Home Radiator BTU Calculator
Find the ideal radiator size for your room using simple choices — no technical knowledge required!
Your Estimated Heating Requirement
This calculator provides estimated values based on typical UK conditions using professional heat loss calculation methods. The ±15% comfort range reflects real-world variations in insulation, draughts, and heat retention.
For accurate heat loss calculations, professional system design, or installations requiring Building Regulations compliance, please consult a qualified heating engineer who can perform detailed calculations following CIBSE standards and UK Building Regulations Part L.
BTU Calculator FAQs
What is BTU and why does it matter?
BTU (British Thermal Unit) measures heat energy. One BTU is the energy needed to raise the temperature of one pound of water by one degree Fahrenheit. For radiators, BTU/hour tells you how much heat a radiator can output. You need enough BTUs to replace the heat your room loses through walls, windows, and floors.
What is Delta T50 and why do we use it?
Delta T50 (ΔT50) is the UK and European standard for measuring radiator output, defined in EN 442. It assumes water temperatures of 75°C flow, 65°C return, and 20°C room temperature. This standard lets you compare radiators from different manufacturers fairly.
How many BTUs do I need to heat my room?
Enter your room length, width and height, your UK region, external walls, wall construction, windows, room level and draught level above. The calculator returns a BTU and watts range with a ±15% margin, so you can pick radiators with a combined output towards the upper end of that range.
Should I choose a radiator with exactly the BTU I need?
No. If you are choosing between sizes, go larger. You can turn a bigger radiator down with thermostatic radiator valves (TRVs), but you cannot make a smaller radiator produce more heat.
How do I convert BTU to watts?
Divide BTU/hr by 3.412 to get watts. For example, 10,000 BTU/hr is about 2,931 watts. The calculator shows both figures for you.
How accurate is this BTU calculator?
It gives estimates based on typical UK building conditions and simplified heat loss calculations. For Building Regulations compliance or complex projects such as extensions and conversions, consult a qualified heating engineer who can perform detailed calculations following CIBSE standards.