What Size Pipe Do I Need? Domestic Plumbing Pipe Sizing Guide

Summary

Correct pipe sizing is fundamental to every domestic plumbing and heating installation. Undersized pipes cause poor flow rates, noisy pipework, and customer complaints; oversized pipes waste materials and increase heat loss. In the UK, copper pipe to BS EN 1057 (formerly BS 2871 Table X) remains the dominant material for internal domestic pipework, available in standard metric sizes: 8mm, 10mm, 15mm, 22mm, 28mm, 35mm, and 42mm. For water supply, sizing is governed by the design flow rates in BS 6700 / BS EN 806-3, the number of outlets served, and available mains pressure. For central heating, pipe size is determined by the heat load (kW) the pipe must carry, the pump head available, and the system temperature differential.

Key Facts

• 15mm is the workhorse of domestic plumbing -- used for individual cold and hot water branches, radiator tails, and single fixture feeds
• 22mm is standard for the rising main (internal), hot water cylinder feeds, bath cold feeds, and primary heating flow and return in systems up to approximately 15 kW
• 28mm is used for larger heating circuits, gravity-fed primary circuits, and boiler connections on systems above approximately 20 kW
• External supply pipes from the boundary stopcock are typically 25mm blue MDPE (polyethylene) to BS EN 12201, buried at a minimum depth of 750mm
• Copper pipe to BS EN 1057 has replaced the old BS 2871 Table X/Y/Z classifications but the same nominal sizes and wall thicknesses apply
• The Water Supply (Water Fittings) Regulations 1999 require that pipe sizing delivers adequate flow without risk of contamination from backflow, stagnation, or excessive velocity
• Maximum recommended water velocity in copper pipe is 3 m/s for cold water and 2 m/s for hot water to prevent erosion and noise (BS 6700)
• For heating, design to a maximum velocity of 1.0-1.5 m/s in copper to avoid noise; size pipes to carry the required kW at the system temperature differential

Detailed Guidance

What are the standard UK copper pipe dimensions?

All domestic copper pipe sold in the UK conforms to BS EN 1057. The "size" refers to the outside diameter (OD) -- the internal diameter depends on wall thickness.

When should I use 15mm pipe?

Use 15mm for individual branch runs to single fixtures and for radiator connections. Standard applications:

• Basin hot and cold taps
• WC cistern supply
• Individual shower feeds (where supply pressure is adequate)
• Kitchen sink hot and cold (short runs under approximately 3m)
• Radiator flow and return tails
• Outside tap supply
• Washing machine and dishwasher supply valves

Limitations: 15mm struggles to deliver adequate flow over long runs (above 8-10m) or where multiple fixtures draw simultaneously. If you are running a long branch to a kitchen or en-suite, consider upsizing to 22mm and reducing down at the fixture.

When should I use 22mm pipe?

Use 22mm for distribution pipework -- any pipe that feeds more than one fixture or carries a significant flow rate:

• Internal rising main from the incoming supply
• Cold water distribution from the rising main to bathroom groups
• Hot water distribution from the cylinder or combi boiler
• Bath cold feed (baths demand 0.3 l/s per BS 6700 -- too much for a long 15mm run)
• Primary heating flow and return up to approximately 15 kW
• Hot water cylinder connections (both cold feed in and hot out)
• Feed and expansion pipework on open-vented systems

When should I use 28mm pipe?

Use 28mm where the heat load or water demand exceeds what 22mm can handle:

• Heating flow and return on systems above approximately 20 kW
• Boiler connections on larger system boilers (check manufacturer requirements)
• Gravity-fed primary circuits (low velocity demands larger bore)
• Cold water distribution in properties with multiple bathrooms drawing simultaneously
• Open-vent pipe and cold feed on vented heating systems (check manufacturer specs)

What are the design flow rates for domestic fixtures?

These are the design flow rates from BS 6700 used to size pipework. The "minimum" column is the lowest acceptable flow at the outlet.

Note: These are flow rates at the point of use, not necessarily the flow in the supply pipe. Simultaneous demand calculations reduce the total because not all fixtures run at once.

How do I size pipes for central heating?

Central heating pipe sizing depends on three factors: the heat load (kW) downstream, the temperature differential between flow and return, and the pump head available.

Modern condensing boilers are designed for a 20 degree C temperature differential (e.g. 70 degree C flow / 50 degree C return). Heat pumps typically work at a 5 degree C differential, requiring significantly larger pipes.

Heating pipe capacity -- copper pipe at 1.0 m/s velocity, 20 degree C differential (gas boiler)

Practical heating pipe sizing rules

Important: These are working guidelines for copper pipe with a gas boiler at 20 degree C DT. For heat pump systems at 5 degree C DT, you need roughly four times the pipe capacity -- typically one or two sizes larger. Always check the heat pump manufacturer's installation manual for specific requirements.

How does mains pressure affect pipe sizing?

Available mains pressure directly affects what pipe size will deliver adequate flow. UK mains pressure typically ranges from 1 bar to 4 bar (10-40m head), with most properties receiving 2-3 bar.

Key considerations:

• Low pressure areas (below 1.5 bar): Consider upsizing distribution pipes from 15mm to 22mm. Combi boilers may struggle -- check the minimum inlet pressure requirement (typically 0.5-1.0 bar)
• High pressure areas (above 4 bar): A pressure reducing valve (PRV) is required by the Water Supply Regulations. Size pipes as normal
• Long supply pipe runs: Every metre of pipe, every fitting, and every change of direction causes pressure loss. A 25m run of 15mm pipe at 0.3 l/s loses approximately 0.5 bar
• Height: Every 1m of vertical rise costs 0.1 bar. A first-floor bathroom is already 0.3 bar down on ground-floor pressure

What size should the rising main be?

