What size radiator do I need? (BTU guide)

“What size radiator do I need?” is really a question about heat, not dimensions. The size that matters is the heat output, and the right output depends on the room you are heating. This guide explains what a BTU is, what makes one room need more heat than another, how to use a BTU calculator to get your number, and what to do with it once you have it. The aim is simple: a radiator that warms the room properly, without being so oversized that it wastes energy.

What is a BTU?

A BTU is a unit of heat. BTU stands for British Thermal Unit, and radiator output is given either in BTUs per hour or in watts (1 watt is roughly 3.41 BTU per hour). The higher the figure, the more heat the radiator can put into a room. Every radiator has a stated output, and every room has a heat requirement; the job of sizing is to match the two so the radiator can comfortably meet what the room needs.

You will see both units used, sometimes on the same listing, which can be confusing. Older guidance and many UK calculators talk in BTUs, while watts are the metric measure and increasingly common, especially on European and designer ranges. They describe the same thing, so you can convert freely, and the only rule that matters is to compare the radiator's figure and the room's requirement in the same unit. Once you have your room's number in either unit, you can shop confidently across any range.

What affects the BTU you need

Two rooms of the same floor area can need very different amounts of heat, because output depends on how much heat the room loses. The main factors a calculation takes into account are:

• Room size. Length, width and height: bigger rooms, and rooms with high ceilings, need more heat.
• Windows. The number and size of windows, and whether they are single or double glazed, since glass loses heat faster than wall.
• External walls. How many walls face outside; a room with several external walls loses more heat than an internal one.
• Insulation. Wall, loft and floor insulation, and the age of the property: a well-insulated modern room needs far less than a draughty older one.
• Room type. A lounge you want cosy, a bathroom that needs to feel warm, or a hallway you only pass through all call for different target temperatures.

Because so much depends on these, there is no single “right size” for a given room area. The calculation weighs them up for your specific room, which is why a calculator is the sensible way to get an accurate figure rather than guessing from floor space alone.

A few less obvious things nudge the figure too. North-facing rooms and those above an unheated space such as a garage tend to lose more heat. Rooms with high ceilings hold a larger volume of air to warm, so they need more than their floor area suggests. French doors and large picture windows behave like big areas of glass and push the requirement up. And conservatories are a special case, losing heat so fast through glass on all sides that they often need far more output than their size implies, or a heating solution of their own. A good calculator asks about these so the number reflects the real room, not an average one.

Use the BTU calculator

The quickest way to get your number is to use a BTU calculator. You enter the room's measurements and answer a few questions about windows, walls, glazing and room type, and it returns the heat output the room needs, usually in both BTUs and watts. Do this room by room, because each one differs: a north-facing lounge with big windows will need much more than a small internal study. Note the figure for each room before you start choosing radiators, so you are shopping to a target rather than hoping a radiator is powerful enough.

Use the BTU calculator on this page for each room (calculator wired in at build), then carry your figure into the radiator listings, which show each model's output.

A good calculator does the weighing for you, so you do not need to understand the physics, just answer honestly about the room. Be realistic with the inputs: if the loft insulation is thin or the windows are old single glazing, say so, because optimistic answers produce an undersized radiator that leaves the room cold. If you are between options, for example unsure how much glazing counts as “large”, choose the answer that gives the higher figure, since a little extra output is easy to turn down but missing output cannot be added without changing the radiator.

What to do with your number

Once you have the BTU figure for a room, choosing becomes straightforward: pick a radiator whose output meets or comfortably exceeds it. A small margin above the requirement is sensible, because it lets the room warm up quickly and copes with very cold days, and you can always turn a slightly larger radiator down with its valve. What you want to avoid is undersizing, where the radiator simply cannot heat the room, and gross oversizing, where you pay for heat and energy you never use. If one radiator cannot meet a large room's figure, two radiators that together hit the number work well.

Two things change the picture worth knowing. If your home runs on a heat pump, the water reaching the radiators is cooler, so each radiator gives out less than its headline figure and you need a higher-output or larger radiator to compensate. And a slim designer radiator may need sizing up, or choosing in a double-panel or aluminium version, to reach the output a chunkier radiator would manage in the same space.

For low-temperature systems, aluminium radiators make hitting the figure easier; if you love a slim designer model, check its output at the size you want.

