How to Use the Boiler Size Calculator
UK mode (kW)
- Choose the boiler type you're considering, or "Not sure" to get a recommendation.
- Enter the number of bedrooms, bathrooms and radiators.
- Enter the floor area in m² if you know it (it's on your EPC). This gives a more accurate heating estimate than radiator count.
- Choose the property age and insulation level.
- Optionally enter your mains flow rate in L/min from a bucket test, so the calculator can check whether a combi will work.
USA mode (BTU/h)
- Enter the heated floor area in square feet.
- Choose your climate zone and insulation level.
- Choose heating only or combi (heat + hot water), and the number of showers that may run at once.
- Enter the boiler's AFUE (efficiency) to see the input rating you should shop for.
What Size Boiler Do I Need? Quick Answer
Typical sizes for UK homes with average insulation:
| Home | Bathrooms | Radiators | Combi boiler | System / regular boiler |
|---|---|---|---|---|
| 1–2 bed flat | 1 | Up to 8 | 24–25 kW | 12–15 kW |
| 2–3 bed house | 1 | 8–12 | 28–30 kW | 15–18 kW |
| 3–4 bed house | 1–2 | 12–15 | 32–35 kW | 18–24 kW |
| 4+ bed house | 2+ | 15–20 | 35–40 kW (if mains allows) | 24–30 kW |
| Large 5+ bed | 3+ | 20+ | Not recommended | 30–40 kW |
Combi boilers look "bigger" than system boilers for the same house because their kW figure is set by instant hot water, not heating. A 30 kW combi might only need 8–10 kW for the radiators, but needs all 30 kW to heat mains water fast enough for a good shower.
How Boiler Size Is Calculated
A boiler has two jobs, and the right size is whichever job is bigger.
1. Heating load (heat loss)
Your home constantly loses heat through walls, windows, roof, floor and draughts. On the coldest design day, the boiler must replace that heat as fast as it escapes. Engineers calculate this room by room using BS EN 12831 (UK) or ACCA Manual J (USA). A good estimate uses floor area multiplied by a heat loss rate for your type of home:
| UK property type | Typical heat loss |
|---|---|
| Victorian or Edwardian, solid walls | 120–150 W/m² |
| 1930s semi, cavity walls | 90–110 W/m² |
| 1970s–1990s house | 70–85 W/m² |
| Post-2000 or well-upgraded | 50–65 W/m² |
| New build (2020s) | 45–55 W/m² |
Example: a 90 m² 1930s semi at 100 W/m² needs about 9 kW of heat on a cold day. That's far less than most people expect, and why oversized boilers are so common.
If you don't know your floor area, radiator count is a rougher guide. The calculator allows about 1.5 kW per radiator, which tends to overestimate in homes with good insulation.
2. Hot water demand
- Combi boilers heat water on demand as it flows through. The kW needed is: kW = flow (L/min) × temperature rise (°C) × 0.0698. Heating mains water from about 10°C to 45°C (a 35°C rise) at 10 L/min needs 24.4 kW; at 12 L/min, 29.3 kW; at 14 L/min, 34.2 kW.
- System and regular boilers heat a stored cylinder of hot water. They only need 2–3 kW extra on top of the heating load, because the cylinder does the heavy lifting at the tap.
Combi Boiler Sizing: kW vs Flow Rate
| Combi output | Hot water flow (35°C rise) | Typically suits |
|---|---|---|
| 24–25 kW | about 10 L/min | Flat or small house, 1 shower |
| 28–30 kW | about 12 L/min | 2–3 bed house, 1 bathroom |
| 32–35 kW | about 14 L/min | 3–4 bed, bathroom plus en-suite (not used at once) |
| 38–40 kW | about 16 L/min | Larger home, but only if the mains can supply it |
The mains supply is the limit. A 40 kW combi rated at 16 L/min is wasted if your incoming main only delivers 13 L/min. Do a bucket test at the kitchen tap with the Pipe Flow Rate Calculator before choosing. If the cold feed is 15mm, the Pipe Size Calculator will show whether upgrading to 22mm would help.
Combi vs System vs Regular Boiler
Combi boiler
- Heats water on demand; no tank or cylinder.
- Best for flats and homes with one bathroom and good mains pressure (around 1.5 bar or more working pressure, 12+ L/min).
- Struggles when two showers run at once: the flow is shared.
System boiler
- Heats a sealed unvented cylinder; no loft tank.
- Best for homes with two or more bathrooms and decent mains pressure.
- Boiler output is usually 15–24 kW, plus a 150–250 litre cylinder.
Regular (heat-only / conventional) boiler
- Uses a cold water tank in the loft and a vented cylinder.
