Radiator BTU Calculator
Work out exactly how much heat output your room needs — then get a free, no-obligation quote from Potter's Home Improvements.
Heat requirement
Enter your room's length, width and height to see the radiator output you need.
How the BTU calculator works
1. Measure your room
Enter the length, width and height. The calculator works out the room volume — the starting point for how much heat it needs.
2. Fine-tune (optional)
Add details like glazing, external walls and insulation for a more accurate figure. Sensible defaults are used if you skip this.
3. Get your output
See the BTU and Watt output your radiators need at ΔT50 — the same rating shown on radiator spec sheets — and request a quote.
Frequently asked questions
What is a BTU?
BTU stands for British Thermal Unit — the standard measure of a radiator’s heat output. The more heat a room loses, the higher the BTU output your radiators need to keep it warm.
How do I measure my room for the calculator?
Measure the length, width and height of the room in metres (or feet). The calculator multiplies these to find the room volume, then applies factors for glazing, walls, insulation and your heating system.
What is ΔT50 and why does it matter?
Every radiator sold in the UK publishes its output at ΔT50 — a 50°C difference between the average water temperature and the room. Our headline figure is at ΔT50, so it matches the numbers on radiator spec sheets.
Do I need bigger radiators for a heat pump?
Yes. Heat pumps run at lower water temperatures, so a radiator gives out less heat. Switch on heat-pump mode and the calculator shows the larger ΔT50-rated output you should specify to get the same warmth.
How accurate is this BTU calculator?
It gives a reliable guide estimate suitable for planning. For an exact specification — accounting for your specific property, system and layout — we recommend a free survey from Potter’s Home Improvements.