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BTU Heat Loss Calculator

BTU Heat-loss Calculator (ΔT50)

Our BTU calculator helps you estimate the amount of heat your room loses per hour as well as how much heat is needed to comfortably warm your room. Results are shown using the UK standard measurement of ΔT50 BTU/h, which reflects the output of radiators when used with a typical gas-powered central heating system.

If you are are in a remote area, in the highlands and islands, we would recommend using extreme mode to allow for colder minimum temperatures.

Why are Heat pump systems different?

If you’re using a heat pump, radiators operate at lower water temperatures, so we also provide an equivalent ΔT30 calculation. This allows you to easily compare outputs and choose a radiator that will perform effectively with your heating system, whatever type you use.

Radiator heat outputs are measured using a temperature difference known as Delta T. Delta T50 is the traditional UK standard and reflects the higher water temperatures produced by gas central heating systems. Heat pumps operate at much lower flow temperatures, which is why their performance is measured at Delta T30. Because radiators emit less heat at these lower temperatures, a radiator sized for Delta T50 will produce significantly less output when connected to a heat pump. To fully replace the heat lost from a room when using a heat pump, radiators must therefore be larger or more efficient, ensuring they can deliver the required heat output at Delta T30 for comfortable, consistent warmth.

Your Saved Rooms:

Click on the room to reload the BTU results, any changes will be automatically saved

    Boiler type
    Your Room
    Dimensions
    Window type
    External walls (0–4)
    Wall type
    What is below the room?
    What is above the room?
    Exposure

    Extreme mode uses a colder outside temp + higher ventilation rate to align with harsher conditions, for example remote areas and Scotland.

    Understanding your results

    We calculate your heat requirement using your room size, construction details and how the room is used.

    Different rooms are typically kept at different temperatures — for example, bathrooms are usually warmer than bedrooms or living areas (22°C vs 18°C).

    The way your room is built, along with the style of windows, affects how quickly heat is lost, so these details help us estimate how much warmth is needed to keep the space comfortable.

    We also factor in an estimated outside temperature to reflect typical winter conditions (Default: -3°C).

    Together, these assumptions help us recommend a radiator size that will comfortably heat your room, even on colder days.

    BTU Calculator Disclaimer

    This BTU calculator provides an estimated heat requirement based on a typical UK home at Delta ΔT50. Actual performance may vary depending on factors such as installation quality, building construction, insulation, air changes, and the overall efficiency of your heating system.

    For best results, we recommend rounding your calculated BTU requirement up to ensure a comfortable heat output. If you have a complex layout, an unusual property type, or require a precise calculation, please consult a qualified heating engineer.

    All radiator outputs listed on our website use Delta ΔT50 for standard UK central heating systems and Delta ΔT30 for typical domestic air-source heat pumps. If you require a different Delta T rating, please use an appropriate conversion tool.

    FREQUENTLY ASKED QUESTIONS

    Questions from our beloved customers

    • What are BTUs and why are they important?
      BTU stands for British Thermal Unit, which is a measurement of the heat energy required to warm a room. Calculating your BTUs ensures you don't "under-size" your heating (leaving you cold) or "over-size" it (wasting energy and money). At Radiators 4u, our calculator uses precision algorithms to ensure you get the perfect balance of comfort and efficiency.
    • How does the calculation differ for a gas boiler vs. a heat pump?
      Gas boilers typically run at high "flow temperatures" (around 70–80°C), whereas Air Source Heat Pumps (ASHPs) operate most efficiently at lower temperatures (35–55°C). Because the water is cooler with a heat pump, you will usually need a radiator with a higher BTU rating or a larger surface area to achieve the same room temperature.
    • What is "Delta T" and which one should I use?
      Delta T (ΔT) is the difference between the average water temperature in your radiator and the desired room temperature. ΔT50: The UK standard for modern gas boiler systems. ΔT30 or ΔT35: The standard for low-temperature systems like heat pumps. Radiators 4u provides technical data for multiple Delta T ratings to ensure your new purchase is future-proofed for any heat source.
    • Do I need bigger radiators if I switch to a heat pump?
      Usually, yes. Since heat pumps provide a gentler, lower-level heat, you need more radiator surface area to emit that heat into the room. Our expert team can help you identify high-efficiency aluminium radiators, which conduct heat faster - making them ideal for low-temperature systems.
    • Should I calculate BTUs differently for a bathroom?
      Yes. Bathrooms often feel colder due to tiling and humidity. We generally recommend adding 10% to your BTU requirement for bathrooms. Explore our range of high-output heated towel rails that combine luxury with the performance needed to keep towels dry and the room warm.
    • How do "U-values" and insulation affect my result?
      The better your insulation (double glazing, cavity wall, loft insulation), the lower the BTU requirement. If you live in a period property with solid walls, your heat loss will be higher. Our calculator allows you to input your house type to account for these variables accurately.
    • Can I use one large radiator or two smaller ones?
      If a room requires a very high BTU count, two smaller radiators often provide more even heat distribution than one giant unit. Radiators 4u stocks thousands of column radiators in various widths, allowing you to split the load without compromising on style.
    • Why should I choose aluminium radiators for high BTU needs?
      Aluminium is a "super-conductor". It heats up almost instantly and is incredibly efficient at low flow temperatures. Radiators 4u is one of the UK’s leading suppliers of eco-friendly aluminium radiators, offering a 100% recyclable heating solution that lowers your carbon footprint.
    • What if I want a traditional look, like cast iron?
      Cast iron radiators take longer to heat up, but retain heat for much longer after the boiler or heat pump has turned off. We offer bespoke cast iron radiators that can be built to your exact BTU requirements and finished in a wide range of colors, including authentic Farrow & Ball paints.
    • Does the ceiling height matter?
      Absolutely. Heat rises, so a room with high ceilings (common in Victorian homes) requires a higher BTU output to maintain warmth at "living level". Our calculator includes a height variable to ensure your tall rooms stay cosy.
    • Do north-facing rooms need more heat?
      Yes. Rooms that receive little to no sunlight are naturally colder. We recommend selecting a radiator with a slightly higher BTU output than the "minimum" for north-facing rooms to compensate for the lack of solar gain.
    • Is it okay to "over-spec" my BTU requirements?
      It’s better to have a radiator that is slightly too powerful than one that is too weak. You can always turn a Thermostatic Radiator Valve (TRV) down, but you can't force a small radiator to work harder. Radiators 4u offers a massive selection of TRVs to help you maintain precise control.
    • Can I get a high-BTU radiator delivered quickly?
      Yes! We know that a broken radiator is an emergency. That’s why Radiators 4u offers next-day delivery on thousands of stocked items for qualifying orders placed before 3pm.
    • What if I’m replacing a radiator and the pipework is already there?
      If your pipework is fixed, use our Radiator Size Finder tool alongside the BTU calculator. We can help you find a modern, high-output radiator that fits your existing "pipe centres", often using telescopic extensions to avoid expensive plumbing work.
    • Why trust Radiators 4u with your heating project?
      Established in 2002, we ‘re specialists, not just general retailers. From our Portsmouth base, we provide expert technical support over the phone and offer an industry-leading range of finishes, sizes, and exclusive brands that you won't find in standard DIY stores.
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