How many BTUs do I need for a room in the UK?

A useful starting point is around 400 BTU per square metre for a typical UK room, adjusted for insulation, occupancy and heat-producing appliances. A professional survey is needed before installation.

What size heat pump do I need for my house?

Many UK homes fall between 5kW and 12kW, but floor area alone is not enough. An MCS installer should complete a room-by-room heat-loss calculation before specifying a system.

Is a bigger heat pump better?

No. An oversized system can short-cycle, cost more and run less efficiently. Correct sizing is based on the property heat loss at the local design temperature.

Free tool

What size heat pump do I need?

Work out what size heat pump or air conditioner your home needs. The calculator gives a quick room-cooling estimate in BTU; the whole-home guide below shows typical heat pump sizes in kW. These are two different calculations, and an MCS heat-loss survey is essential before buying a whole-house system.

Quick answer: a typical 20 m² UK room often needs roughly 8,000–9,000 BTU of cooling. A typical 2–3 bedroom home may need a 5–8 kW heat pump, but insulation and heat loss matter more than floor area alone.

Room or floor area (m²)

Insulation quality

People usually in the room

It's a kitchen

You should look for around

8,000 BTU

≈ 2.3 kW output · 9,000 BTU class unit

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How this BTU calculator works

We start from roughly 400 BTU per m² of floor area — a widely used rule of thumb for UK rooms with standard ceilings — then adjust for insulation quality, the number of people (each person adds body heat), and whether it's a kitchen (appliances add a lot). For an exact figure, a MCS-certified installer will do a full heat-loss survey, which is the gold standard before installing a heat pump.

What size air source heat pump for a whole house?

For whole-home heat pumps, the sizing depends on your property's heat loss (insulation, glazing, number of rooms). As a rough guide:

1-2 bed flat / terraced (up to 80 m²): 3–5 kW heat pump
2-3 bed semi-detached (80–120 m²): 5–8 kW heat pump
3-4 bed detached (120–180 m²): 8–12 kW heat pump
Large detached (180+ m²): 12–16 kW or a cascade system

These figures assume average insulation. A well-insulated home needs less; a Victorian property with solid walls may need more. Your installer will calculate the exact size during a site survey. If you are comparing equipment, see our ranked guide to the 10 best heat pumps and our guide to heat pumps in older homes.

Room-by-room guide

Small bedroom / office (up to 15 m²): ~5,000–7,000 BTU (1.5–2 kW)
Average bedroom / living room (15–30 m²): ~7,000–12,000 BTU (2–3.5 kW)
Large living room / open plan (30–45 m²): ~12,000–18,000 BTU (3.5–5.3 kW)

Does a heat pump's size affect the Boiler Upgrade Scheme grant?

Yes — the Boiler Upgrade Scheme gives £7,500 regardless of heat pump size, but the total installation cost will vary. A larger heat pump costs more to install, so your out-of-pocket cost after the grant will be higher. Getting the right size is important — oversizing wastes money and reduces efficiency.

Frequently asked questions

How many BTUs do I need for a room?

Start at roughly 400 BTU per square metre, then adjust for insulation, occupancy and appliances. Use the calculator above for a quick estimate.

Can I size a whole-house heat pump from floor area?

Only approximately. Correct sizing requires a room-by-room heat-loss survey that accounts for walls, windows, ventilation and the local winter design temperature.

Is a bigger system safer?

No. Oversizing can cause short cycling, higher upfront costs and poorer seasonal efficiency.

Ready for an accurate figure? Compare survey-backed quotes from MCS-certified installers and check whether the £7,500 Boiler Upgrade Scheme applies.

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This tool gives a ballpark for guidance only and isn't a substitute for a professional heat-loss survey from an MCS-certified installer.