Estimate radiator output in BTU and watts using room size, room type, EPC and heating system details. Use simple mode for a quick answer or advanced mode for a more refined room-by-room estimate.
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Live resultUpdates as you type so you can quickly compare room sizes and heating choices.
BTU + WattsUseful whether radiator listings show traditional BTU or modern watt output.
EPC-awareApplies a whole-home efficiency overlay without replacing the core room calculation.
Layout guidanceHelps show whether one radiator, two radiators or a larger emitter may make more sense.
Start with the room dimensions, then choose whether you want a quick estimate or a more detailed result. This tool is designed to be practical, readable and useful for normal homes.
Simple modeQuick estimate using room size, room type, EPC and heating system.
Advanced modeAdd windows, glazing, insulation, room position and other heat-loss details.
Room size
Home efficiency and system
EPC is used as a whole-home efficiency overlay rather than a replacement for room dimensions.
Lower flow temperatures usually mean larger radiators are needed for the same comfort level.
Advanced room factors
Estimated radiator target
—
Enter your room details to see the recommended output.
Heating demand: —
EPC: not used
System: standard
Heating demand meter
This helps show whether the room sits at the lower or higher end of heating demand.
Room volume—
Base requirement—
Total adjustment—
Suggested target range—
Room type factor—
EPC adjustment—
Heating system adjustment—
Warmth preference—
Advanced room factors—
Final recommendation—
Suggested radiator direction
Enter your room details to see a suggested radiator direction.
Layout suggestion
Best fit—
Bathroom / practical note—
Practical guidance
A slightly larger radiator is usually safer than undersizing, especially in colder rooms or lower temperature heating systems.
How this calculator thinks about heat
Room size first
The starting point is always the room itself. Length, width and height create the base heat requirement because bigger spaces simply need more heating output.
EPC as an overlay
EPC does not replace the room calculation. It acts as a whole-home efficiency adjustment so the estimate better reflects how the property tends to retain heat overall.
System temperature matters
Lower flow temperatures often mean the radiator has to be physically larger to deliver the same comfort. That is why the heating system setting can increase the target output.
BTU stands for British Thermal Unit. It measures heat output. In simple terms, the BTU figure tells you how much heat a radiator can provide. Larger, colder or less efficient rooms usually need a higher BTU output to feel warm.
What is EPC and how does it affect heating?
EPC stands for Energy Performance Certificate. It rates how energy efficient a property is from A to G. Better EPC ratings generally mean less heat loss. In this calculator, EPC is used as an adjustment layer to help reflect the general efficiency of the home.
Why do I still need room size if I already know my EPC?
EPC applies to the whole home, but a radiator heats a specific room. The room dimensions are still needed to work out the base heating demand. EPC then fine-tunes the result rather than replacing it.
Does the boiler affect radiator sizing?
The room still needs the same amount of heat regardless of boiler age, so the main sizing should always be room-based. However, the heating system and flow temperature do affect how much heat a radiator can deliver, which is why this calculator includes a heating system adjustment.
Why are both BTU and watts shown?
Many UK radiator listings still show BTU, while others use watts. Showing both makes it easier to compare products and match the result to what you see on retailer or manufacturer pages.
How are radiators rated in the USA, Canada and Australia?
The UK commonly uses BTU and watts. In the United States, heating output is often shown in BTU per hour, especially in wider HVAC contexts. Canada commonly uses both BTU and watts. Australia more often uses watts for heating products, and electric heating is more common in many homes. If this calculator proves popular in those countries, country-specific versions could be built later.
Is it better to oversize or undersize a radiator?
Undersizing is usually the bigger risk because the room may never feel properly warm. A slightly larger radiator is often the safer choice, especially in older homes, colder rooms or lower temperature systems.
How accurate is this calculator?
This calculator gives a practical estimate for typical homes. It is more realistic than a basic volume-only formula because it includes room type, EPC and optional advanced heat-loss factors, but it is still not a full professional heat-loss survey.
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