What this wizard works out
This wizard multiplies room volume by a watts-per-cubic-metre figure chosen from the insulation standard, giving an approximate heat requirement in watts and the equivalent in BTU per hour.
It is a rule of thumb, and a coarse one. A proper heat loss calculation accounts for external wall area, window size and glazing type, floor and ceiling construction, and the temperature you actually want to reach — none of which volume alone can capture.
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Before you buy a radiator
- Measure the room's internal dimensions including the ceiling height, which is what makes this a volume calculation rather than a floor area one.
- Choose the insulation band honestly. Solid walls with single glazing is poor; a modern well-insulated house with good glazing is good.
- Rooms with large windows, external corners or above unheated spaces need more than this figure suggests.
- Radiator outputs are quoted at a stated temperature difference, usually Δ50. Modern condensing boilers often run cooler, which reduces actual output.
Radiator outputs are quoted in both. One watt is 3.412 BTU per hour, so a 1,500 watt radiator is about 5,100 BTU/h. Comparing a watts figure against a BTU figure without converting is a factor-of-three error, and it is a common one.
How this calculator works
Volume times a heat requirement per cubic metre:
Room volume = length × width × height
Watts = volume × watts per m³
BTU per hour = watts × 3.412
60 W/m³ poor · 50 W/m³ average · 40 W/m³ goodThe bands are broad. A living room you want at 21°C needs more than a bedroom at 18°C, and a north-facing room with two external walls needs more than an internal one of the same size.
For a whole-house system, or where you are replacing a boiler, a room-by-room heat loss calculation is the right approach. Many heating engineers will produce one, and it matters considerably more for a heat pump than for a gas boiler.
Worked example: a 4 m by 3 m room
Using the default figures — 4 m by 3 m by 2.4 m with average insulation:
- Room volume: 28.8 m³
- At 50 watts per m³: 1,440 watts
- Equivalent: 4,913 BTU per hour
Change the insulation to poor and the requirement rises to 1,728 watts; to good and it falls to 1,152. That is a 50% spread from one dropdown, which is a fair reflection of how much the building fabric matters — and a reminder that improving insulation reduces the radiator you need as well as the fuel you burn.
Common mistakes
- Comparing watts against BTU without converting. The factor is 3.412.
- Using floor area instead of volume. Ceiling height matters, particularly in older houses.
- Ignoring the quoted temperature difference. A radiator rated at Δ50 delivers less on a system running cooler.
- Sizing for a heat pump using boiler assumptions. Lower flow temperatures need substantially larger radiators.
- Treating a rule of thumb as a design. A real heat loss calculation is a different thing.
Frequently asked questions
What is Δ50 and why does it matter?
Radiator outputs are quoted at a stated difference between the average water temperature and the room temperature — commonly Δ50, meaning water averaging 70°C in a 20°C room. A system running at a lower flow temperature, which modern condensing boilers and especially heat pumps do, produces a smaller difference and therefore less output from the same radiator. The correction is substantial: at Δ30 a radiator delivers roughly half its Δ50 figure.
Do heat pumps need bigger radiators?
Usually yes, and often considerably. Heat pumps work most efficiently at low flow temperatures, typically 35 to 45°C rather than 60 to 70°C. That reduces radiator output sharply, so radiators are frequently upsized or replaced with larger or fan-assisted types. This is a major part of the cost of converting a house to a heat pump and should be assessed properly rather than estimated.
Where should a radiator go?
Traditionally under a window, because that was the coldest surface and the rising warm air counteracted the downdraught. With modern double glazing that logic is weaker, and placement is more flexible. What has not changed is that curtains hanging over a radiator, or furniture pushed against it, send the heat where it is not wanted.
Is one big radiator better than two smaller ones?
Two smaller radiators generally distribute heat more evenly in a large or awkwardly shaped room, and give more flexibility in placement. One large radiator is simpler and cheaper to install. For a square room of ordinary size either works; for a long room, two is usually better.
Is what I enter stored?
No. Room dimensions are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Approved Document L, heating system guidance and heat pump schemes
- Health and Safety Executive — safe working on heating systems and hot surfaces
Sources are checked at publication and can change — how I choose and check references.