Boiler Size Calculator — kW Output for Your Home
Find the right boiler output in kilowatts by combining your home's space heating demand — based on floor area and insulation — with the domestic hot water load from your bathrooms.
m²
Insulation level
rooms
radiators
Nearest standard combi/system boiler size at or above calculated need
- 1
Space heating load
100 × 60 ÷ 1000 = 6 - 2
Hot water demand
1 × 3 = 3 - 3
Heat demand with 10% margin
(6 + 3) × 1.10 = 9.9The recommended boiler is the nearest standard size at or above this figure.
How does this calculator work?
Multiply heated floor area (m²) by your insulation heat-loss rate (25–80 W/m²) and divide by 1000 for space heating kW. Add 3 kW per bathroom for hot water. Apply a 10% margin, then pick the nearest standard boiler size at or above that total. Typical UK homes need 18–32 kW.
Formula
How this is calculated
A boiler must cover two demands simultaneously: space heating (warming all rooms via radiators or underfloor heating) and domestic hot water. The space heating load is estimated by multiplying the total heated floor area by a heat loss rate expressed in watts per square metre. That rate depends on insulation quality — poorly insulated older homes lose around 80 W/m², while a modern well-insulated home loses 40 W/m², and an excellent Passivhaus-style build may be as low as 25 W/m². These figures are 2020s estimates for typical European climates and are editable in the insulation selector.
Each bathroom or wet room adds roughly 3 kW of instantaneous hot water demand for a combi boiler (which heats water on demand rather than using a storage cylinder). A 10% design margin is added on top of the calculated total to ensure the boiler is not running continuously at its limit, which extends its lifespan. The nearest standard commercial boiler size at or above that figure is then suggested.
As a cross-check, the number of radiators is shown: a typical radiator outputs around 1.5 kW, so the total radiator capacity should broadly match the space heating figure. These are planning estimates; a Gas Safe (UK) or equivalent certified engineer should perform a full heat loss calculation before installation.
Frequently asked questions
A typical UK 3-bedroom semi-detached home of about 85–100 m² with average insulation and one bathroom usually needs a 24–28 kW combi boiler. Enter your actual floor area and bathroom count for a personalised estimate.
Combi (combination) boilers heat water on demand and are space-efficient, suitable for most homes up to 3 bedrooms with 1–2 bathrooms. System boilers work with a hot water cylinder and suit larger homes or homes with higher simultaneous hot water demand.
Insulation directly determines how fast heat escapes your home. A poorly insulated property loses heat twice as fast as a well-insulated one of the same size, so the boiler must work twice as hard — meaning you need a larger, more expensive unit that also costs more to run.
Also known as
TG we-Calculate Editorial Team. (2026). Boiler Size Calculator — kW Output for Your Home [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/boiler-size-calculator
TG we-Calculate Editorial Team. "Boiler Size Calculator — kW Output for Your Home." TG we-Calculate. 2026. https://we-calculate.com/calculator/boiler-size-calculator.
TG we-Calculate Editorial Team, "Boiler Size Calculator — kW Output for Your Home," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/boiler-size-calculator
@misc{wecalculate_boiler_size_calculator, title = {Boiler Size Calculator — kW Output for Your Home}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/boiler-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?