Radiator BTU Output Table by Room Size
A room-size table gets you a ballpark radiator output in seconds. It is a starting point, not an answer — a proper heat-loss calculation is still the only figure worth trusting on a real job.
When someone asks 'what size radiator do I need?', they usually want a number now, not a heat-loss survey. A rule-of-thumb table keyed to room size gives that — a rough BTU or watt output for a typical room of that footprint — and it is genuinely useful for a first sanity check or a rough quote.
The catch is that room size alone ignores everything that actually drives heat loss: glazing, insulation, external walls, ceiling height and the flow temperature your system runs at. Treat the table below as a starting point, then confirm the real figure with a heat-loss calculation before you order anything.
Key takeaways
- Room size gives a quick rough output — handy for a first estimate, not for ordering radiators.
- The figures assume a typical UK home heated to around 21°C at a conventional flow temperature.
- Lounges, bay-windowed, north-facing, high-ceilinged and poorly insulated rooms need more than the table suggests.
- Low-temperature and heat-pump systems can need roughly double the output for the same room.
- Always confirm with a heat-loss calculation before you buy — the table is a sanity check, not the answer.
Why room size only gives a rough figure
A radiator has to replace exactly the heat a room loses to the outside, and floor area is only one part of that. Two rooms of identical size can lose very different amounts of heat depending on how much glazing they have, whether the walls are solid or insulated cavity, how many walls face outside, and how high the ceiling is.
That is why a room-size table can only ever be a rule of thumb. It bakes in an 'average' room and gives you the output such a room typically needs. Real rooms deviate — sometimes a lot — so the table is best used to check whether a more careful figure looks sensible, not to replace it.
Radiator output by room size
The table below gives approximate outputs for common room sizes in a typical UK home. Figures are rounded and expressed in both BTU per hour and watts (1 W is about 3.412 BTU/h, so watts are roughly the BTU figure divided by 3.412). Use them as a rough starting point only.
Notice that lounges and living spaces carry a higher output per square metre than bedrooms — they tend to have more glazing and are heated for longer — and that a large room may well need two radiators to hit the total rather than one oversized emitter.
| Room size | Room type | Approx output (BTU/h) | Approx output (W) |
|---|---|---|---|
| 2 × 2 m (4 m²) | Small bathroom | ~2,000 | ~600 |
| 3 × 3 m (9 m²) | Small bedroom | ~3,500 | ~1,000 |
| 3 × 4 m (12 m²) | Double bedroom | ~4,500 | ~1,300 |
| 3 × 3 m (9 m²) | Kitchen | ~4,000 | ~1,200 |
| 4 × 4 m (16 m²) | Average lounge | ~7,000 | ~2,050 |
| 4 × 5 m (20 m²) | Kitchen-diner | ~8,500 | ~2,500 |
| 5 × 5 m (25 m²) | Large lounge | ~10,000 | ~2,950 |
What pushes a room above the rule of thumb
Several common features mean a room loses more heat than its footprint suggests, so the real output needed climbs above the table. Large or bay windows, single glazing, north-facing rooms that get no solar gain, high or vaulted ceilings, rooms with several external walls, and poorly insulated solid-wall or older properties all push the figure up.
The biggest shift, though, is the system itself. On a low-temperature or heat-pump system the water runs far cooler, so a given radiator delivers much less heat — often around half its catalogue output. For those systems the room-size table understates what you need badly; you may need roughly double the emitter, or underfloor heating, to meet the same heat loss.
The table is a starting point, not a substitute
Use it to get in the right ballpark, then run a proper heat-loss calculation — especially on heat-pump, low-temperature or hard-to-heat rooms, where a rule of thumb can be well off.
From the estimate to a real sizing
Once the table has given you a ballpark, move to the reliable method: work out the room's heat loss from its actual construction, then find the corrected radiator output at your system's real delta-T, and pick the radiator that meets it with a little headroom. Our radiator sizing guide walks through that properly, and a BTU calculator does the arithmetic in seconds.
Record the sizing against the job so the numbers are there if a room is ever questioned, and so any quote reflects the emitters the design actually needs rather than a rule-of-thumb guess that comes back to bite you.
Frequently asked questions
What size radiator do I need for a bedroom?
As a rough rule of thumb, a typical 3 × 3 m single bedroom needs around 3,500 BTU/h (about 1,000 W), and a 3 × 4 m double around 4,500 BTU/h (about 1,300 W). Those assume an average, reasonably insulated UK room at about 21°C — confirm with a heat-loss calculation before ordering.
What size radiator do I need for a living room?
An average 4 × 4 m lounge needs roughly 7,000 BTU/h (about 2,050 W) as a starting figure, rising to around 10,000 BTU/h (about 2,950 W) for a large 5 × 5 m room. Living rooms have more glazing and are heated for longer, so they carry a higher output per square metre — and large rooms often need two radiators.
Can I size a radiator from room dimensions alone?
Only roughly. Room size ignores glazing, insulation, external walls, ceiling height and your flow temperature, all of which change the heat loss. A room-size table is fine for a first estimate, but you should confirm the real output with a proper heat-loss calculation before you buy.
Do these figures work for a heat pump?
No — treat them as far too low for a heat pump. Because a heat pump runs at a low flow temperature, a radiator delivers much less than its catalogue output, so you often need roughly double the emitter for the same room, or underfloor heating. Always size heat-pump systems from a full heat-loss calculation at the design flow temperature.
From guidance to action
Related guides
Radiator Sizing Guide (Heat Loss & BTU)
Room heat loss, BTU output and delta-T corrections — how to size radiators that actually heat the room.
Air Source Heat Pump Installation Guide
System design, siting, commissioning and the Benchmark and MCS paperwork for an ASHP install.
Underfloor Heating Installation Guide
Wet vs electric UFH, screed and pipe spacing, commissioning and handover documentation.
