BTU Calculator: What Size Radiator Do I Need?
Getting radiator size right is the difference between a room that never quite warms up and one that heats fast and holds temperature. This free BTU calculator works out the heat output your room needs in BTU/hr and kW, then recommends a radiator to match. It uses the official BS EN 12831 design heat-loss method with CIBSE design temperatures and U-values — the same standard MCS-certified heat-pump surveys use — so the figure is defensible, not a rule-of-thumb guess. Once you have your number, browse our designer & traditional radiators: cast iron style column radiators, flat panel radiators and oval column radiators.
🔥 BTU & Radiator Size Calculator
Enter your room details for the 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 exactly as a heating engineer would under BS EN 12831, the official UK/European design heat-load standard:
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.
Official design temperatures used
Target room temperatures follow the CIBSE Domestic Heating Design Guide; outside design temperatures follow CIBSE Guide A (the 99.6% winter figure for each city).
| Room | Target °C | City | Outside design °C |
|---|---|---|---|
| Lounge / living / dining | 21 | London | −3.0 |
| Study / office | 21 | Manchester | −4.5 |
| Bedroom / kitchen / hall | 18 | Birmingham | −5.1 |
| Bathroom | 22 | Glasgow | −5.6 |
U-values behind the numbers (W/m²K)
| 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. 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
Most consumer calculators use an old "room volume × factor" rule of thumb that assumes high air leakage and deliberately oversizes. This tool uses the official 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%).
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.
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.
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.
Standards & sources
- BS EN 12831-1:2017 — method for calculation of the design heat load (the calculation framework).
- CIBSE Guide A: Environmental Design (2015), Table 2.5 — UK external design temperatures.
- CIBSE Domestic Heating Design Guide (2026), Tables 2-2, 2-18, 2-42 — internal design temperatures, air-change rates and element U-values.
- MCS Heat Load Calculator & MGD 007 — the UK certification scheme's BS EN 12831 reference tool.
- RdSAP 10 / SAP 10.2 (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 →