Estimate only. Final sizing should be confirmed by a qualified heating engineer, especially for heat pumps and unusual rooms.
How to size a radiator correctly
Choosing the right radiator size is the difference between a room that feels comfortable and one that is always too cold or too hot. A radiator that is undersized will struggle in winter, while an oversized unit can short-cycle your boiler or create uneven heat. This calculator estimates the required output in watts and BTU/hr, then converts that need into an approximate radiator length.
What this radiator calculator includes
This tool uses a practical heat-loss estimate based on the variables that matter most in most homes:
- Room volume (length × width × height)
- Insulation quality (poor to excellent)
- Climate severity (mild, average, cold)
- Number and quality of windows (single, double, triple glazing)
- Number of exterior walls (greater exposure means more heat loss)
- Target indoor temperature
- Flow temperature of your heating system (important for heat pumps)
Why flow temperature matters so much
Most radiator catalogs list outputs at ΔT50 (high-temperature systems). If your system runs cooler water, each radiator gives off less heat. That means you need a larger radiator surface area to deliver the same room warmth. This is especially important with low-temperature boilers and air-source heat pumps.
Step-by-step: using the calculator
- Enter your room dimensions in meters.
- Select insulation quality and climate.
- Add window count and glazing type.
- Choose exterior wall count.
- Select radiator panel type and heating system temperature.
- Click Calculate Radiator Size.
You will get a required heat output (watts and BTU/hr) plus a recommended total radiator length for the chosen type.
Example radiator sizing result
For a medium-sized living room in an average climate with standard insulation and double glazing, you might need around 1,400–2,200 W depending on ceiling height and exterior exposure. At high system temperature, that could be one medium Type 22 radiator. At low temperature, the same room may need a much longer radiator or two separate units.
Practical advice before buying
- If required radiator length is very long, split output across two radiators for better heat distribution.
- Under windows is still a common placement because it offsets downdrafts.
- Use thermostatic radiator valves (TRVs) for room-by-room control.
- Bleed radiators and balance the system after installation.
- For renovations, confirm pipe sizes and pump capacity.
When to get a full heat-loss survey
This calculator is ideal for quick planning. For final specification—especially in older properties, open-plan spaces, rooms with large glazing areas, or heat pump systems—get a professional room-by-room heat-loss calculation. A proper survey can include infiltration rates, exact U-values, and design outdoor temperatures for your location.
Watts vs BTU: quick reference
Radiators are sold in both watts and BTU/hr. The conversion is:
- 1 watt = 3.412 BTU/hr
- 1,000 watts = 3,412 BTU/hr
Use whichever unit your radiator supplier provides, but keep ratings at the same temperature regime when comparing products.