Last updated on November 11, 2026 by Emma Collins
The answer to “what size radiator do I need?” is not “the same width as the old one”. A radiator has to put back the heat a room loses each hour through its walls, floor, ceiling, windows and ventilation. Get the output right and the room warms properly; get it wrong and you can have the heating on full and still reach for a jumper.
This guide shows a simple way to estimate the output a room needs in watts, explains what BTU and ΔT50 mean on a radiator listing, and covers why heat pumps need much larger radiators. It also covers what a replacement typically costs in 2026 and when a rule of thumb is not enough.
Watts, BTUs and ΔT50: what the numbers mean
Radiator output is measured in watts (W). Older listings and some online calculators still use BTU/h (British Thermal Units per hour). Converting is easy:
- BTU/h to watts: divide by 3.412. For example, 5,000 BTU/h is about 1,465 W.
- Watts to BTU/h: multiply by 3.412.
The other number you will see is ΔT50 (“delta T fifty”). It is the difference between the average water temperature in the radiator and the room temperature, and UK catalogues usually quote outputs at ΔT50, which roughly matches a traditional gas boiler running hot. If your system runs cooler, as a condensing boiler set to a low flow temperature or a heat pump does, the same radiator gives out noticeably less heat than its catalogue figure.
Radiator maker Myson publishes these correction factors:
| ΔT | Typical system (Myson’s examples) | Correction factor |
|---|---|---|
| 50°C | Traditional gas boiler (80/60) | × 1.0 |
| 30°C | Eco boiler (55/45) | × 0.52 |
| 22.5°C | Heat pump system (45/40) | × 0.35 |
In practice, a radiator listed at 2,000 W at ΔT50 gives out only about 2,000 × 0.35 = 700 W on a heat pump running at ΔT22.5. To get a real 2,000 W at ΔT22.5, you would need a catalogue rating of about 2,000 ÷ 0.35 = 5,714 W. If the manufacturer of the radiator you are looking at publishes its own conversion figures, use those instead.
A simple way to estimate a room’s heat loss
Myson’s guide gives a quick starting method based on room volume and the age of the property. These are typical starting points, not a legal standard or a guarantee.
- Measure the room in metres: length, width and height.
- Work out the volume: length × width × height = cubic metres (m³).
- Multiply by a starting factor: about 50 W per m³ for homes built in the 1990s and 2000s, or about 40 W per m³ for homes from the 2010s and 2020s.
- Think about the room itself: insulation, wall construction, which way it faces and how it is used. Myson notes that a bathroom may need extra warmth after showers, while a bedroom can run a little cooler.
- Correct for your system’s temperature using the factors above.
Step 4 is why online radiator and BTU calculators ask about windows, external walls and insulation. Myson does not publish a percentage to add for each of these, so be wary of any rule that says “add 10% for a north-facing wall”. If your home is older than the 1990s, has solid walls or has had major changes, the starting factors may not fit, and a proper heat-loss calculation is the safer route.
If your result falls between two radiator sizes, Myson says it is usually better to go slightly larger. A thermostatic radiator valve (TRV) lets you turn it down, whereas an undersized radiator cannot be turned up past its limit.
A small habit that makes this easier: before I look at any calculator, I note the room’s measurements, the number of external walls and the window sizes in my phone. Every calculator asks for the same things, and it saves running back with the tape measure.
Worked example: a 4.0 × 3.5 × 2.4 m room
Take a rectangular room 4.0 m long, 3.5 m wide and 2.4 m high.
- Volume: 4.0 × 3.5 × 2.4 = 33.6 m³
- 1990s–2000s home: 33.6 × 50 = 1,680 W (about 5,732 BTU/h)
- 2010s–2020s home: 33.6 × 40 = 1,344 W (about 4,586 BTU/h)
| Property age band | Starting watts | Approx. BTU/h | Catalogue ΔT50 needed on a heat pump at ΔT22.5 (÷ 0.35) |
|---|---|---|---|
| 1990s–2000s | 1,680 W | 5,732 BTU/h | About 4,800 W |
| 2010s–2020s | 1,344 W | 4,586 BTU/h | About 3,840 W |
With a traditional boiler at ΔT50, the 1990s–2000s version of this room needs a radiator, or two radiators combined, of at least 1,680 W at ΔT50. Heat the same room with a heat pump at ΔT22.5, and the catalogue ΔT50 rating needs to be about 4,800 W, nearly three times as much. That is why a straight like-for-like swap often leaves rooms cold after a heat-pump conversion.
Treat this as a starting example only. A real room has its own windows, insulation and external walls.
Why heat pumps need larger radiators
Heat pumps work most efficiently with cooler water than a traditional boiler uses. Cooler water means each radiator gives out less heat, so to deliver the same warmth you need more surface area: a wider or taller radiator, a double-panel model in place of a single, or an extra radiator in the room.
When you compare radiators for a heat pump:
- find the radiator’s ΔT50 output on the listing
- apply the manufacturer’s correction for your system’s operating temperature
- check the corrected figure against the room’s estimated heat loss
For a full heat-pump installation, do not rely on a rule of thumb at all. An MCS-certified installer should carry out a room-by-room heat-loss calculation and specify the radiators to suit the heat pump. If you are still comparing a heat pump with a new boiler, our guide to new boiler costs in the UK covers the boiler side.
