Use our free BTU calculator to work out how much heat every room in your home actually needs. It calculates to BS EN 442 (Δt 50 °C) — the standard all radiator outputs are quoted to — and adjusts for room size, glazing, insulation, exposure and more, for as many rooms as you like.
Once you have your figures, match them to our cast iron radiators — or have your own originals restored at our Greater Manchester workshop.
Work out each room’s BTU requirement
Add every room you’re heating and tune the details — the readout updates as you type. When you’re done, get the results emailed to you as a free PDF report.
Your heating system changes what a radiator delivers, not what the room needs. Every radiator we sell lists its output at both T50 (boilers) and T30 (heat pumps) — compare your figures with the column that matches your system.
We show the room’s true requirement — some calculators quietly pad every figure by 20%. If you’d like a buffer, add it yourself here.
Calculated at Δt 50°C to BS EN 442. We recommend choosing a radiator with output 5–10% above the calculated figure; an oversized radiator can always be regulated with a thermostatic valve (TRV). Figures are guidance — for a precise specification, call us on 0161 843 8163.
Get your results as a free PDF report
We put your figures into a written report and email it to you — free, no obligation.
- Every room’s heat requirement in BTU and watts
- Calculated to BS EN 442 (Δt 50°C) — the figure your plumber or installer needs
- Yours to keep for planning, quotes and comparisons
Pop your details in below and your report will be with you within one working day.
We’ll only use your details to send your report and follow up on your enquiry — see our privacy policy.
How this calculator works
The calculator starts with your room’s volume — length × width × height — and applies a base requirement of 153 BTU per cubic metre, the figure for a typical room heated to 21 °C, measured at Δt 50 °C to BS EN 442. It then adjusts for everything that changes how fast a room loses heat: glazing type, insulation age, what’s above and below the room, how exposed the property is, the number of external walls and windows, and the temperature the room is designed to reach.
Δt 50 °C is the European standard test condition for radiator outputs: the heat delivered when the radiator’s average water temperature is 50 °C above the room temperature. Radiators deliver less heat at lower water temperatures, so every radiator we sell lists its output at both T50 (Δt 50 °C — boilers) and T30 (Δt 30 °C — heat pumps). Your room figures stay the same whichever system you run — just compare them with the column that matches it, and the numbers line up like for like.
Different rooms are heated to different design temperatures — the calculator sets these automatically when you choose a room type:
| Room type | Design temperature |
|---|---|
| Living room | 21 °C |
| Bedroom | 18 °C |
| Kitchen | 18 °C |
| Bathroom | 22 °C |
| Hallway | 18 °C |
| Dining room | 21 °C |
| Conservatory | 21 °C |
| Home office | 21 °C |
Choosing radiators to match your figures
Pick a radiator — or a combination — whose output at Δt 50 °C meets or slightly exceeds each room’s figure. We recommend 5–10% of headroom: an oversized radiator can always be turned down with a thermostatic valve (TRV), but an undersized one can never catch up on the coldest days of the year. In larger rooms, two smaller radiators often heat the space more evenly than one large one.
Running a heat pump, or planning to? Your room figures stay the same — but a radiator delivers less heat at heat-pump water temperatures, so compare your figures against each radiator’s T30 output, listed on every product page alongside the standard T50 figure. Cast iron’s large water content and thermal mass suit steady, low-temperature running. If in doubt, check the numbers with your installer.
Browse our cast iron radiators — each product page shows the heat output for your chosen size — or restore the originals you already own. Not sure? Call us on 0161 843 8163 and we’ll size it with you.
Contact Radrestore
Phone: 0161 843 8163
Email: info@radrestore.co.uk
Workshop: Unit 8, Station Road Industrial Estate, Reddish, Stockport, Greater Manchester SK5 6ND
Social: @radrestore · facebook.com/radrestoreuk
Collection and delivery available UK-wide. Showroom open Mon–Fri 8:00am–4:00pm, weekends by appointment; phone and WhatsApp every day 8:00am–8:00pm.
BTU and radiator sizing FAQs
How many BTUs do I need to heat a room?
It depends on the room’s volume and how quickly it loses heat. As a guide, a 4 m × 3.5 m living room with a 2.4 m ceiling, double glazing, average insulation and one external wall needs around 5,950 BTU/h (about 1,750 W). Glazing, insulation, exposure and room type all shift the figure significantly — use the calculator above for a per-room answer in seconds.
What is a BTU?
A BTU (British Thermal Unit) is the traditional unit of heat energy. Radiator outputs are quoted in BTU per hour (BTU/h) or in watts — 1 watt is about 3.4 BTU/h — and our calculator shows both, so you can compare any radiator on either figure.
What does Δt 50 °C mean?
Δt 50 °C is the standard test condition for radiator outputs under BS EN 442: the heat a radiator delivers when its average water temperature is 50 °C above the room temperature. All reputable UK radiator outputs — including ours — are quoted at Δt 50 °C, so you can compare products like for like. Every radiator we sell also lists its output at Δt 30 °C (T30) — the figure that matters for heat pumps and other low-temperature systems.
Do cast iron radiators heat as well as modern radiators?
Yes — cast iron outputs are measured to the same BS EN 442 standard as any modern radiator, so a 6,000 BTU/h cast iron radiator delivers the same heat as a 6,000 BTU/h steel one. The difference is how it feels: cast iron’s high thermal mass gives a gentler, more radiant warmth and keeps releasing heat after the system switches off.
Do cast iron radiators work with heat pumps?
Yes — cast iron radiators are compatible with heat pumps and modern condensing boilers. Heat pumps run at lower water temperatures, so compare your calculator figures against each radiator’s T30 (Δt 30 °C) output — we list it on every product page alongside the standard T50 figure. Cast iron’s large water content and thermal mass suit steady, low-temperature running.
What size radiator should I choose if nothing matches my figure exactly?
Go 5–10% above the calculated figure. An oversized radiator can always be turned down with a thermostatic valve (TRV); an undersized one can’t catch up on the coldest days of the year. In larger rooms, splitting the output across two radiators often heats the space more evenly than one large one.
How accurate is this calculator?
It uses the BS EN 442 (Δt 50 °C) method with adjustment factors for glazing, insulation, floor type, what’s above the room, exposure, external walls, windows and room temperature — accurate enough to size radiators confidently in a typical home. For unusual spaces, or a full heat-loss survey, call us on 0161 843 8163 and we’ll work through it with you.
Can I get my results as a PDF?
Yes — free. Calculate your rooms, then pop your details into the form on this page and we’ll email you a personalised PDF report of every room’s requirement within one working day, ready to hand to your plumber or installer.