BTU Calculator: What Size Radiator Do I Need?
Choosing the correct radiator size means your room heats up quickly, reaches a comfortable temperature and holds it without wasting energy. This free BTU calculator estimates the heat output your room requires in BTU/hr and kW, then recommends a radiator to match. It is built on the principles of the BS EN 12831 design heat-loss method, using CIBSE design temperatures and U-values — the same standard MCS-certified heat-pump surveys are based on — so the figure is defensible and far more realistic than a volume-based rule of thumb. Once you have your number, browse our designer & traditional radiators: cast iron style column radiators, flat panel radiators and oval column radiators.
Last reviewed: August 2026 · Rackerman technical team
🔥 BTU & Radiator Size Calculator
Enter your room details for the estimated heat output (BTU/hr and kW) and an indicative radiator size. Results are indicative — confirm with a qualified heating engineer before buying.
Our cast iron style column radiators list BTU output on every size — match it to your calculated figure above.
How this BTU calculator works
Every room loses heat two ways, and this calculator adds both together using the same principles a heating engineer applies under BS EN 12831, the UK/European design heat-load standard. It is a simplified, single-room estimate rather than a full survey — but it captures the factors that dominate real heat loss:
Fabric loss — heat escaping through walls, windows, doors, roof and floor, calculated element by element as U-value × area × temperature difference. Ventilation loss — heat carried out by air changes, calculated as 0.33 × air-changes-per-hour × room volume × temperature difference. The two are summed, a design margin added, and the result converted to BTU/hr (1 watt = 3.412 BTU/hr).
The temperature difference (ΔT) is the gap between your room's target temperature and the coldest outside design temperature for your area — which is why a lounge in Glasgow needs more output than the same lounge in Plymouth, and a Victorian solid-wall room needs far more than a modern new build.
Room target temperatures
Target room temperatures follow the CIBSE Domestic Heating Design Guide.
| Room | Target °C |
|---|---|
| Lounge / living / dining / study | 21 |
| Bedroom / kitchen / hall / landing | 18 |
| Bathroom | 22 |
Outside design temperatures used (CIBSE Guide A)
Outside design temperatures follow CIBSE Guide A (the 99.6% winter figure for each location). Every city in the calculator is listed below.
| City / region | Outside design °C | City / region | Outside design °C |
|---|---|---|---|
| London | −3.0 | Nottingham | −3.9 |
| Cardiff | −3.1 | Norwich | −4.6 |
| Belfast | −3.2 | Swindon | −4.6 |
| Leeds | −3.3 | Manchester | −4.5 |
| Newcastle | −3.7 | Southampton | −4.8 |
| Plymouth | −1.5 | Birmingham | −5.1 |
| Edinburgh | −5.4 | Glasgow | −5.6 |
U-values behind the numbers (W/m²K)
U-value benchmarks follow SAP 10.2 / RdSAP and Building Regulations Approved Document L. Real values vary with construction, so these are representative figures for each era.
| Element | Construction | U-value |
|---|---|---|
| Wall | Solid brick (pre-1919) | 2.1 |
| Wall | Uninsulated cavity (1919–1975) | 1.6 |
| Wall | Insulated cavity (1976–2010) | 0.45 |
| Wall | Modern new build (post-2010) | 0.18 |
| Window | Single / double / triple glazed | 4.8 / 2.8 / 2.1 |
| External door | Solid wood | 3.0 |
Typical radiator outputs (ΔT50, BS EN 442)
Radiators are rated at ΔT50 under BS EN 442. Match one with output equal to or slightly above your figure. Heat pumps run at ΔT30 (about 51% of rated output), so heat-pump rooms need radiators roughly 1.95× larger — the calculator applies this automatically. The figures below are typical; actual outputs vary slightly between manufacturers (e.g. Stelrad, Myson, K-Rad), so always check the rating on the specific radiator. Compare outputs across our column, flat panel and oval column ranges.
| Radiator (H×W mm) | Type 11 | Type 21 | Type 22 | Type 33 |
|---|---|---|---|---|
| 600 × 800 | 2,480 | 3,600 | 4,650 | 6,400 |
| 600 × 1000 | 3,100 | 4,500 | 5,800 | 8,000 |
| 600 × 1200 | 3,720 | 5,400 | 7,000 | 9,600 |
| 600 × 1400 | 4,340 | 6,300 | 8,150 | 11,200 |
| 600 × 1800 | 5,580 | 8,100 | 10,450 | 14,400 |
Found your BTU? Pick your radiator
Shop designer & traditional radiators in anthracite, black and white — every size lists its BTU output at ΔT50, with free UK delivery.
