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.
Central Heating Pipe Size Calculator results
Fill in the details and select Calculate to see your figures.
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.
How to use the Central Heating Pipe Size Calculator
- Enter the heat load the pipe run carries in kW: for a branch, just the radiators it feeds; for the main, the whole circuit.
- Choose 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.
- Read the water flow rate in litres per second and the minimum copper pipe size that carries it within sensible velocity limits.
- Size 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
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Frequently asked questions
What size pipe do I need for central heating?
It depends on the heat load and the system's temperature difference. As a rough guide at the standard ΔT of 11K and the ~1 m/s maximum flow velocity used in the CIBSE Domestic Heating Design Guide, 15mm copper carries up to about 6.7kW, 22mm up to about 15kW, and 28mm up to about 24kW. Size the main runs for the full boiler output and reduce to 15mm on branches feeding one or two radiators.
How do I work out the water flow rate?
Flow rate in litres per second equals the heat load in kW divided by (4.18 × ΔT). So a 10kW circuit at ΔT 11K needs 10 ÷ (4.18 × 11) = about 0.22 l/s. That flow rate is what determines the pipe size and the pump duty.
What is ΔT and why does it matter?
ΔT is the temperature difference between the flow and return of the heating water. Traditional radiator systems design around 11K. A larger ΔT (such as 20K) means the system carries the same heat with less water flow, which allows smaller pipes and a smaller pump, but the radiators must be sized for the lower mean water temperature.
Can I run 22mm the whole way to save time?
You can, but it's wasteful; oversized pipe holds more water, so the system is slower to warm up and costs more in materials. Good practice is to size the main flow and return for the full load, then step down to 15mm for the final branches to individual radiators.
Does this work for heat pumps?
Heat pumps usually run at a lower flow temperature and a smaller ΔT (often 5K), which means higher flow rates and frequently larger pipework than a boiler system for the same heat. This calculator's 11K/20K options are aimed at boiler systems; a heat-pump design needs the manufacturer's flow rates and a full pipe-sizing exercise.
How can I keep heating jobs profitable?
Sizing pipe and radiators right first time avoids callbacks and cold rooms that eat your margin. Tradehand keeps your quotes, materials and job notes together, and your assistant chases the quote and follows up customers for you, so nothing slips. Get a free demo to see it on your own jobs.