Emma’s tip: When I compare radiator listings, I write the ΔT50 figure and the corrected figure side by side in my notes. Shops tend to put the bigger ΔT50 number in the headline, and it is easy to forget the correction once you have a basket open.
One radiator or two, and where to put them
Most rooms need only one radiator if it is the right size. If wall space is tight, two smaller radiators that add up to the required output work just as well. Myson’s point is that the total output is what matters.
Position matters too. A radiator hidden behind a sofa or under a deep shelf cannot heat the room evenly, so leave the front and top clear. Keeping a new radiator in the same spot as the old one is usually the cheapest option, because moving it means new pipework.
Emma’s tip: If a sofa really has to sit in front of a radiator, I pull it a hand’s width away from the wall so warm air can rise behind it. It is not as good as a clear wall, but it makes a noticeable difference.
If you are swapping like-for-like on the same wall, measure the distance between the centres of the two pipes coming out of the floor or wall before you shop. A new radiator with matching pipe centres is a simpler fit than one that needs the pipes moved.
Emma’s tip: Before I ask for a quote, I take a photo of the old radiator with a tape measure across the pipe centres. Sending that to the plumber saves a site visit and avoids buying a radiator that will not line up.
When a rule of thumb is not enough
A volume-based estimate is fine for choosing a replacement in an ordinary room. Get a proper heat-loss calculation from a heating engineer when:
- you are installing a heat pump (an MCS installer should do this as part of the design)
- the room is poorly insulated, has solid walls or is in an older or listed building
- it is an extension, a converted loft or an open-plan space
- there is a lot of glazing, such as bifold doors or a conservatory
- an existing radiator already struggles to heat the room
Before you replace a radiator that “never gets warm”, rule out simpler causes. A radiator that is cold at the top usually has trapped air, so try bleeding the radiators first. If the whole system feels weak, check that your boiler pressure is in the normal range.
What it costs to replace a radiator in 2026
These are Checkatrade’s UK averages, from its table last updated in June 2026. They are ballpark figures, not quotes, and London and the South East may cost more.
| Job | Typical cost | Typical time |
|---|---|---|
| Remove a small radiator | About £80 | About 1 hour |
| Remove and replace a double-panel radiator | About £200 | 1–2 hours |
| Replace a large radiator | About £300 | About 2 hours |
| Move a radiator | About £275 | Varies |
| Remove up to six radiators and flush the system | About £350 | Varies |
| Radiator type | Typical supply-and-fit cost |
|---|---|
| Single panel | £150–£250 |
| Double panel | £180–£300 |
| Horizontal | £160–£280 |
| Vertical | £200–£350 |
| Column | £220–£400 |
| Heated towel radiator | £180–£320 |
Checkatrade puts typical labour for a standard radiator swap at £200–£300. A single radiator can often be changed without draining the whole system, but replacing several may need a full drain. Get two or three written quotes and check whether each includes the radiator, valves, removal of the old one and any pipework changes.
Frequently asked questions
How do I convert radiator BTUs to watts?
Divide the BTU/h figure by 3.412. For example, 5,000 BTU/h is about 1,465 W. To go the other way, multiply watts by 3.412.
Do I need bigger radiators for a heat pump?
Often, yes. Heat pumps run at lower water temperatures, so a radiator gives out less heat than its ΔT50 catalogue figure. Using Myson’s factors, a radiator gives about 35% of its ΔT50 output at ΔT22.5. An MCS installer should size the radiators as part of the heat-pump design.
Can a radiator be too big?
A slightly oversized radiator is rarely a problem, because a thermostatic radiator valve can turn it down. Myson recommends going slightly larger when your result falls between two sizes. A radiator that is far too large mainly costs more to buy and takes up more wall space.
How much does it cost to replace a radiator in the UK?
Checkatrade’s June 2026 figures put supply-and-fit at about £150–£250 for a single-panel radiator and £180–£300 for a double panel, with vertical and column styles costing more. Labour alone for a standard swap is typically £200–£300.
Is an online BTU calculator accurate enough?
For a like-for-like replacement in an ordinary room, a good calculator with accurate measurements gives a sensible starting figure. It is not a substitute for a proper heat-loss calculation for a heat pump, an extension or a poorly insulated room.
Conclusion
Size a radiator to the room’s heat loss, not to the old one’s width. Measure the room, multiply the volume by Myson’s 50 or 40 W per m³ starting factor, think about insulation and use, then correct for your system’s temperature. On a traditional boiler that gives a ΔT50 figure you can shop with. On a heat pump, expect to need roughly three times the catalogue output and get an MCS heat-loss survey. If a room is cold and draughty while you sort it out, our tips on keeping the house warm without central heating may help.
Emma Collins is the editor of The Homely Way, a UK home and lifestyle magazine. She writes practical, plain-English guides on home improvement, cleaning, heating and saving money around the home — always with UK prices, UK products and real-world costs, and no jargon. When she is not writing, she is planning the next guide.
