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Choosing a new radiator isn't just about finding one that fits the wall. If it's too small, the room will never feel warm and your boiler will work harder than it needs to. If it's far too big, you've paid for heat you don't need.
The secret to getting it right is one number: BTU. In this guide, we'll explain what BTU means, show you how to calculate the radiator size you need for any room, and share the expert tips that make the difference between a cold room and a cosy one.
BTU stands for British Thermal Unit.
It's the standard way of measuring how much heat a radiator gives out. In the UK, radiator heat output is shown in BTUs per hour, and often in watts too. Every radiator we sell shows its heat output on the product page, so once you know your room's BTU requirement, choosing the right one is easy.
3.412 BTU A 1,000W radiator gives out about 3,412 BTU.
A simple step-by-step guide
Measure the room
Measure the length, width and ceiling height in metres.
Calculate the volume
Length × Width × Height gives room volume in m³.
Work out watts needed
Use approximately 35–60 watts per m³ depending on insulation and room type.
Convert to BTU
Watts × 3.412 gives your estimated BTU requirement.
Choose the higher end of your estimate, or add a little extra, if the room has:
Typical room calculations
Living Room
4.5m × 3.5m × 2.4m · average double-glazed home
Double Bedroom
3.5m × 3m × 2.4m · average double-glazed home
Bathroom
2.5m × 2m × 2.4m · using 55 watts per m³
These figures are estimates to help you choose. For a whole-house project, and especially if you're switching to a heat pump, ask a heating engineer for a full room-by-room heat loss calculation.
Radiator types explained
Two radiators of exactly the same height and width can give out very different amounts of heat, depending on how they're built. Panel radiators are usually described by type:
Single panel
- Single panel
- Single convector
The slimmest option with the lowest output. Good for small rooms or where space is tight.
Double panel
- Double panel
- Single convector
More heat without much extra depth.
Double convector
- Double panel
- Double convector
The most popular choice in UK homes, with high output for its size.
Maximum output
- Triple panel
- Triple convector
Maximum output, but deeper from the wall. Ideal for large rooms with limited wall space.
Column and designer radiators add real style to a room, and vertical radiators are perfect when wall width is limited. Output varies a lot between styles, so always check the BTU figure rather than judging by size.
Browse our full range of radiators →Get the most from your radiators
A larger radiator can be turned down with a TRV. One that's too small cannot properly heat the room.
Two radiators on different walls can spread heat more evenly.
Where possible, this helps counter cold draughts.
Chrome radiators and towel rails can give out less heat than painted alternatives.
Include room for valves and pipework when measuring your wall.
Find valves and fittings in our heating & plumbing range.
Heated towel rails
In bathrooms, many people choose a heated towel rail instead of, or as well as, a radiator. Towel rails usually give out less heat than a radiator of the same size, and towels hanging on them block some heat too. If a towel rail is your only heat source, choose one with plenty of output.
Shop Towel Rails →Quick answers
What size radiator do I need for a 3m × 3m room?
With a 2.4m ceiling, the room volume is 21.6m³. In an average double-glazed home, that's roughly 864W, or about 2,950 BTU. For a bedroom, around 3,000 BTU would be a good starting point. Choose more if the room has external walls or large windows.
Can a radiator be too big for a room?
A slightly oversized radiator isn't usually a problem because you can control it using a thermostatic radiator valve. Undersizing is the bigger risk.
Is it better to have one big radiator or two smaller ones?
In larger rooms, two smaller radiators placed on different walls usually give more even heat than one large radiator.
Do vertical radiators give out as much heat?
They can, as long as the BTU output matches the room's requirements. Always compare the stated BTU figure.
How many BTU per square metre do I need?
It depends on the room and insulation, which is why working from room volume is more accurate. As a rough guide, an average living room with a standard ceiling may need around 100 BTU or more per square metre.
Find the right radiator for your room
Now you know your BTU figure, you're ready to choose. At Tradeplumbing, we stock hundreds of radiators and towel rails in every size, style and finish, with the heat output clearly shown on every product. All at competitive prices, with free delivery to the UK mainland.
Heating & Plumbing
Valves, fittings and everything needed to complete the install.
Shop Heating & Plumbing →Are you a heating engineer, developer or contractor? We supply trade customers and large projects too, from hotels and pubs to property developers. No job is too big or too small.
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