Radiator BTU calculator

How to size a radiator for a UK room

Getting radiator sizing right is one of the most important decisions a heating engineer or plumber makes on a domestic installation. Too small and the room never reaches temperature. Too large and the boiler short-cycles, reducing efficiency and increasing wear.

The standard approach in the UK is to calculate the room's heat loss in BTU per hour (or watts), then select a radiator with at least that output. Heat loss depends on the volume of the room, how well it is insulated, how many windows it has and their glazing type, and how many of its walls face outside.

The BTU formula explained

The core calculation starts with room volume (length × width × ceiling height in metres) multiplied by a base heat loss factor per cubic metre. This factor varies by room type — bathrooms need more heat per m³ than bedrooms, and conservatories significantly more than both.

The base figure is then adjusted upward for poor insulation and downward for good insulation. Windows add a fixed BTU allowance per window depending on glazing type — single glazing loses roughly four times more heat than triple glazing. External walls add further allowance because they lose more heat than internal partition walls.

Delta T 50 vs Delta T 60

Radiator output is rated at a standard temperature difference (Delta T) between the mean radiator water temperature and the room air temperature. In the UK, radiator output is typically quoted at Delta T 50 (mean water temperature of 70°C in a 20°C room) for traditional gas boiler systems.

If you are installing for a heat pump system, the flow temperature will be lower — typically 45-55°C — so you need to derate the quoted radiator output accordingly. At Delta T 30 (mean water temp 50°C, room 20°C), a radiator may only deliver 50-55% of its stated Delta T 50 output. For heat pump installations, always specify radiators sized at the actual operating temperature.

Typical BTU requirements for UK rooms

To give you a sense of scale, here are typical BTU requirements for common UK room sizes in an average-insulated property with double glazing:

• Small bedroom (3m × 3m × 2.4m): approximately 2,500–3,500 BTU/hr
• Double bedroom (4m × 3.5m × 2.4m): approximately 4,000–5,500 BTU/hr
• Living room (5m × 4m × 2.4m): approximately 6,000–9,000 BTU/hr
• Large open-plan kitchen/diner (7m × 4m × 2.5m): approximately 8,000–12,000 BTU/hr
• Bathroom (2.5m × 2m × 2.4m): approximately 2,200–3,500 BTU/hr

These are approximate ranges. Use our calculator above for a more precise figure based on your specific conditions.

Choosing the right radiator type

Once you know the BTU requirement, you need to select a radiator that meets it. Common types include:

• Single panel (Type 11): Lower output, suitable for smaller rooms or supplementary heating.
• Single panel with convector fins (Type 21): Mid-range output. Good for bedrooms and bathrooms.
• Double panel, double convector (Type 22): High output. The most common choice for living rooms and large spaces.
• Towel rail radiators: Lower output than panel radiators of similar dimensions — make sure to check the BTU output, not just the size.

If you cannot fit a large enough radiator in the available space, consider a vertical radiator (which gives more output in a narrower footprint) or a fan convector unit for rooms with very high heat demand.

Compliance and sign-off

In England and Wales, replacing a like-for-like radiator does not require Building Regulations notification. Installing a new heating system, adding a boiler, or installing underfloor heating does. Gas boiler work must be carried out by a Gas Safe registered engineer. Oil heating work requires OFTEC registration.

For plumbers and heating engineers quoting jobs, accurate BTU calculations help you specify the right radiator first time — avoiding return visits and unhappy customers. Our quote generator can help you put together a professional quote once you have sized the system.