Estimate a room’s heating requirement in watts and BTU per hour, with an allowance for insulation, glazing, external walls and heating-system temperature.
Estimate radiator output
Estimated radiator requirement
| Base heat-demand rate | |
|---|---|
| Room-type factor | |
| External-wall factor | |
| Glazing factor | |
| System output factor |
Use the catalogue rating when choosing a radiator tested at ΔT50. Manufacturer correction factors and actual design temperatures can differ.
How the radiator estimate works
The calculator multiplies room volume by a planning heat-demand rate based on insulation. It then adjusts the estimate for room type, external walls and glazing. A 10% allowance is shown separately. This is a screening estimate rather than a full room-by-room heat-loss survey.
Insulation rates used
| Insulation selection | Planning rate | Typical interpretation |
|---|---|---|
| Very poor | 60 W/m³ | Older, draughty fabric with limited insulation |
| Poor | 50 W/m³ | Some improvements but substantial heat loss |
| Average | 40 W/m³ | Typical insulated UK room |
| Good | 30 W/m³ | Well-insulated room with modern glazing |
| Very good | 25 W/m³ | High-performance fabric and airtightness |
Why system temperature changes radiator size
Radiator output falls when the average water temperature moves closer to room temperature. A radiator that produces its catalogue output at ΔT50 may deliver roughly 51.5% at ΔT30 or 30% at ΔT20. The calculator therefore increases the required ΔT50 catalogue rating for lower-temperature systems.
Energy Saving Trust guidance says an accurate room-by-room heat-loss calculation is fundamental when designing low-temperature heating. Review its detailed heat-loss calculation guidance before choosing emitters for a heat-pump installation.
Worked example
A 4.5 m × 3.8 m living room with a 2.4 m ceiling has a volume of 41.04 m³. With average insulation, two external walls and double glazing, the calculator applies its room and fabric factors, then adds a 10% selection margin. A lower-temperature system will need a larger catalogue-rated radiator than a traditional ΔT50 system.
What a full survey considers
A professional calculation measures individual walls, floors, ceilings, windows and doors, assigns U-values, includes ventilation and air leakage, and uses local outdoor design temperature. It also checks pipework, water flow, boiler or heat-pump output and the selected radiator manufacturer’s correction table.
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Frequently asked questions
Should radiator size be based on watts or BTU?
Either unit can be used if the conversion is consistent. UK radiator catalogues commonly show watts, BTU/h or both.
Why does a heat pump need a larger radiator?
Lower water temperatures reduce radiator output, so a larger surface area may be needed to match the room heat loss.
Does double glazing reduce the estimate?
Yes. This calculator applies a smaller glazing uplift for double glazing than for single glazing.
Can one large radiator replace two smaller radiators?
The total output may match, but room shape, comfort, pipework and heat distribution also matter.
Is the 10% margin always suitable?
No. It is a simple selection allowance. A heating designer may use a different margin after a detailed survey.