Radiator Pipe Sizing Calculator — Cumulative Load

You can step down to 15 mm pipe once the cumulative load it carries falls to 6 kW (about 20,500 BTU) or less at an 11 °C ΔT — the maximum the HVCA Central Heating Design Guide allows for 15 mm copper within the 1.5 m/s velocity ceiling recommended by CIBSE Guide G (the same limit appeared in the now-withdrawn BS 5449:1990). 22 mm copper is rated to 13 kW (about 44,000 BTU) at the same ΔT, so keep 22 mm while the section is above 6 kW. These limits scale with ΔT: at a 20 °C drop the capacities rise by roughly 1.8x. Use our Pipe Sizing Calculator for general water supply pipe sizing, or our Radiator Sizing Calculator to find each radiator's BTU load.

Starting from the boiler, add the BTU output of each radiator along the pipe run. The section of pipe between the boiler and the first tee carries the total load of all radiators it serves. After each tee, subtract the BTU of the radiator branching off. The pipe can reduce in size as the load decreases.

CIBSE Guide G caps water velocity at about 1.5 m/s for domestic heating copper pipework (the now-withdrawn BS 5449:1990 used the same ceiling); below that, the HVCA Central Heating Design Guide gives quick-sizing capacities that this calculator applies directly. Higher velocities cause noise (water rush), erosion-corrosion at bends and fittings, and air-entrainment issues. Microbore (8 and 10mm) carries higher friction per metre, so its HVCA limits are low (8mm 1.5 kW, 10mm 2.5 kW at 11°C ΔT) and it is best kept to short single-radiator branches.

Microbore (8 or 10mm) is for individual radiator connections from a manifold or tee. The HVCA Central Heating Design Guide caps 8mm at 1.5 kW (about 5,100 BTU) and 10mm at 2.5 kW (about 8,500 BTU) at 11°C ΔT; many fitters keep each microbore radiator below 3,000-4,000 BTU for quiet running. It must not be used for main runs carrying multiple radiators - this calculator only ever offers microbore for the single furthest radiator branch. Microbore is commonly used with manifold distribution systems.

The HVCA Central Heating Design Guide rates 15mm copper tube (13.6mm internal bore, BS EN 1057 R250) at about 6 kW (roughly 20,500 BTU) at an 11°C ΔT, within the 1.5 m/s velocity ceiling recommended by CIBSE Guide G. That corresponds to a flow of about 7.8 L/min (a gentle 0.9 m/s through the 13.6mm bore, so it is the pressure-drop limit rather than velocity that binds). Capacity scales with ΔT, so on a modern 20°C-drop design the same tube carries roughly 1.8x as much. For very quiet installations many fitters work to about 4-4.5 kW per 15mm section.