Shop all radiators → Cast iron style columns →Frequently asked questions
Why is my figure lower than other BTU calculators?
Most consumer calculators use an old "room volume × factor" rule of thumb that assumes high air leakage and deliberately oversizes. This tool uses the BS EN 12831 design method with realistic air-change rates, which gives a more accurate — usually lower — figure. For a more conservative result on an older or draughty property, switch the "Sizing basis" to Comfort (+20%).
BTU or watts — which should I use?
They measure the same thing: 1 kW = 3,412 BTU/hr. UK central-heating radiators are usually catalogued in BTU; electric radiators and towel rails in watts. This calculator gives both.
What is ΔT50?
The test condition radiators are rated at — the radiator surface averaging 50°C above room temperature (BS EN 442). Heat pumps run cooler (ΔT30), delivering ~51% of the rated output, so they need larger radiators.
Can a radiator be too big?
A modest oversize (10–20%) is helpful — the room warms up faster and, with a thermostatic valve, simply switches off once it reaches temperature, so you don't waste energy. A very large oversize mainly wastes wall space and money, and on a boiler the radiator will cycle on and off more often. On a heat pump, sizing generously is actually beneficial (see below).
Should I oversize radiators for a heat pump?
Yes. A heat pump runs at a lower flow temperature (ΔT30), so a radiator delivers only about 51% of its ΔT50 rating. To give out the same heat it must be roughly 1.95× larger. Select "Air source heat pump" in the calculator and it sizes the radiator correctly for you. Size to the room's heat loss — not an arbitrary big number — and confirm with your MCS installer.
What if I have large patio or bi-fold doors?
Big glazed areas are often the single largest source of heat loss in a room and can easily double the figure. Set the "Window size" option to Large or Patio / bi-fold, and count each glazed door set as a window. If most of one wall is glass, lean towards the Comfort (+20%) sizing basis.
Does ceiling height matter?
Yes. A taller room has more air volume (so more ventilation loss) and more external wall area (so more fabric loss). A Victorian room at 2.7–3.0m needs noticeably more output than the same floor area at a modern 2.4m — always enter your real ceiling height.
Do bathrooms need more heat?
Yes — they are designed to 22°C rather than 18°C because occupants are wet, so the same-size bathroom needs more output than a bedroom.
Why is my radiator cold at the bottom?
Cold at the bottom (but warm at the top) usually means sludge or debris has settled inside — the fix is a chemical flush or power-flush, not a bigger radiator. Cold at the top with warm at the bottom means trapped air — bleed it. Neither is a sizing problem, so don't assume the radiator is undersized.
Standards & sources
- BS EN 12831-1:2017 — method for calculation of the design heat load (the framework this tool is based on).
- CIBSE Guide A: Environmental Design — UK external design temperatures.
- CIBSE Domestic Heating Design Guide (2026) — internal design temperatures, air-change rates and element U-values.
- MCS MIS 3005-D (Heat Pump Design Standard) & the MCS Heat Load Calculator — the UK certification scheme's BS EN 12831 reference for heat-pump sizing.
- SAP 10.2 / RdSAP (BRE) & Building Regulations Approved Document L — construction U-value benchmarks.
- BS EN 442 — radiator heat-output testing at ΔT50.
Related radiators & accessories
- Cast iron style column radiators — 2, 3 & 4 column, anthracite, black & white.
- Flat panel radiators — single & double panel, vertical & horizontal.
- Oval column radiators — sculptural oval profile, single & double depth.
- Angled radiator valves — manual & thermostatic (TRV) pairs.
- Cast iron radiator legs — floor-standing support feet.
- Browse the full designer & traditional radiator range →