What Size Radiator Do I Need?

Choose a radiator by the room's estimated heat loss in watts or BTU/h, not by width and height alone.

Measure room volume, account for external walls, glazing, insulation and the desired temperature, then check the radiator's published output at the temperatures your heating system will actually use.

Calculate my radiator requirement

Output, not dimensions

Two radiators with the same height and width can produce different heat because panel count, convector fins, water content and construction differ. Compare the manufacturer's stated watts or BTU/h for the exact model and test condition.

One watt of heat output is approximately 3.412BTU/h.

Measure the room

Record internal length, width and height. Volume gives a better starting point than floor area, but a reliable design also needs the fabric of the room: wall, floor, roof, windows and air leakage.

Note how many walls face outdoors or unheated spaces and whether the room sits above a garage or beneath a roof.

Glazing and insulation

Large or older windows generally lose more heat than a well-insulated wall. Building age is only a clue; extensions, replacement glazing and later insulation can change performance.

Do not assume a period property is uninsulated or a new-looking room meets a particular standard without evidence.

Room type and target temperature

Bedrooms are often designed for a different target temperature from living rooms or bathrooms. The calculator uses room type as an explicit assumption, but personal preference and professional system design can differ.

Open-plan areas must be treated as connected volumes, and frequently opened external doors add an uncertain ventilation load.

System temperature matters

Radiator output depends on the difference between mean water temperature and room temperature. A radiator rated at ΔT50 will emit less when supplied by a lower-temperature system.

Energy Saving Trust explains that heat pumps commonly use lower water temperatures and may need larger-surface-area, double or triple panel radiators.

See its heat-pump radiator guidance.

Treat online sizing as an estimate

A simple calculator is useful for shortlisting but cannot inspect construction, air leakage, pipework, flow rate or whole-system balance. Heat-pump design particularly needs a room-by-room heat-loss calculation.

Energy Saving Trust's installer guidance calls that calculation fundamental to adequate low-temperature heating.

Use the SetupSelect radiator calculator for a transparent starting range.

Then read radiator BTU explained, Type 21 vs Type 22, Delta T explained, bedroom sizing, living-room sizing and vertical vs horizontal radiators.

Confirm the final design with a competent heating professional.

Turn the estimate into a product shortlist

Keep the room heat requirement separate from the selected radiator output. The first describes the building; the second describes an emitter at a stated test condition. Record both in watts, then note the Delta T beside every product figure.

This avoids accidentally comparing a ΔT50 catalogue value with a lower-temperature requirement.

If one radiator cannot meet the output within the available wall space, possible design responses include a deeper panel type, a longer or taller emitter, more than one radiator, or improvements to heat loss. None is automatically best.

Pipe capacity, valve position, usable walls, furniture and the rest of the heating system all matter.

Information to give an installer

• Internal room dimensions and target temperature.
• Window and external-door sizes and types.
• Which walls, floors and ceilings border outdoors or unheated spaces.
• Known insulation and any uncertainty about hidden construction.
• Proposed flow and return temperatures.
• Existing pipe sizes, radiator positions and controls.
• Any planned fabric improvements or room-layout changes.

Common errors include using an online room-area table as a final design, adding an arbitrary percentage twice, and selecting by the radiator's dimensions. It is also easy to overlook a permanent opening into another room.

For a heat pump or major system change, ask for the assumptions and room-by-room result rather than only a total property figure. Energy Saving Trust's detailed heat-loss guidance explains why fabric and ventilation losses are calculated for each space.

A transparent estimate is useful for research, while final performance remains a system-design responsibility.