What Size Radiator Do I Need?
Heat output decides whether a room stays warm so size your radiator by BTUs before you think about how it looks on the wall.
Written by Richard Morgan
Updated
The short answer
The radiator size you need depends on the heat output the room needs, measured in BTUs or watts, not on the radiator's width or height. Work out the room's BTU target with a radiator calculator using its size, windows and walls, then choose a radiator that meets it. A double panel radiator gives far more heat than a single panel of the same dimensions.
Choosing a radiator because it fills the space under a window is how rooms end up cold all winter. The physical size only matters as far as the wall goes. What decides whether the room feels warm is the heat output, measured in BTUs or watts.
We've fitted radiators in our own homes, including a big double panel in place of an old single and a tall vertical radiator between two doors. Below is what we weigh up before buying, for a straight swap when replacing a radiator yourself or a brand new one being plumbed in.
Work Out The Heat Output In BTUs First
BTU stands for British Thermal Unit, a traditional measure of heat, and every radiator lists its output in BTUs, watts or both. The higher the figure, the more heat it puts into the room. One watt is roughly 3.41 BTUs, which helps when a calculator and a product listing use different units.
To find out what a room needs, use a radiator calculator. B&Q's BTU radiator calculator takes the size of the room along with other factors that affect heat loss and gives you a target figure. Rooms with older solid walls, big windows or a cold floor lose heat faster and need a higher output than a room of the same size with cavity walls and good insulation.
That target doesn't have to come from a single radiator. Two radiators in the same room add together, and splitting the output across two walls warms a long room more evenly than one large radiator at the far end.
Single Panel Or Double Panel Radiators
Standard panel radiators come in single and double versions, and most have convector fins welded to the back of the panels to add surface area. A double panel with fins gives out far more heat than a single panel of the same width and height. Merchants list them by type number. Type 11 is a single panel with one set of fins, types 21 and 22 are doubles, and type 22 is the one to look for when you need the most heat from a small space.
Double panels cost more and sit further off the wall, but they're the answer when there's only room for one radiator or the space under a window is short. A single panel giving the same output has to be much longer or taller to match it.
The photo above shows the large 1800 x 600 mm double panel we fitted in place of an old single panel radiator, and the room was noticeably warmer for it.
How The Room Changes The Size You Need
Where the radiator goes matters as much as the floor area. Hallways lose warmth every time the front door opens, and a big open-plan kitchen has far more air to heat than a bedroom, so both need more output than their measurements suggest. People overlook this when they pick sizes in summer and only notice once the colder months arrive.
Rooms with lots of glass are the hardest to keep warm, and our guide to the best conservatory heaters covers ways to top up a radiator that can't keep pace. Insulation pulls the figure the other way, and our guide to floor insulation explains how much heat a room over a cold void loses through the floor.
Designer Radiators Give Out Less Heat
Designer radiators, including flat-panel horizontal and vertical styles like the one at the top of this page, look great on the wall. Most of them give out less heat than a standard panel radiator of a similar size, because the slim, flat faces have less surface area for the heat to come off.
If you've set your heart on a designer model, compare its BTU figure with the room's target and ignore the dimensions until that's settled. You'll need a bigger designer radiator, or a double version, to match what a plain double panel gives. Our pick of the best vertical radiators is a good place to compare tall designs that suit narrow walls.
Sizing A Radiator To Fit The Space
Sometimes the wall decides the size. In his own house, Richard, who wrote this guide, wanted a vertical radiator that sat in the gap between two doors and finished level with the top of the door architraves. It meant tailoring the pipework to suit, but measuring the gap before ordering paid off, and the radiator sits neatly between the doors.
Leave room for the valves at the bottom, and check the radiator clears window boards, sockets and furniture once it's on its brackets. For tall radiators in a narrow gap, decide on the pipe positions before you order, since some models take their connections at the bottom centre and others need a valve at each end.
Getting The Radiator Size Right
The radiator size you need comes down to the room. Work out its BTU target first, then find a radiator that meets it in a shape that suits the wall. If you have the space, go a little over the target rather than under, because a cold room is miserable and a radiator with spare output can always be turned down.
That's where thermostatic radiator valves earn their keep, holding a bigger radiator at a comfortable temperature instead of letting it run flat out. For control from your phone, room by room, smart radiator valves take the same idea further.
Quick Questions And Answers
How many BTUs do I need to heat a room?
The BTUs a room needs depend on its floor area and ceiling height, how many external walls and windows it has, the type of glazing and the insulation. Put those details into a BTU radiator calculator to get a target figure. Add a little extra for rooms that lose heat quickly, such as hallways, glazed extensions and rooms above a cold floor.
Is it better to have a bigger radiator?
A bigger radiator is better as long as its output meets or slightly exceeds what the room needs. Spare output warms the room faster and copes better on the coldest days, and a thermostatic radiator valve stops it overheating. A radiator far larger than the room needs is wasted money, so size it to the BTU target first.
Can I swap a single panel radiator for a double panel?
Yes, you can swap a single panel radiator for a double panel to get more heat from the same wall space. The double sits further off the wall, so the brackets will change, and the pipework needs adjusting if the valve positions differ. We made this swap with an 1800 x 600 mm double panel and the room was noticeably warmer.
Do vertical radiators give out less heat?
Many vertical radiators give out less heat than a standard panel radiator covering the same wall area, because slim flat-panel designs have less surface for heat to leave. The height itself isn't the problem. Check the BTU figure against the room's target, and choose a double vertical radiator if a single one falls short.
Can't I just buy a radiator that fills the space under the window?
Only if its output also meets what the room needs. Width and height say nothing about heat, and a radiator picked purely to fill the wall is how rooms end up cold all winter. Get the BTU target first, then look for something in that output that suits the space.
Is it better to go over the BTU target or under it?
Over, if the wall has room for it. A radiator with spare output can be turned down, while one that falls short leaves you cold on the worst days. A thermostatic radiator valve keeps the bigger radiator at a comfortable temperature instead of letting it run flat out.
Can two smaller radiators replace one big one?
Yes. Their outputs add together, and two radiators on different walls warm a long room more evenly than one big one at the far end. Just check the two figures add up to the room's total BTU target.
How much difference did the big double panel make to your room?
A big one. Once the 1800 x 600 mm double panel replaced the old single, which we did while the floorboards were still lifted, the room was noticeably warmer. That's the extra heat a second panel and fins give over a single panel of the same width and height.
Richard has spent years renovating properties and turning them into rentals and Airbnbs. Like a lot of people he started out as a beginner, and these days there isn't much DIY he won't take on.