Reading a radiator's output figure

When you compare radiators, there is one detail that trips people up: the output figure is quoted at a particular temperature difference between the radiator and the room, written as delta T or ΔT. The common standard is ΔT50, meaning the radiator averages 50 degrees hotter than the room. That matters because a radiator only hits its headline figure when the water flowing through it is hot enough to maintain that difference. Run the system cooler and the radiator gives out less than the number on the listing. So when you match a radiator's output to your room's requirement, make sure you are comparing figures at the same delta T, and remember the headline figure assumes a fairly hot system.

This is exactly why heat pumps change the sums. A heat pump runs at a much lower flow temperature than a gas boiler, so the temperature difference is smaller and every radiator gives out well below its ΔT50 figure, often little more than half. To deliver the room's required heat at that lower temperature, you need a radiator with a much higher headline output, which in practice means a larger or higher-output radiator than a boiler system would use. If you are sizing for a heat pump, size for the low flow temperature, not the ΔT50 number, or get a heating engineer to do the heat-loss calculation properly.

Common sizing mistakes

A few mistakes come up again and again, and they are easy to avoid once you know them:

• Buying on looks alone. Choosing a radiator because it suits the room, without checking its output meets the BTU figure, then finding the room never warms up.
• Sizing on floor area only. Two rooms of the same size can need very different heat depending on windows, walls and insulation, so a rule of thumb misses the mark.
• Ignoring delta T. Comparing a radiator quoted at one temperature difference with a room figure worked out at another, or assuming the headline output applies on a cool-running system.
• Forgetting the heat pump. Reusing boiler-era radiators on a heat pump and finding they cannot deliver enough heat at the lower flow temperature.
• Choosing slim without checking. Picking a slim designer radiator without confirming the output, when a double-panel or aluminium version may be needed to hit the figure.

How to work out radiator size

1. Measure the room: length, width and height.
2. Note the windows: how many, what size, and single or double glazed.
3. Note how many walls are external, and the property's insulation and age.
4. Decide the room type and how warm you want it.
5. Enter these into a BTU calculator to get the output the room needs.
6. Choose a radiator whose output meets or comfortably exceeds that figure, allowing more if you run a heat pump.
7. Repeat for each room, and size large rooms with two radiators if one cannot meet the figure.

BTU FAQs

How do I work out what size radiator I need?

Work out the room's heat requirement with a BTU calculation, using the room size, windows, external walls and insulation, then choose a radiator whose output meets or exceeds that figure. The size in millimetres matters only insofar as it delivers the output the room needs and fits the wall.

What happens if a radiator is too small?

The room never reaches a comfortable temperature, because the radiator cannot put in as much heat as the room loses. It will run constantly and still feel cool on cold days. The fix is a higher-output radiator: larger, a double panel, or a higher-output material such as aluminium.

Can a radiator be too big?

It can. A much oversized radiator costs more, takes up more space, and can waste energy, though a thermostatic valve lets you turn it down. A small margin over the requirement is useful for fast warm-up; gross oversizing is not worth paying for. Aim to meet the figure with a little headroom, not to overwhelm it.

Do I need a bigger radiator for a heat pump?

Usually yes. Heat pumps run at lower flow temperatures, so a radiator gives out less than its headline output, and you need a higher-output or larger radiator to deliver the room's BTU figure. Aluminium and oversized panels suit low-temperature systems.

Is BTU the same as watts?

They measure the same thing, heat output, in different units. One watt is about 3.41 BTU per hour, so figures convert directly. Radiator listings often show both; use whichever your calculator gives and compare like with like.

What is delta T and why does it matter?

Delta T, written ΔT, is the temperature difference between the radiator and the room that the output figure assumes, commonly ΔT50. A radiator only reaches its headline output when the system runs hot enough to maintain that difference; run cooler, as a heat pump does, and it gives out less. Always compare radiator and room figures at the same delta T, and size up for a low-temperature system.

Should I round my BTU figure up or down?

Up, by a sensible margin. A little headroom over the requirement lets the room warm quickly and cope with the coldest days, and you can turn a slightly larger radiator down with its valve. Rounding down risks a radiator that cannot keep up. Avoid gross oversizing, but a modest margin is the safer side to land on.

Do bathrooms and towel rails need sizing too?

Yes. A bathroom has its own BTU requirement, and a heated towel rail has a heat output like any radiator, but towels draped over it reduce the heat it releases into the room. In a larger or colder bathroom, check the rail's output against the room and add a panel radiator or underfloor heating if the rail alone falls short of the figure.

Find your number with the BTU calculator above, then shop radiators to your output, or read how to choose the right radiator for the full decision (linking when live). Trusted since 1999, with free UK delivery and 365-day returns. Big brands, small prices.