When it comes to choosing the right cast iron radiator, it’s not just about looks — it’s also about ensuring your space is warm and comfortable. That’s where heat output comes in. But how do you work out how much heat a room actually needs?
This guide explains everything in simple terms, so you can confidently choose the correct radiator size for your home.
🔢 What Is Heat Output and Why Does It Matter?
Heat output is measured in BTUs (British Thermal Units) or Watts. It tells you how much heat a radiator will give off to warm your room. If your radiator is too small, your room won’t get warm enough. Too big, and you’re wasting money on oversized heating.
The key is to find the ideal balance for your specific space.
🧮 How to Calculate the Heat Output You Need
To calculate the required BTU or Watt output for a room, you need to consider:
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Room size (length x width x height in metres)
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Type of windows and glazing
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External walls
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Floor level (ground floor, basement, upper floor, etc.)
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Type of room (bathroom, kitchen, living room, etc.)
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Insulation quality
💡 The easiest way? Use a free online BTU calculator — just search “BTU calculator UK” and plug in your room details.
📏 Example Calculation
Let’s say your room is:
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4 metres long
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3 metres wide
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2.4 metres high
That’s 28.8 cubic metres of air to heat. Depending on insulation, you may need between 50–80 Watts per m³, or 170–270 BTUs per m³.
So:
28.8 m³ × 70 Watts = 2,016 Watts
28.8 m³ × 240 BTUs = 6,912 BTUs
Now you’ll need one or more radiators that add up to this heat output, using the Delta rating relevant to your system (Δt 30, 50 or 60 – see our previous post for help!).
🧊 Bathrooms & Heated Towel Rails
Bathrooms often need extra warmth due to ventilation and tiled surfaces. Add around 20% more heat output to your BTU calculation for bathrooms or cloakrooms — and consider a dedicated cast iron towel rail or radiator with hanging space.
❄️ Cold Rooms, Extensions or Period Properties
Older homes or rooms with high ceilings, poor insulation, or large windows may need more heating. Always round up, not down, when choosing your radiator output.
Need Help With Sizing?
Our expert team is happy to help you calculate the right radiator size for your space — just give us your room dimensions and a few details, and we’ll do the rest.
📞 Call us on 01723 321 333
📧 Or email info@periodstyle.uk
