Radiator Buying Guide: What you actually need to know

When it’s time to swap out a radiator, everyone asks the same things: "How big does it need to be?" or "Which one's the best bet?" We’ve been shifting radiators for decades, so we’ve heard it all. We’re here to give you the straight talk on getting your heating sorted without the faff.

What size radiator should I buy?

After years of selling these things, we’ve got a solid handle on the best way to choose. The truth is, the physical size isn't where you start. It’s all about the "grunt"—the heat output needed to actually warm the room. Forget generic online calculators; I’ll show you how to crunch the numbers yourself using Watts and BTU (British Thermal Units).

Working out the room volume step-by-step

First, you need the volume of the space in cubic metres (m³). Grab a tape, measure the length, width, and how high the ceiling goes. Multiply them all together.

Let’s take a simple example: a room is 5m long, 4m wide, and has a height of 2.8m. The calculation looks like this: 5 × 4 × 2.8 = 56 m³.

How to calculate the required power?

For a room with average insulation, we usually allow for about 40 watts per cubic metre. To get the BTU figure, which is the standard measure in the UK, you simply multiply the watts by 3.412.

Using our example of 56 m³:

Power: 56 × 40 = 2240 watts.
In BTU: 2240 × 3.412 ≈ 7643 BTU.

This means you’d need a radiator with an output of roughly 2200–2300 watts (or about 7600 BTU) to keep that room warm. It shows that an average 20 m² room needs that much power, provided the ceilings are a standard 2.8m.

Of course, these numbers change if your insulation is a bit dodgy:

Poor insulation: If the walls are thin or windows are ancient, count on 50 watts per m³ (2800W / 9554 BTU).
Modern insulation: If you have high-spec walls and double glazing, 35 watts is plenty (1960W / 6687 BTU).

Why is insulation so vital?

Insulation is key because a radiator doesn't just "make heat"—it replaces the heat your house is losing. In a typical uninsulated house, heat loss usually looks like this:

External walls: 30–40%
Roof/Loft: 20–30% (since heat rises)
Windows and doors: 15–25%
Floors: 10–15%

The walls are the biggest culprits simply because they have the largest surface area. Focus on the heat output. A well-chosen radiator gives off steady heat without being pushed to its limit. If you’re unsure, it’s always better to go slightly bigger than to find out later that your radiator can't cope.

Note: These are guidelines. For a whole house, talk to a heating engineer to factor in your boiler or heat pump specs.

Which radiator should I buy?

The first requirement is simple: buy one that actually hits the wattage and BTU we calculated. The second is: make sure it physically fits.

A classic mistake is picking a radiator that fills the wall space exactly. This is a bad move. You need about 10cm of clearance on all four sides. Radiators work by circulating air: drawing cold air in at the bottom and releasing warm air at the top. If it’s too close to the floor or a windowsill, the air can't move and the room stays chilly. Also, don't forget you need room for the valves and pipework!

Common mistakes to avoid

Only counting floor space: Ceiling height matters just as much.
The "Perfect Fit" trap: Measuring to the centimetre and leaving no room for airflow.
Fashion over function: Buying a tiny "discreet" radiator that leaves you shivering in January.
Ignoring the windows: Old, single-glazed windows mean you need a lot more power.