BTU Calculator
Calculate the heating requirement for any room in BTU/hr and kW. Based on CIBSE heat loss methodology — accounts for insulation, glazing, property age, room type and site exposure. Includes radiator size recommendation.
Affects target temperature and ventilation rate used in the heat loss calculation.
Sets wall U-value and pre-fills glazing type and above/below — you can override any of these after selecting. For properties with retrofitted insulation, choose the age profile closest to your current wall construction.
Heat pumps run at lower flow temperatures — radiators must be significantly larger (~1.95×) to output the same heat as at ΔT50.
| Radiator | BTU/hr | % of Req. |
|---|
Results are indicative estimates for radiator sizing guidance only. This calculator does not replace a full room-by-room heat loss survey to BS EN 12831, which is mandatory for heat pump system specification and commissioning. Always consult a Gas Safe or MCS-certified engineer before specifying heating equipment.
Heating Guide
How to Calculate Radiator BTU
Choosing the wrong radiator size leads to a permanently cold room or wasted energy. Accurate sizing means calculating the actual heat loss from every surface of the room — not just a rough volume estimate.
| Room size | Volume @ 2.4m | Modern (post-2010) | 1980s+ insulated | Pre-1919 solid wall | Suggested radiator |
|---|---|---|---|---|---|
| 5 m² | 12 m³ | 1,500–2,000 | 2,000–2,500 | 3,000–3,800 | Type 11, 600×600 |
| 10 m² | 24 m³ | 2,800–3,500 | 3,500–4,500 | 5,500–7,000 | Type 21, 600×800 |
| 12 m² (small bed) | 29 m³ | 3,200–4,000 | 4,000–5,200 | 6,500–8,500 | Type 21, 600×900 |
| 15 m² (double bed) | 36 m³ | 4,000–5,000 | 5,200–6,500 | 8,000–10,500 | Type 22, 600×1000 |
| 18 m² (small lounge) | 43 m³ | 5,000–6,200 | 6,500–8,000 | 10,000–13,000 | Type 22, 600×1200 |
| 20 m² (lounge) | 48 m³ | 5,800–7,000 | 7,500–9,000 | 11,500–15,000 | Type 22, 600×1400 |
| 25 m² (large lounge) | 60 m³ | 7,200–8,800 | 9,500–11,500 | 14,500–19,000 | Type 33, 600×1400 / 2× Type 22 |
| 30 m² (open plan) | 72 m³ | 8,600–10,500 | 11,500–14,000 | 17,500–23,000 | 2× Type 22 / Type 33, 700×1600 |
| 40 m² (large open plan) | 96 m³ | 11,500–14,000 | 15,000–18,500 | 23,000–30,000 | 2× Type 33 / split-load |
| Radiator (H×W mm) | Type 11 (single) | Type 21 (double + 1 conv.) | Type 22 (double + 2 conv.) | Type 33 (triple) |
|---|---|---|---|---|
| 600 × 600 | 1,860 BTU | 2,700 BTU | 3,500 BTU | 4,800 BTU |
| 600 × 800 | 2,480 BTU | 3,600 BTU | 4,650 BTU | 6,400 BTU |
| 600 × 1000 | 3,100 BTU | 4,500 BTU | 5,800 BTU | 8,000 BTU |
| 600 × 1200 | 3,720 BTU | 5,400 BTU | 7,000 BTU | 9,600 BTU |
| 600 × 1400 | 4,340 BTU | 6,300 BTU | 8,150 BTU | 11,200 BTU |
| 600 × 1600 | 4,960 BTU | 7,200 BTU | 9,300 BTU | 12,800 BTU |
| 600 × 1800 | 5,580 BTU | 8,100 BTU | 10,450 BTU | 14,400 BTU |
| 600 × 2000 | 6,200 BTU | 9,000 BTU | 11,600 BTU | 16,000 BTU |
| 700 × 1600 | 5,800 BTU | 8,400 BTU | 10,850 BTU | 14,950 BTU |
The Professional Standard — BS EN 442
Modern heating specification in the UK is governed by BS EN 442, which ensures radiator outputs are tested under standardised conditions — typically ΔT50°C (mean water temperature of 75°C, room temperature of 20°C). When selecting a radiator, always match its catalogue output to the BTU requirement calculated here. Our tool aligns with BS EN 442 test benchmarks and the CIBSE 2026 Domestic Heating Design Guide.
