Heat Loss Calculator
Calculate how much heat your building loses — and how much heating power you need to stay warm.
Want to see savings from better insulation? Insulation Savings Calculator →
Project
Temperatures
Design outdoor temperature is used for peak heat load (heating system sizing). HDD is used for annual energy demand. Typical HDD: Oslo ~4300, London ~2700, Paris ~2600, Munich ~3400.
Building Elements
Add each building element: walls, roof, floor, windows, doors. Enter its area and U-value.
Results
Peak load covers conduction only — add 10–20% for ventilation and infiltration. Annual energy assumes continuous heating during heating season.
Typical U-values for reference
Common questions about heat loss & heating system sizing
What size boiler do I need for my house?+
Boiler size is determined by your home's peak heat loss in kilowatts. A typical 3-bedroom semi-detached house with cavity walls and double glazing loses around 5–8 kW on the coldest day. Add 20% for ventilation losses and domestic hot water, giving a recommended boiler size of 7–12 kW. Older uninsulated homes can need 12–20 kW.
What size heat pump do I need?+
Heat pumps are sized to peak heat loss, just like boilers — but they work best when slightly undersized and run continuously rather than cycling. A well-insulated 100 m² home typically needs a 5–8 kW heat pump. Older uninsulated homes may need 12–16 kW. Improving insulation before installing a heat pump significantly reduces the required size and running cost.
How do I calculate heat loss through a wall?+
Heat loss (W) = U-value × Area (m²) × ΔT (°C). Example: a 20 m² wall with U-value 1.5 W/m²K and ΔT of 31°C (inside 21°C, outside −10°C) loses 20 × 1.5 × 31 = 930 W. Add all building elements together for the total peak load.
How much does it cost to heat a house per year?+
A typical 100 m² home in central Europe with moderate insulation needs around 10,000–15,000 kWh/year for space heating. At €0.28/kWh with a gas boiler (90% efficiency): roughly €3,100–€4,700/year. The same home with a heat pump at COP 3.5 would cost €800–€1,200/year in electricity — a saving of €2,000–€3,500 annually.
What is a U-value?+
A U-value measures how much heat passes through a building element per m² per degree of temperature difference (W/m²K). Lower = better insulation. Single glazing: ~5.7. Double glazing: 1.4–2.8. Well-insulated wall: 0.18. Passivhaus wall: 0.10. The reference table on this page lists typical values for common construction types.
How to calculate building heat loss
The formula
Heat loss (W) = U-value × Area × ΔT
Where U-value is in W/m²K, area in m², and ΔT is the temperature difference between inside and outside in Kelvin (same as °C difference).
Add up the heat loss through every building element — walls, roof, floor, windows, doors — to get the total peak heat load.
Peak heat load vs. annual energy demand
Peak heat load (watts) tells you the minimum size of your boiler or heat pump — it must cover the worst-case winter day. Annual heat demand (kWh) tells you what you'll spend on heating over a year, which is where heating degree days come in.
What are heating degree days?
Heating degree days (HDD) count how many degrees below a base temperature (usually 15.5°C or 18°C) the average daily temperature falls. A year with HDD 3000 means the heating season is 3000 degree-days long. Annual energy demand = heat loss per degree × HDD × 24 hours.
What about ventilation losses?
This calculator covers conduction losses only. In a real building, ventilation and infiltration (draughts) add another 10–30% on top. For a full Manual J / EN 12831 calculation, use a specialist tool or consult a building services engineer.