Choosing a radiator that looks right is the fun part. Making sure it actually heats the room is the important part. Here's how to get the size right first time.
BTU and watts: what's the difference?
Both measure heat output. BTU (British Thermal Units per hour) is the figure most UK retailers quote, and watts is what most manufacturers print on spec sheets. They're interchangeable: 1 watt = 3.412 BTU. So a 1,000W radiator gives out about 3,412 BTU.
What affects how much heat a room needs?
- Room size: length × width × ceiling height. Bigger rooms, and especially higher ceilings, need more heat.
- What the room is used for: living rooms and bathrooms are kept warmer than bedrooms and kitchens.
- Outside walls: every wall that faces outside loses heat. A corner room needs more than a room in the middle of the house.
- Insulation: a newer home with filled cavity walls needs far less than an older house with solid brick walls.
- Windows and doors: glass loses heat faster than walls, so more windows, single glazing or patio doors all push the figure up.
- Above and below: a room under an uninsulated loft, or over a garage, needs a bit more.
The quick way: use our calculator
Our Find my size calculator takes all of the above into account in under a minute, then shows you the radiators in our range that will heat the room, with live prices.
Got a heat pump? Read this bit
Radiator outputs are tested at ΔT50, which assumes hot water from a traditional gas or oil boiler. Heat pumps and low-temperature systems run cooler, so the same radiator gives out roughly half the heat. If you have a heat pump, look for a radiator rated at around twice the output your room needs, or use two radiators. Just choose "Heat pump" in the calculator and it will do this for you.
Top tips
- If you're between two sizes, go for the bigger one. You can always turn a radiator down, but you can't turn it past full.
- Big room or limited wall space? Two smaller radiators often heat a room more evenly than one large one.
- For bathrooms, remember that towels hanging on a towel rail block some of its heat.
- Tall, vertical radiators are a great way to get plenty of output on a narrow wall.
Still not sure? Get in touch and we'll help you choose.