How Much Radiator Do I Need? BTU & kW by Room
Quick answer: size by room volume — about 4 BTU per cubic foot — plus 1,000 BTU per window, adjusted for insulation. A 12×12 room with one window needs about 5,600 BTU (1.6 kW).
The rule of thumb
The classic radiator rule: 4 BTU per cubic foot of room volume — about 32 BTU/sq ft at 8-ft ceilings — plus 1,000 BTU per standard window. Multiply by an insulation factor: 0.8 well-insulated, 1.0 average, 1.25 older homes. It's a rule of thumb, not a Manual J heat-loss analysis, but it lands within 10% for most rooms.
Room-by-room table
| Room | Area | BTU/hr | kW | Typical unit |
|---|---|---|---|---|
| 8×8 ft | 64 sq ft | 3,000 | 0.9 | Single panel / 3 column |
| 10×10 ft | 100 sq ft | 4,100 | 1.2 | Double panel 600–800 mm |
| 12×12 ft | 144 sq ft | 5,600 | 1.6 | Double panel 1,200–1,400 mm |
| 14×12 ft | 168 sq ft | 6,400 | 1.9 | Large double / 4–5 column |
| 16×14 ft | 224 sq ft | 8,100 | 2.4 | Double panel + vertical |
| 20×16 ft | 320 sq ft | 10,900 | 3.2 | Two radiators |
Figures assume one window, 8-ft ceilings and average insulation. Add ~15% for a cathedral ceiling, a patio door, or a below-grade cold room.
Windows, insulation & ceilings
- Windows: +1,000 BTU per 2×3-ft window, +1,500 for patio doors. North-facing windows leak more in winter.
- Ceiling height: every foot over 8 ft adds ~10% — volume already captures this if you measure to the actual ceiling.
- Insulation: well-insulated (0.8), average (1.0), uninsulated or old homes (1.25). Add 10–15% for rooms on concrete slabs or over garages.
- Radiation vs convection: panel radiators convect; cast-iron radiates. Both are rated at ΔT 50°C — compare at the same ΔT.
BTU, kW & EDR
1 kW = 3,412 BTU/hr. UK radiator specs list both; US cast-iron radiators are often rated in EDR (equivalent direct radiation) — 1 sq ft EDR ≈ 240 BTU/hr. Multiply EDR by 240 to compare with this guide's numbers. Whichever unit you shop in, the target is the same: the radiator's rated output at your water temperature must beat the room's heat loss.
Sizing mistakes
- Oversizing "to be safe" — a too-big radiator short-cycles, overheats the room and wastes fuel.
- Sizing by square feet alone — tall rooms need volume-based math.
- Ignoring the window wall — a radiator under a big window must cover that loss.
- Mixing ΔT ratings — a 2,000-BTU unit at ΔT 50°C delivers less at ΔT 30°C.
- One radiator, two rooms — the far room always runs cold; unit per room wins.
Size it once, stay warm
Run your room through the radiator size calculator, compare at the same ΔT, and pick a unit within 10–15% of the load. Right-sized radiators heat steady and quiet — and your boiler cycles like it should.