How Much Should Radiator Output Exceed Room Heat Loss?
Radiator output should exceed calculated room heat loss by 10% as the minimum design margin — standard UK heating engineering practice and included automatically in our calculator. The margin compensates for radiator fouling over time, occasional sub-design outdoor temperatures, and minor calculation variances.
| Scenario | Recommended margin over heat loss |
|---|---|
| Standard gas boiler, well-insulated property | +10% (minimum) |
| Standard gas boiler, older or draughty property | +15–20% |
| Combi boiler in cold or exposed location | +15% |
| Air Source Heat Pump (already × 1.95) | +10% on top of ΔT30 correction |
| Hard-to-reheat space (utility, conservatory) | +25–30% |
| Bathroom (heated by radiator only) | +20% (higher target temperature) |
Mild oversizing has no downside if the radiator has a TRV fitted — it heats the room faster, then closes. Significant oversizing (40%+) can cause boiler short-cycling in well-insulated rooms, reducing condensing efficiency. Never undersize: an underpowered radiator runs flat-out on cold days, leaves the room cold, and forces the boiler to overwork.
Why Property Age Matters So Much
A Victorian terraced house loses heat 4–8× faster than a modern new build. Pre-1919 solid brick walls have a U-value of ~2.1 W/m²K — modern insulated cavity walls achieve 0.18 W/m²K. The same 5×4m living room needs roughly 3,500 BTU/hr in a new build but over 8,000 BTU/hr in an unimproved Victorian house. Always select the correct property age and use the above/below selectors to account for unheated spaces.
Heat Pumps Require Larger Radiators
Standard radiator outputs are rated at ΔT50. Air Source Heat Pumps run at lower flow temperatures (typically ΔT30), where a radiator only delivers ~51.3% of its ΔT50 rating. This means ASHP-heated rooms need radiators approximately 1.95× larger than the raw BTU calculation suggests. Our calculator applies this correction automatically when you select ASHP. This is the single most common specification error when retrofitting heat pumps.
Heat Pump Radiator Sizing — Worked Examples
Air Source Heat Pumps run at ΔT30 rather than ΔT50, meaning each radiator delivers approximately 51.3% of its catalogue BTU rating. For the same room, an ASHP-heated system needs radiators roughly 1.95× larger than a gas boiler system.
| Room | Heat loss | Gas boiler radiator (ΔT50) | ASHP radiator (ΔT30) |
|---|---|---|---|
| Modern 15 m² bedroom | 4,500 BTU | Type 22, 600×800 (4,650 BTU) | Type 22, 600×1600 (~9,300 rated) |
| 1980s 20 m² lounge | 8,000 BTU | Type 22, 600×1400 (8,150 BTU) | Type 33, 600×1600 (12,800) or 2× Type 22 |
| Pre-1919 25 m² lounge | 16,000 BTU | Type 33, 600×2000 (16,000 BTU) | 2× Type 33, 600×1800 — or insulation retrofit first |
For ASHP retrofits the most common error is keeping the original radiators sized for ΔT50, leaving a heat pump that cannot maintain target temperatures on cold days. An MCS-certified installer must complete a room-by-room heat loss survey to BS EN 12831 — our calculator provides the design figure, not the final specification.
BTU vs Watts — Which Should You Use?
UK central heating radiators are almost always catalogued in BTU/hr. Electric radiators and towel rails are usually rated in Watts. The conversion is simple: 1 kW = 3,412 BTU/hr. Our calculator provides both. For professional specifications and heat pump design, kW is the preferred unit.
Are Two Small Radiators Better Than One Large One?
Often yes. In a large or long room, placing two smaller radiators at opposite ends distributes heat far more evenly than one massive radiator, eliminating cold spots. A single large radiator creates a strong convection current on one side of the room only. For rooms over about 20 m², our calculator automatically suggests splitting the load across two radiators.
Common Questions