Boiler size estimator

Quote too small a boiler and the customer's shower runs cold the moment someone fills the kettle. Quote too big and they're paying for capacity they'll never use. This calculator gives a starting kW range from bedrooms, bathrooms and radiator count — the same three numbers most quick-sizing guides work from — so you've got a figure to open the conversation with before you survey the property properly.

Why bedrooms and bathrooms drive the range

Bedroom count is a rough proxy for house size, and therefore total heat demand. Bathroom count matters separately, because it drives hot water demand — specifically, how many taps or showers could realistically run at once. A combi has to heat water on demand, so a house with two or three bathrooms needs more headroom than one with a single family bathroom, even at the same bedroom count.

Where radiator count comes in

Bedrooms and bathrooms alone can undersell a house with an open-plan extension, a converted loft, or more radiators than typical for its size. Once the radiator count runs meaningfully ahead of what's expected for the bedroom count, the estimate nudges the range up to cover the extra heating load.

When to stop trusting the range

Past a certain point — generally three or more bathrooms, or once the range pushes toward the top of what combis are commonly specified at — a system boiler with a hot water cylinder usually makes more sense than a combi. A cylinder stores hot water in advance, so simultaneous demand from multiple bathrooms doesn't leave anyone with a trickle. The calculator flags this, but it's a prompt to check the property, not a final answer.

Still just a starting point

This doesn't replace a proper survey: pipework, existing radiator sizes, incoming water pressure and flow rate, and the property's actual heat loss all affect the real spec. Use the range to scope the job and set expectations before you visit — then confirm the final output on site.