- Best for low mains pressure areas, large homes, or when replacing a like-for-like system without major pipework changes.
If you have two bathrooms and a family that showers at the same time, a system boiler with a cylinder usually beats a big combi.
Why a Bigger Boiler Isn't Better
Oversizing is the most common boiler sizing mistake.
- Short cycling. An oversized boiler hits temperature, switches off, cools, and fires again, many times an hour. This wears components and wastes gas.
- Lower efficiency. Condensing boilers are most efficient at low return temperatures and long, steady burns.
- Modulation limits. A boiler can only turn down to a minimum output, often 3–6 kW. If your home needs 4 kW on a mild day and the boiler's minimum is 6 kW, it will cycle constantly. Look for a wide modulation ratio (1:7 or better).
- Higher upfront cost for capacity you'll never use.
Undersizing is rarer but means rooms that never reach temperature on cold days, and a combi that can't run a decent shower. The aim is a boiler that covers your peak demand with a small margin.
USA: Sizing a Boiler or Furnace in BTU
In the USA, boilers and furnaces are rated in BTU per hour. The correct method is an ACCA Manual J load calculation, which most HVAC contractors can run. For a planning estimate, use BTU per square foot by climate zone:
| IECC climate zone | Example areas | Typical BTU per sq ft |
|---|---|---|
| 1–2 | South Florida, Gulf Coast, Phoenix | 25 |
| 3 | Atlanta, Dallas, Southern California | 35 |
| 4 | Washington DC, Kansas City, Seattle | 40 |
| 5 | Chicago, Denver, Boston | 45 |
| 6 | Minneapolis, Vermont, Montana | 50 |
| 7 | Northern Minnesota, high mountains | 55 |
Example: 1,800 sq ft in zone 5 at average insulation needs about 1,800 × 45 = 81,000 BTU/h of output. With a 95% AFUE condensing boiler, the input rating is about 81,000 ÷ 0.95 = 85,000 BTU/h.
Three US rating terms to know:
- Input: the gas the boiler burns.
- Output (heating capacity): input × efficiency.
- Net AHRI rating: output minus an allowance for piping losses. Match your heat loss to the net rating.
For a combi boiler in the USA, hot water decides the size: BTU/h = GPM × 500 × temperature rise (°F). One 2.5 GPM shower with a 70°F rise needs 87,500 BTU/h; two showers at once (about 4 GPM) need 140,000 BTU/h. That's why US combis are often rated 120,000–199,000 BTU/h input.
Thinking About a Heat Pump?
Heat pumps are sized from the same heat loss calculation, but more carefully, because they run at lower water temperatures and don't cope well with oversizing. In England and Wales, the Boiler Upgrade Scheme offers £7,500 towards an air or ground source heat pump, and £9,000 for eligible off-gas-grid homes from 21 July 2026 to 31 March 2027. Installers must be MCS certified and carry out a room-by-room heat loss survey. Radiators often need upsizing: the Radiator BTU Calculator shows how much.
Frequently Asked Questions
What size boiler do I need for a 3-bedroom house?
Usually a 28–30 kW combi for one bathroom, or a 15–18 kW system boiler with a cylinder if you have two bathrooms. Older solid-wall homes may need more heating capacity.
Is a 24 kW combi boiler big enough?
For a flat or small house with one shower and up to about 10 radiators, yes. It delivers about 10 L/min of hot water, which feels weak with a rainfall showerhead or a bath.
What's the difference between a 30 kW and 35 kW combi?
Mainly hot water flow: about 12 L/min versus 14 L/min. The heating difference rarely matters, because most homes need well under 15 kW for radiators.
How many kW boiler do I need per radiator?
A rough rule is 1.5 kW per radiator, but floor area and insulation give a better answer. A well-insulated home may need under 1 kW per radiator.
Can a boiler be too big?
Yes. An oversized boiler short-cycles, runs less efficiently, and wears out faster. Choose one with a low minimum output and a wide modulation range.
What size boiler do I need for a 2,000 sq ft house in the USA?
Roughly 70,000–110,000 BTU/h output depending on climate and insulation. A Manual J calculation from a contractor gives the exact figure.
Should I get a combi or system boiler for two bathrooms?
A system boiler with an unvented cylinder is usually better, because two showers can run at full flow at the same time. A large combi only works if your mains flow is 16+ L/min and showers rarely overlap.
How do I check my mains flow rate for a combi?
Open the kitchen cold tap fully, time how long a 10-litre bucket takes to fill, and divide 600 by the seconds. Compare the result with the combi's rated hot water flow.
Get quotes from Gas Safe engineers or HVAC contractors
A heat loss survey and mains flow test confirm the right size. Compare at least three quotes before you buy.