Boiler Size Calculator
Estimate the right boiler size in kW for your home from its floor area, insulation quality, and climate. Enter your house area and conditions to get a recommended boiler output with a safety buffer.
Formula
Boiler kW = Area × HeatLoss(W/m²) / 1000 × 1.25 (25% buffer)
120 m² home, average insulation, temperate climate
Inputs
- House Area: 120 m²
At 90 W/m², heat loss is 120 × 90 = 10,800 W. With a 25% buffer the recommended boiler is about 13.5 kW.
Frequently asked questions
How is boiler size estimated?
It multiplies your floor area by a heat-loss rate set by insulation and climate, then adds a 25% buffer for reliable performance.
Why does insulation matter so much?
Better insulation lowers heat loss per square metre, so a well-insulated home needs a smaller boiler than a poorly insulated one of the same size.
What is the BTU/hr figure?
It is the same output expressed in British Thermal Units per hour, common on boiler and heating equipment specs (1 kW ≈ 3,412 BTU/hr).
Is this enough to choose a boiler?
It gives a solid starting estimate. A heating engineer should confirm sizing using a full heat-loss survey and your hot-water demand.
Does this include hot water demand?
No. Combi boilers also need capacity for hot water; add allowance for the number of bathrooms and simultaneous use.