon + low-e | 0.9 |
| Floor over heated storey | 1.3 |
| Floor over cold attic | 0.6 |
| Floor on ground | 0.45 |
| Floor over cold basement | 0.6 |
Calculation formulas
Envelope heat loss: Q_env = Σ (k_i × A_i × ΔT).
Ventilation loss: Q_vent = 0.34 × V × ΔT × n, where V is the room volume (m³), n is the air change rate (h⁻¹), 0.34 is the heat capacity of air (Wh/m³·K).
Total: Q_total = (Q_env + Q_vent) × 1.1 (10% margin).
Specific indicator: q = Q_total / S_floor, W/m². Benchmarks: well-insulated modern house — 30–50 W/m²; average insulation — 50–80 W/m²; uninsulated — 90–150 W/m².
How to use the calculator
- Enter the floor area and ceiling height.
- Select the region or enter the design winter temperature directly.
- Specify construction types: walls, windows, floors.
- Adjust the air change rate if necessary.
- Get heat losses by element, the total value and a heating power recommendation.
Common mistakes
- Calculating by area without climate zone — the result for Scandinavia and Southern Europe differs by 1.5–2 times.
- Ignoring ventilation — it is 20–40% of all losses in a sealed modern house.
- Not accounting for thermal bridges at corners and junctions — add a 10–15% margin.
- Counting windows by opening area without coefficient — glass loses 5–8 times more than a wall of the same area.
- Choosing the boiler strictly by heat loss without margin — on the coldest days a reserve is needed.
Heat loss FAQ
What is the heat transfer coefficient k?
It is the amount of heat in watts passing through 1 m² of construction at a temperature difference of 1 K. The lower the k, the better the insulation: for a wall with 150 mm insulation k ≈ 0.27 W/m²·K, for an uninsulated 380 mm brick wall — 1.6 W/m²·K, 6 times more.
What heat loss is considered normal for a house?
For a modern energy-efficient house — 30–50 W/m² of heated area at the design temperature. Average houses of the 2000s — 60–90 W/m². Old uninsulated houses — 100–150 W/m². A 100 m² house with 70 W/m² losses requires a 7 kW boiler + margin = 8–9 kW.
How to reduce heat loss fastest?
By effectiveness: attic/roof insulation (15–25% of losses), window replacement (up to 20%), wall insulation (20–30%), floor insulation (up to 10%), sealing and ventilation with heat recovery (up to 30% on ventilation). Payback of most works is 3–7 years.
Why is the design temperature important?
Heating power is selected for the coldest period, not the average temperature. For Central Europe it is -15…-25 °C, for Scandinavia -25…-30 °C. Underestimating gives cold rooms at peak frost.
Do I need heat loss calculation to choose underfloor heating?
Yes: underfloor heating can work as the main heating only if its power (usually up to 150–180 W/m²) covers the room's specific heat loss. Otherwise a combination with radiators is needed — the calculator shows whether the power is sufficient.