A bedroom radiator should be sized to the heat the room loses at the temperature you want to hold, not to the wall space you happen to have. A well-insulated 10 m² box room and a draughty 10 m² room with two external walls can need quite different outputs, so floor area alone is only a starting point.
This guide builds on our radiator size guide and applies it to bedrooms specifically.
What changes the output a bedroom needs
- Target temperature. Bedrooms are commonly designed to around 18°C, lower than living rooms. A lower target means a lower required output.
- External walls and windows. More external wall and more glazing means more heat loss. Single glazing loses substantially more than modern double or triple glazing.
- Insulation. Cavity wall, loft and floor insulation all reduce the output you need.
- Ceiling height. A tall room has more air to heat than its floor area suggests.
- The room above and below. A bedroom under an unheated loft loses more than one sandwiched between heated rooms.
Step 1: Get a heat-loss figure
The most reliable route is a room-by-room heat-loss calculation from your heating installer. For a quick estimate, our BTU calculator takes room dimensions and key factors and returns an approximate requirement. Treat any online calculator as a guide, and have the final figure confirmed if you are heating a whole house or replacing a boiler.
Step 2: Compare it to the manufacturer's output
Manufacturers publish outputs in watts (W) and often BTU/h at a stated ΔT. One watt is about 3.412 BTU/h, so the two are easy to convert. The critical point is that the output must be compared at the right ΔT for your system. We explain this in a separate guide on what ΔT50 and ΔT30 ratings mean.
Step 3: Choose a shape that fits the room
Once you know the output, shape becomes a design choice. A vertical radiator suits a bedroom with limited wall width, while a horizontal or mirror radiator can sit under a window or beside a wardrobe. Browse designer radiators, or look at bedroom radiators. Examples include the Terma Rolo mirror designer radiator and the Eucotherm Nova horizontal oval tube radiator. Always check the output at your system's ΔT on the product page before ordering.
Measurements to take before you order
- Available wall width and height, allowing space for skirting, sockets and curtains.
- Distance between existing pipe centres and whether pipes come from the floor or the wall.
- Floor or wall clearance beneath and beside the radiator.
- Radiator projection from the wall, especially beside a bed or door.
Common mistakes
- Sizing from floor area alone and ignoring glazing and insulation.
- Comparing a ΔT50 output with a lower-temperature system without adjusting.
- Oversizing so heavily that the room overheats, which is rarely comfortable for sleeping.
- Placing a radiator where furniture will block heat.
In short
Work out the room's heat loss, match it to a published output at the right ΔT, then choose the shape that suits your wall. If you are unsure, ask a heating engineer to confirm the figure before you buy.