Plumbing pipe size calculator
Calculate the right pipe diameter for your flow rate and run length. Covers copper, plastic and steel pipe for cold water supply, hot water, and central heating.
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How to size plumbing pipe correctly
Pipe sizing is one of the most fundamental skills in plumbing and heating. Too small and the system cannot deliver the required flow — taps run weakly, showers lose pressure, radiators do not heat properly. Too large and you waste money on materials, create unnecessarily long heat-up times in hot water circuits, and may end up with velocities too slow to keep debris in suspension.
The core principle is to select a pipe diameter where the resulting flow velocity sits within an acceptable range — fast enough to prevent sediment or Legionella risk, slow enough to avoid noise, erosion, and excessive pressure drop.
Flow velocity limits
UK standards set clear limits on flow velocity in domestic plumbing:
- Cold water supply (BS EN 806): Maximum 2.0 m/s at peak demand points. 3.0 m/s is permitted for short lengths at draw-off points.
- Hot water systems (BS 6700): Maximum 1.5 m/s to avoid noise and corrosion.
- Central heating circuits: Maximum 1.5 m/s. Typically 0.5–1.0 m/s in practice for most domestic systems.
- Minimum velocity: 0.5 m/s in cold water supplies to prevent sediment build-up and reduce Legionella risk in stagnant sections.
Standard UK pipe sizes and applications
| Pipe size | Typical application | Flow range |
|---|---|---|
| 8–10mm copper | Microbore radiator connections | Up to 150 l/hr |
| 15mm copper | Basin taps, WC, radiators up to 2kW | 60–600 l/hr |
| 22mm copper | Showers, baths, main heating circuit | 600–2000 l/hr |
| 28mm copper | Primary boiler circuit, large systems | 1500–4000 l/hr |
| 35–54mm copper | Commercial, high-demand domestic | 3000 l/hr+ |
Pressure drop and pump head
Every metre of pipe and every fitting creates friction resistance, which reduces the pressure available at the end of the run. In mains pressure systems, you need to ensure sufficient residual pressure for each draw-off point. In heating circuits, the pump must have enough head to overcome the total circuit resistance.
The Darcy-Weisbach equation is the industry standard for calculating friction loss. For a rough guide, in 15mm copper pipe carrying 600 l/hr, expect approximately 300–400 Pa per metre of pipe. In 22mm pipe carrying the same flow, this drops to around 50–70 Pa per metre. This is why upsizing pipe dramatically reduces pressure loss over long runs.
When calculating total pressure drop, add an allowance of 15–20% for fittings (elbows, tees, valves). For systems with many fittings, increase this allowance.
Copper vs plastic pipe
Both materials are widely used in UK domestic plumbing. Copper has a longer track record, is compatible with all fittings and solvents, and has slightly lower friction losses due to its smoother internal surface. Plastic (PEX, polybutylene, or barrier pipe) is lighter, faster to install, less prone to burst from freezing, and increasingly preferred for underfloor heating and heating circuits.
For central heating systems, always use oxygen barrier pipe if using plastic — standard plastic allows oxygen permeation which accelerates corrosion of boiler heat exchangers and radiators.
Relevant British Standards
- BS EN 806-3: Design for water supply inside buildings, including pipe sizing
- BS 6700: Specification for design, installation, testing and maintenance of hot and cold water systems
- BS EN 12828: Heating systems in buildings — design for water-based heating systems
- WRAS Water Fittings Regulations: All fittings and materials in contact with drinking water must be WRAS approved
For plumbers putting together quotes, our quote generator helps you create professional quotes that break down materials and labour clearly. See also our CIS deduction calculator if you work as a subcontractor in the construction industry.
Frequently asked questions
What pipe size do I need for a UK shower?
Most electric showers and thermostatic showers require a 15mm supply pipe. High-flow showers (above 12 litres per minute) may benefit from 22mm to maintain adequate flow pressure. If you have multiple showers on a single branch, 22mm is usually recommended for the main run feeding them.
What is the maximum velocity for water in a copper pipe?
The recommended maximum flow velocity in copper pipe for cold water supply is 2.0 m/s. For hot water and central heating systems, 1.5 m/s is recommended to avoid noise, erosion corrosion, and accelerated wear on fittings. These limits are set by BS EN 806 and BS 6700.
What size pipe do I need for a central heating radiator?
Individual radiators of up to 1.5kW are typically fed with 8mm or 10mm microbore pipe. Standard 15mm copper or plastic is used for radiators up to about 3kW and for most main heating runs. 22mm is used for main flow and return circuits connecting multiple radiators. Large systems with boilers over 20kW often use 28mm or larger on the primary circuit.
What is the difference between nominal pipe size and internal diameter?
Copper and plastic pipe is sold by its nominal outside diameter (e.g. 15mm, 22mm). The internal diameter depends on the wall thickness. A standard 15mm copper pipe has an internal diameter of around 13mm. Flow calculations must use the internal diameter, not the nominal size — which is why using a calculator like this is important.
Do I need to size pipes differently for a heat pump vs a gas boiler?
Heat pump systems typically run at lower flow temperatures and higher flow rates than gas boilers to deliver the same heat output. This often means upgrading pipework from 15mm to 22mm on radiator circuits for heat pump retrofits. The principles of velocity-based sizing still apply, but the flow rates required per kW of heat are higher.
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