Boiler Size Calculator
Estimate the right kW output for a new boiler from your floor area, insulation level, boiler type and number of bathrooms — the same rule of thumb used by Energy Saving Trust and the major UK boiler manufacturers.
Recommended boiler size
24 kW
- Heating demand
- 6.5 kW
- Hot-water demand
- 24.0 kW
- Total demand
- 24.0 kW
- Insulation factor used
- 65 W/m²
- Floor area
- 100 m²
Heating demand = floor area × insulation factor (W/m²). For a combi boiler the rating is set by hot-water demand (24 kW base + 6 kW per extra bathroom), because the boiler heats water on demand. For system and heat-only boilers (separate hot-water cylinder) the rating is the heating demand plus about 3 kW for cylinder reheat. The result is rounded up to the next standard UK boiler size. This is a consumer estimate — for new builds or unusual properties a Gas Safe engineer should perform a full BS EN 12831 heat-loss calculation.
How to use this calculator
Enter the total floor area of your home in square metres (a typical UK 3-bed semi is around 90–110 m², a 4-bed detached is around 130–180 m²). Pick the insulation level that best describes the property: well insulated for a post-2000 build with double glazing and cavity-wall insulation, average for a typical mixed-era UK home, or poorly insulated for an older property with single glazing or no cavity. Choose the boiler type — combi if you want hot water on demand without a cylinder, system if you have a sealed hot-water cylinder, or heat-only (regular / conventional) if you have an open-vent cylinder in the loft. Finally enter the number of bathrooms (showers and ensuites count separately). The calculator returns the recommended boiler output in kW, rounded up to the nearest standard UK size.
How the calculation works
Heating demand is estimated as floor area × an insulation factor in watts per square metre: 50 W/m² for a well-insulated modern home, 65 W/m² for an average UK property, and 85 W/m² for an older, poorly insulated home. Divide by 1,000 to get kilowatts. For a combi boiler, the recommended rating is set by hot-water demand, not heating demand — combis heat water on demand, so the boiler must be large enough to deliver an acceptable shower flow rate. The DHW model is 24 kW base for a one-bathroom property plus 6 kW for each extra bathroom. For system and heat-only boilers the hot-water cylinder is reheated separately, so the rating is the heating demand plus about 3 kW for cylinder reheat. The result is rounded up to the next standard UK boiler size (12, 15, 18, 24, 25, 28, 30, 32, 35, 40, 42 or 50 kW) so the recommendation matches a model you can actually buy.
Worked example
Take a typical 100 m² three-bed semi-detached UK home with average insulation, one bathroom and a combi boiler. Heating demand is 100 × 65 / 1000 = 6.5 kW. Hot-water demand for one bathroom on a combi is 24 kW. The boiler rating is the larger of the two — 24 kW — so the recommended size is 24 kW (which matches a Worcester Bosch Greenstar 25i, Vaillant ecoTEC plus 824, Ideal Logic Combi 24 and most entry-level combi models). Add a second bathroom and the DHW demand rises to 30 kW, so the calculator recommends a 30 kW boiler. If you switched the same house to a system boiler with a cylinder, the rating would be 6.5 kW heating + 3 kW reheat = 9.5 kW, rounded up to 12 kW — system boilers are smaller because the cylinder buffers the hot-water peak.
Frequently asked questions
Why are combi boilers rated by hot-water demand instead of heating demand?
A combi boiler heats hot water on demand — there is no cylinder storing pre-heated water — so the boiler has to deliver the full bathroom flow rate the instant you turn the tap on. A typical UK shower needs about 10–12 litres per minute at 35 °C rise, which takes around 24 kW of instantaneous heat transfer. The heating demand of a small UK home is usually only 4–10 kW, so the DHW load is the binding constraint. System and heat-only boilers heat a cylinder slowly in the background, so the boiler itself only needs to cover heating plus a few kW of reheat — they are typically smaller for the same property.
What insulation factor (W/m²) should I use for my home?
Use 50 W/m² if the property was built after 2000 (or has been fully retrofitted), with double or triple glazing, cavity-wall insulation, 250–300 mm of loft insulation and a SAP / EPC rating of B or above. Use 65 W/m² for a typical UK property — most 1950s–1990s semi-detached and terraced homes with partial cavity insulation, double glazing and an EPC of C–D. Use 85 W/m² for older properties (pre-1980), single-glazed homes, solid-wall properties with no internal or external wall insulation, or anything below an EPC of E. If in doubt, ask a Gas Safe engineer for a heat-loss survey — they will use BS EN 12831 to calculate the exact figure room by room.
Are bigger boilers always better?
No — oversizing a boiler is one of the most common UK heating mistakes. A boiler that is too big short-cycles (fires up, heats the water, switches off, repeats) instead of running at a steady low output, which wastes gas, increases wear on the heat exchanger and pump, and reduces seasonal efficiency by 5–15%. Modern condensing boilers are most efficient when they modulate down to a low percentage of their rated output, which they cannot do if the rated output is already far above demand. Pick the smallest boiler that meets both heating demand and required hot-water flow — the calculator rounds up to the next standard size, which already provides a small safety margin.
Does the calculator account for radiator output or pipe sizing?
No — it estimates whole-home heating demand from floor area and insulation, which is what determines the boiler's rated kW. Radiator sizing is a separate step: each room's radiator output should match its individual heat loss, not the whole-house total. Pipe sizing depends on flow rate, pump pressure and the layout of the run. For a like-for-like boiler swap on an existing system the existing radiators and pipework are usually fine, but if you are extending the system or moving to underfloor heating you will need a room-by-room heat-loss calculation and a hydraulic design.
How many bathrooms count — do showers, ensuites and cloakrooms each add 6 kW?
Count each room that has a shower or bath as a bathroom — ensuites, family bathrooms and any second-floor bathrooms all count. A downstairs cloakroom with only a basin and WC does not, because the basin tap load is negligible compared to a shower. The 6 kW per extra bathroom rule assumes the simultaneous-use risk grows with each bathroom: in a busy family home two showers may run at once, and the boiler must size for the worst case. If your household never runs more than one shower at a time you can size at the lower end of the recommendation, but most installers prefer to keep the headroom.
Should I use this calculator for a new build or unusual property?
No — for new builds, listed buildings, very large homes (over 250 m²), homes with significant single-storey or vaulted areas, or any property with non-standard fabric, get a Gas Safe engineer to perform a full BS EN 12831 heat-loss calculation. The W/m² rule of thumb assumes a typical UK two-storey home with 2.4 m ceiling height and roughly average glazing ratios — it can be wrong by 20–30% for atypical buildings. This calculator is a fast first estimate suitable for a like-for-like swap or a planning conversation with an installer; it is not a substitute for a survey.