Radiator Size Calculator — BTU & kW per Room
Enter room dimensions, windows and insulation to get the radiator output you need in BTU/hr and kW. No sign-up, no data stored.
Each standard window adds ~1,000 BTU of heat loss; ceiling over 8 ft adds 10% per foot.
Radiator output needed
How radiator size is determined
- Room volume = length × width × ceiling height in cubic feet.
- Base heat load = volume × 4 BTU/ft³ (≈ 32 BTU/sq ft at 8-ft ceilings) for a normally insulated room.
- Windows: +1,000 BTU per standard window (2×3 ft), +1,500 for patio doors.
- Insulation factor: 0.8 well-insulated, 1.0 average, 1.25 older homes.
- Convert: 1 kW = 3,412 BTU/hr.
This is the classic "rule of thumb" sizing (4 BTU per cubic foot base). For a professional calculation use a full Manual J heat-loss analysis — but this gets the radiator size right for 90% of rooms.
Room sizes → radiator output
Common rooms, average insulation, one window, 8-ft ceilings.
| Room | BTU/hr | kW |
|---|---|---|
| 8×8 ft (64 sq ft) | 3,000 | 0.9 |
| 10×10 ft (100 sq ft) | 4,100 | 1.2 |
| 12×12 ft (144 sq ft) | 5,600 | 1.6 |
| 14×12 ft (168 sq ft) | 6,400 | 1.9 |
| 16×14 ft (224 sq ft) | 8,100 | 2.4 |
| 20×16 ft (320 sq ft) | 10,900 | 3.2 |
Last updated: October 2026 · Rule-of-thumb estimates — verify with a heat-loss calculation for extreme climates or cathedral ceilings. Radiator output is rated at ΔT 50°C (UK) or standard EDR (US).
Radiator sizing FAQ
30–45 BTU/sq ft for a normally insulated home with 8-ft ceilings. Cold climates, poor insulation or large windows push it to 50+ BTU/sq ft. Size by volume for tall rooms.
About 5,600 BTU (1.6 kW) with one window and average insulation. In UK terms that's a double panel radiator around 1,200–1,400 mm, or a 4-column cast-iron in US homes.
Oversizing by more than ~20% makes the room overheat in cycles and wastes energy; undersizing runs the boiler flat out and still chills. Match the load within 10–15%.
US cast-iron radiators are rated in EDR (equivalent direct radiation): 1 sq ft EDR ≈ 240 BTU/hr. Multiply EDR by 240 to compare with this calculator's BTU output.
Only with an open plan and a single large unit sized to the combined volume — and even then the far room runs cooler. Separate units per room are the reliable answer.