Boiler Size Calculator

Selecting the correct boiler size, measured in kilowatts (kW), is crucial for efficiently heating your home and providing hot water. An undersized boiler will struggle to meet your needs, resulting in lukewarm radiators and showers. Conversely, an oversized boiler will use more fuel than necessary, leading to higher energy bills. This is a concept known as "short-cycling", where the boiler heats up too quickly and shuts off before distributing heat properly.

The ideal size depends on your home's heat loss, which is influenced by factors like insulation, window quality, and the number of radiators. Our Calculator Site tool simplifies this by focusing on the key drivers: the number of radiators for heating demand and the number of  bathrooms for hot water demand. For more detailed heat loss calculations, you might use a BTU calculator.

While our calculator provides a tailored estimate, here are some general guidelines for boiler sizing in the UK.

These ranges primarily apply to combi boilers. System and regular boilers for the same properties can often be smaller (e.g., 15-24 kW) because they don't need to produce hot water instantaneously. You can explore running costs with our Gas Cost Calculator.