Free Central Heating Pipe Size Calculator
Size copper central-heating pipe (15, 22 or 28mm) for a given heat load and temperature difference, with the flow rate worked out for you.
Undersized heating pipe means noisy, starved radiators; oversized wastes money and slows warm-up. Enter the heat load a pipe circuit carries in kW and your design temperature difference (ΔT), and this calculator returns the water flow rate and the minimum copper pipe size that carries it within sensible velocity limits.
Central Heating Pipe Size Calculator results
Fill in the details and select Calculate to see your figures.
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Get startedHow to use the Central Heating Pipe Size Calculator
- 1Enter the heat load the pipe run carries in kW: for a branch, just the radiators it feeds; for the main, the whole circuit.
- 2Choose your design temperature difference: 11K is the traditional radiator standard; a larger ΔT (e.g. 20K) carries the same heat at a lower flow, allowing smaller pipes.
- 3Read the water flow rate in litres per second and the minimum copper pipe size that carries it within sensible velocity limits.
- 4Size the main runs for the full load and drop pipe size as the load reduces along branches; don't run 22mm all the way to a single towel rail.
How the Central Heating Pipe Size Calculator works
Pipe size follows the water flow rate, which follows the heat load and the temperature difference the system runs at. A bigger ΔT means less water for the same heat, so smaller pipes.
- 10 kW at ΔT 11K → 10 ÷ (4.18 × 11) = 0.217 l/s.
- 15mm copper carries about 6.7 kW at ΔT 11K, so 10 kW exceeds it; step up.
- 22mm carries about 15 kW at ΔT 11K, so use 22mm for a 10 kW circuit.
- At ΔT 20K the capacities rise ~1.8×, so 15mm would then carry ~12 kW and 22mm ~27 kW.
These capacities are calculated from standard heat-transfer arithmetic (Q = ρ·v·A·cp·ΔT) using BS EN 1057 copper tube bores at the ~1 m/s maximum flow velocity convention set out in the CIBSE Domestic Heating Design Guide, then scaled for ΔT. It's not a full pipe-sizing design; pump head, index circuit and system layout also matter. For a whole-system design use a proper heat-loss and pipe-sizing calculation.
Typical UK benchmarks
| 15mm copper | ≈6.7 kW: calculated from Q = ρ·v·A·cp·ΔT using BS EN 1057 copper tube bore, at the ~1 m/s max flow velocity convention from the CIBSE Domestic Heating Design Guide, ΔT 11K |
| 22mm copper | ≈15 kW: same calculation as 15mm copper, at BS EN 1057's 22mm bore, ~1 m/s, ΔT 11K |
| 28mm copper | ≈24 kW: same calculation as 15mm copper, at BS EN 1057's 28mm bore, ~1 m/s, ΔT 11K |
| Flow rate formula | flow (l/s) = kW ÷ (4.18 × ΔT), where 4.18 kJ/kg·K is the specific heat capacity of water, standard heat-transfer arithmetic (Q = ṁ·cp·ΔT), not a third-party figure |
Disclaimer
This tool gives an estimate for general guidance only, based on typical UK figures and the details you enter. It is not a formal quote, survey, or professional advice. Always confirm with a qualified tradesperson before relying on any figure, as regional prices, site conditions, and current standards vary.
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