BTU Calculator UK – What Size Radiator & Boiler Do I Need?
Work out your room heating requirements in seconds.
Room size, insulation and usage all affect how much heat you need. Use the calculator below to find the correct BTU output, radiator size, and estimated boiler capacity based on your room dimensions.
Get accurate results for bedrooms, lounges, kitchens and bathrooms — ideal for planning heating upgrades, replacing radiators or sizing a new boiler system.
How to Use the BTU Calculator
Enter your room length, width and height, select the room type, and adjust for insulation level if needed. The calculator will estimate the heat output required in BTU and watts, along with a recommended radiator size.
What This BTU Calculator Covers
This BTU calculator UK estimates:
- Room heat requirement (BTU) based on volume and usage
- Watts required for radiator sizing
- Suggested radiator output for the space
- Total heating load for multi-room calculations
- Optional adjustments for insulation and room type
How to Use
- Enter your room length, width and height in metres.
- Select the room type (living room, bedroom, kitchen, bathroom).
- Adjust for insulation level if your property is older or poorly insulated.
- Review the BTU and watt output to determine the correct radiator size.
- Add multiple rooms together to estimate total boiler size (kW).
Why BTU Calculations Matter
Choosing the correct radiator or boiler size ensures your home heats efficiently. Undersized radiators struggle to warm the room, while oversized systems waste energy and increase running costs. Accurate BTU calculations help you achieve the right balance of comfort and efficiency.
Typical BTU Guidelines (UK)
- Living rooms: higher heat output due to larger space and usage
- Bedrooms: moderate BTU requirement
- Bathrooms: higher heat demand due to ventilation and comfort
- Well-insulated homes: lower BTU requirement
Pro Tips for Accurate Results
- Measure correctly: Include full room height for volume
- Consider insulation: Older properties need more heat
- Account for windows: Large glazed areas increase heat loss
- Use multiple radiators: For large rooms, split output across units
- Add a safety margin: 10–15% helps cover colder conditions
Related Tools & Guides
Whether you’re upgrading radiators or planning a full heating system, this BTU calculator gives fast, reliable results to help you size everything correctly.
Calculate Room BTU, Radiator & Boiler Size.
Enter your room dimensions, room type and insulation level. We’ll estimate the BTU required, watts output, radiator size and suggested boiler size.
Room BTU
Watts Required
Radiator Size
Suggested Boiler Size
Room Volume
Radiator Size (mm)
Saved Rooms
Whole House Totals
Notes
- Figures are estimates for radiator and boiler sizing.
- Poor insulation, large windows and exposed rooms may need extra output.
- For whole-house boiler sizing, add all room watt totals together.
Next Steps
- Use the BTU result to choose a radiator with equal or slightly higher output.
- For colder rooms, consider adding 10–15% extra capacity.
- For full property sizing, repeat the calculation for each room and total the watts.
- Always check radiator outputs at the correct ΔT rating when comparing products.
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