What is a BTU?

A BTU (British Thermal Unit) is the standard measure of heat output used for radiators in the UK. One BTU is the energy needed to heat one pound of water by 1°F. In practice, the number that matters is the BTU/hr rating of a radiator — how much heat it pumps into a room every hour. Radiator heat output is also shown in watts: 1 kW is roughly 3,412 BTU/hr.

If a radiator's output is too low for the room, it will never feel warm — the boiler works harder and your bills rise. Too high isn't dangerous, but you'll have paid for a bigger radiator than you need. That's why sizing matters.

How to measure your room

Our calculator uses your room's volume and a few correction factors. To measure:

1

Length and width — measure wall to wall in metres, at floor level.

2

Height — floor to ceiling. Most UK homes are around 2.4m.

3

Windows and insulation — big or single-glazed windows lose heat, so the room needs more BTUs. Rooms under a roof or above a garage also lose more.

Typical BTU requirements by room

Every home is different, but as a rough guide for an average modern UK house:

Room	Typical size	Approx. BTU/hr
Small bedroom	3m × 2.5m	1,500 – 2,500
Double bedroom	4m × 3m	2,500 – 4,000
Lounge / living room	5m × 4m	4,000 – 7,000
Kitchen / diner	4m × 4m	3,000 – 5,500
Bathroom	2.5m × 2m	1,200 – 2,000

Frequently asked questions

What happens if my radiator's BTU is too low?

The room will struggle to reach temperature, especially in winter. Your boiler runs longer and your energy bills go up. Fitting the right size — or adding a second radiator — fixes it.

Is it better to have one big radiator or two smaller ones?

In larger rooms (over ~5,000–6,000 BTU), two radiators on different walls usually heat the space more evenly than one very large unit.

Do double panel radiators give more BTUs?

Yes — a double panel radiator of the same width and height gives roughly 40–50% more output than a single panel, without taking up more wall space.

How accurate is this calculator?

It follows the standard UK sizing method (room volume × correction factors), which is what most retailers and installers use for an estimate. For a precise heat-loss calculation — for example when replacing a whole system — a Gas Safe registered engineer will survey your home.

Tip: radiator sizing is only half the job — correct installation, balancing and clean system water make the real difference. A local radiator engineer can confirm the size and fit it properly.

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Radiator BTU Calculator UK — What Size Radiator Do I Need?