Radiator Sizing

Watts/BTU heat output for a room

Room length

Reading 4 m

Room width

Reading 3 m

Ceiling height

Reading 2.4 m

Room type

Insulation level

External walls

Each adds 10% to heat loss.

System flow temperature

Radiators are rated at ΔT50. Lower flow temperatures need a bigger radiator for the same room.

Results

Heat output

2074W

BTU/hr

7076

Room volume28.8 m³

Base rate60 W/m³ (lounge)

Insulation factor×1

External walls2 (×1.2)

Flow temperatureΔT50 — catalogue rating applies as is

MethodRule-of-thumb W/m³ — a first pass, not a heat loss calculation

7100 BTU (ΔT50) compact double-panel radiator

Round up at the merchant.

1 pc

TRV (15mm) — thermostatic valve

1 pc

Lockshield valve (15mm)

1 pc

Radiator wall bracket pair

1 pair

15mm copper pipe

6 m

Pipe clip 15mm

8 pc

Estimates use standard UK trade rules of thumb. Always check the bag/spec on the day.