Home & Construction📅 8 April 2025⏱ 5 min read
How to Calculate the Right Radiator Size for Any Room
An undersized radiator will never heat the room; oversized wastes energy. Here's how to calculate the BTU or watt output you need using room volume, heat loss, and insulation factors.
JW
James WhitfieldPersonal Finance & Maths WriterJames has written about personal finance, health metrics, and everyday mathematics for over six years. He holds a BSc in Mathematics from the University of Leeds.
Radiator sizing is more precise than most DIY guides suggest. A rule of thumb based only on room area ignores ceiling height, window area, insulation quality, and room exposure — all of which have significant effects on heat loss.
The Core Calculation: Heat Loss
Required output (W) = Room volume (m³) × Heat loss factor (W/m³)
Room volume = Length × Width × Ceiling height
Heat loss factor varies by insulation quality:
Poor insulation (pre-1980s, single glazing): 60–80 W/m³
Average insulation (cavity walls, double glazing): 40–60 W/m³
Good insulation (modern, well-insulated): 25–40 W/m³
Excellent insulation (Passivhaus standard): 15–25 W/m³
Worked Example
Living room: 5m × 4m × 2.4m ceiling
= 48 m³
Semi-detached 1980s house, double glazing, cavity walls:
Heat loss factor: 50 W/m³ (average)
Required output = 48 × 50 = 2,400W
One standard double-panel radiator (600mm × 1200mm)
provides approximately 2,500–3,000W at 50°C ΔT ✓
Room Adjustment Factors
FactorAdjustment
External / north-facing room+10–15%
Large window area (>30% of wall)+10%
Poor loft insulation+15%
Solid stone / uninsulated walls+25–35%
Conservatory or extension+15–20%
Corner room (two external walls)+10%
BTU vs Watts Conversion
1 BTU/hr = 0.2931 Watts
1 Watt = 3.412 BTU/hr
2,400W × 3.412 = 8,189 BTU/hr
(Round up to nearest radiator size)
Standard Radiator Output Reference
Radiator Size (mm)TypeOutput at ΔT50 (approx)
400 × 600Single panel (K1)~500W
600 × 600Single panel (K1)~700W
600 × 1000Double panel (K2)~1,800W
600 × 1200Double panel (K2)~2,200W
600 × 1600Double panel (K2)~2,900W
600 × 2000Double panel (K2)~3,600W
Heat Pump Compatibility Note
Radiator output figures are quoted at ΔT50 — meaning the radiator flow temperature is 70°C and the room is 20°C. Heat pumps typically run at 35–55°C flow temperatures. At ΔT30 (45°C flow temperature), a radiator produces approximately 55% of its ΔT50 rating. If installing a heat pump, size radiators generously — typically 1.5–2× the calculated requirement — or use underfloor heating.