The rising main (the pipe from the internal stopcock up through the property) should be:

• 22mm copper for most domestic properties with a single bathroom
• 22mm copper for properties with two bathrooms if mains pressure is above 2 bar and runs are short
• 28mm copper for larger properties with two or more bathrooms, or where the incoming supply is 32mm MDPE
• Match or upsize from the incoming MDPE supply (25mm MDPE has a similar bore to 22mm copper)

What about plastic pipe (push-fit / press-fit)?

Plastic barrier pipe (e.g. Hep2O, JG Speedfit, Polypipe) is widely used in domestic plumbing and heating. The sizing principles are the same, but note:

• Plastic pipe has a thicker wall than copper for the same nominal OD, meaning a smaller internal bore
• 15mm plastic has an internal diameter of approximately 10mm (vs 13.6mm for copper) -- significantly less flow capacity
• Where you would use 15mm copper, you may need to upsize to 22mm plastic on longer runs
• Always check the manufacturer's flow rate data for the specific product

This reduced bore means pressure drop in plastic pipe is substantially higher than copper for the same nominal size. In practice, many installers upsize plastic by one increment on longer runs.

Frequently Asked Questions

Can I run a bath on 15mm pipe?

In theory, yes -- but in practice it depends on the run length and available pressure. A bath demands 0.30 l/s (BS 6700 design flow rate). A short 15mm run of 1-2m from a 22mm distribution pipe will usually deliver this, but a longer dedicated 15mm run (over 3-4m) will struggle to maintain adequate flow, especially if other fixtures are in use. Best practice is to run 22mm to the bath and reduce to 15mm at the taps only. Your customer will thank you when the bath does not take 20 minutes to fill.

Do I need 28mm pipe for a combi boiler?

Most combi boilers have 22mm flow and return connections, and 22mm is adequate for the vast majority of domestic systems up to approximately 15 kW heating output. The manufacturer's installation manual is the definitive guide -- always check it. Some higher-output combis (35 kW+) may specify 28mm, particularly on the heating side in larger properties with long pipe runs.

What size pipe do I need for an outside tap?

A 15mm branch taken off the nearest cold water pipe is standard. If the run is over 10m, or if you need high flow for a garden hose (0.30 l/s), consider using 22mm and reducing at the tap. The branch must include a double check valve or other backflow prevention device as required by the Water Supply (Water Fittings) Regulations 1999, Schedule 2, Section 6.

Should I use 10mm or 15mm for radiator tails?

Both are acceptable for standard panel radiators. 15mm is the industry default and causes fewer flow restriction issues. 10mm (or 8mm micro-bore) is used in some micro-bore heating systems, but it is more prone to blockage from sludge and has a higher pressure drop. For new installations, 15mm is recommended unless the system has been specifically designed for micro-bore.

How do I size pipes for a heat pump system?

Heat pumps operate at a much lower temperature differential (typically 5 degree C vs 20 degree C for a gas boiler), meaning they need four times the flow rate to deliver the same heat output. This typically means:

• Radiator tails: 15mm (same as conventional, as individual radiator loads are modest)
• Branch pipes: 22mm where you would normally use 15mm
• Main flow/return: 28mm or even 35mm where you would normally use 22mm
• Always follow the heat pump manufacturer's sizing tables

Regulations & Standards

• Water Supply (Water Fittings) Regulations 1999 (SI 1999/1148) -- the primary legal requirement for all water fittings installations in England and Wales. Covers backflow prevention, materials, and installation standards. Enforced by water undertakers.

• Water Byelaws 2014 -- the Scottish equivalent of the above, administered by the Scottish Water Byelaws 2014.

• BS EN 806-3:2006 -- Design of inside installations for hot and cold water. Provides the loading unit method for pipe sizing in multi-residential buildings. Referenced by Building Regulations.

• BS 6700:2006+A1:2009 -- Design, installation, testing and maintenance of services supplying water for domestic use within buildings and their curtilages. Contains the design flow rate tables for individual fixtures.

• BS EN 1057:2006+A1:2010 -- Copper and copper alloys. Seamless, round copper tubes for water and gas in sanitary and heating applications. Replaces BS 2871.

• BS EN 12201 -- Plastics piping systems for water supply. Covers MDPE and other polyethylene pipes for external underground supply.

• Building Regulations Approved Document G (England and Wales) -- Sanitation, hot water safety and water efficiency. References BS EN 806 and BS 6700 for pipe sizing.

• CIPHE Plumbing Engineering Services Design Guide -- The Chartered Institute of Plumbing and Heating Engineering's guide to pipe sizing using loading units, with frequency-of-use factors for different building types.

• CIBSE Guide G: Public Health and Plumbing Engineering -- Comprehensive reference for pipe sizing methods and design flow rates.

• BS 6700 Pipe Sizing -- CIBSE/BIP2177 Chapter 5

• Yorkshire Copper Technical Guide -- BS EN 1057 Pipe Data

• P&H Engineering -- CIPHE Loading Units Method

• P&H Engineering -- Hot and Cold Water Pipework Design

• h2x Engineering -- UK Domestic Mains Water Pipe Size Calculator

• CIBSE Technical Bulletin 01 -- Domestic Water Demand Assessment

• Heatweb Wiki -- Pipework Calculations

• diydata.com -- Checking Central Heating Pipe Work Sizes

• Building Regulations Approved Document G (2024 amendments)

• Water Supply (Water Fittings) Regulations 1999 -- legislation.gov.uk

• Engineering Toolbox -- BS 2871 Copper Tube Data

• City Plumbing -- Pipe Sizes Guide

• waste pipes -- Waste pipe sizing and minimum falls

• hot water systems -- Boiler type selection and sizing

• underfloor heating -- UFH pipe spacing and loop lengths