Boiler Size Calculator — What kW Boiler Do I Need?

A 3-bedroom semi-detached house with 1 bathroom typically needs a 25-29 kW combi boiler, with 27 kW the figure this calculator returns for that property. With 2 bathrooms, go for 29-36 kW. These are rule-of-thumb bands, not a specification. Always have a Gas Safe engineer assess your property.

Combi boilers are best for smaller homes (1–3 bedrooms, 1 bathroom) as they heat water on demand and save space. System boilers are better for larger homes with 2+ bathrooms, as they use a hot water cylinder to supply multiple outlets simultaneously at mains pressure. Use our Hot Water Cylinder Calculator to find the right cylinder size if choosing a system boiler.

A 30 kW combi boiler can typically heat a home with 10–15 radiators, depending on radiator size and property insulation. The central heating output of a combi is usually around 80% of its total output — so a 30 kW combi provides roughly 24 kW for heating. Use our Radiator Sizing Calculator to check the BTU output needed for each room, and our Central Heating Volume Calculator to work out total system water volume.

kW (kilowatts) is the measure of a boiler's heat output. A higher kW rating means the boiler can produce more heat. Combi boilers have two ratings — one for central heating and a higher one for domestic hot water. For example, a "30kW combi" typically delivers 30kW for hot water and 24kW for heating.

Yes, by UK law (Gas Safety (Installation and Use) Regulations 1998), all gas boiler installations must be carried out by a Gas Safe registered engineer. This applies to new installations, replacements, and relocations. The engineer will also notify Building Control on your behalf.

For a typical 3-bed semi-detached house with 1 bathroom and 10 radiators, this calculator's default figures (1.5 kW per radiator, standard property-type adjustment) recommend a 27 kW combi boiler (this calculator's own 25–29 kW output range), or a 19 kW system or regular boiler (17–22 kW range) if you choose stored hot water instead. The combi figure is driven by the 1 bathroom's hot water demand: the estimated 16 kW radiator heating load sits well within the roughly 22 kW central-heating output of a 27 kW combi, so no upsizing is needed. The system or regular figure adds that 16 kW heating load to a 3 kW hot water allowance for the 1 bathroom. These are indicative rule-of-thumb figures, not a specification: final sizing must be confirmed by a Gas Safe registered engineer from a room-by-room heat-loss calculation to BS EN 12831-1. See the worked example in the How This Calculator Works section below for the full maths, and check the flow rate a boiler this size can actually deliver with our Combi Flow Rate Calculator.

Both are sized on central heating load rather than hot water demand, using the same rule of thumb as this calculator: about 1.5 kW per radiator plus a hot water allowance per bathroom. The difference is in the hot water and pipework setup, not the sizing method. A system boiler has an integral pump and expansion vessel and connects straight to a hot water cylinder without needing a tank in the loft, so it suits most modern retrofits. A regular boiler, also called a conventional or heat-only boiler, needs two loft tanks, a cold water storage cistern and a small feed-and-expansion cistern, plus a separate hot water cylinder, and tends to suit older systems being replaced like-for-like or properties with low mains water pressure. If you're choosing a cylinder to pair with either type, see our Unvented Cylinder Calculator for mains-pressure sizing to Part G3.

Yes, the same sizing method (heating load from radiators plus a hot water allowance) applies to oil-fired boilers, since output is measured in kW regardless of fuel. The regulatory picture is different though: Gas Safe registration is a legal requirement for gas boiler work, but there is no equivalent legal requirement for oil. Installation is usually carried out by an OFTEC-registered engineer, who can self-certify that the work meets Building Regulations; OFTEC is one of six competent person schemes listed in Schedule 3 to the Building Regulations 2010 for oil-fired appliance work, and an installer registered with none of them must instead notify local Building Control directly. Once you know the boiler's kW output, use our Oil Tank Sizing Calculator to work out storage tank size and delivery frequency. Always confirm final sizing, tank and flue requirements with a competent heating engineer.

Multiply the kW figure by 3,412 to get BTU per hour (1 kW = 3,412 BTU/hr), the same watts-to-BTU factor this site's Radiator Sizing Calculator uses for room heat loss. For example, the 27 kW combi in the worked example above converts to 27 × 3,412 = 92,124 BTU/hr. UK boiler spec sheets are almost always quoted in kW, so you'll mostly only need the BTU figure when cross-referencing older documentation, US-made appliances, or radiator BTU ratings.