MDPE Pipe Sizes: 20mm, 25mm, 32mm, 50mm — Complete Guide

What Size MDPE Pipe Do I Need?

The MDPE pipe size you need depends on the water demand of your property, the distance from the mains connection, and the number of outlets being served. For a typical single dwelling with standard domestic demand, 25mm MDPE pipe is the most commonly specified size. Properties with higher water demand — such as those with multiple bathrooms, combination boilers, or commercial premises — generally require 32mm or larger. The 20mm size is suitable only for short, low-demand branch connections, while 50mm and 63mm pipes serve multi-occupancy buildings and small commercial supplies.

Choosing the correct pipe diameter is one of the most important decisions in any water supply installation. Too small and you’ll suffer poor flow rates, pressure drops, and complaints from occupants. Too large and you’ve spent unnecessarily on materials without any real benefit. This guide covers the full range of MDPE pipe sizes available in the UK, their technical specifications, and how to match pipe diameter to your project requirements.

What Is MDPE Pipe?

MDPE stands for Medium Density Polyethylene — a thermoplastic polymer with a density range of approximately 0.926 to 0.940 g/cm³. It sits between LDPE (Low Density Polyethylene) and HDPE (High Density Polyethylene) in terms of stiffness and flexibility, making it ideally suited to buried water supply applications.

MDPE pipe is manufactured to BS EN 12201 (Polyethylene pipes for water supply) and is the standard material used for below-ground potable water services throughout the UK. Blue MDPE pipe indicates potable (drinking) water use, while black MDPE with a blue stripe is used for the same purpose but indicates above-ground or specific regional preferences.

Key Properties of MDPE Pipe

• Flexibility — Can be coiled and bent around gradual curves without fittings, reducing joints and potential leak points
• Corrosion resistance — Immune to the electrochemical corrosion that affects copper and iron pipes
• Lightweight — Significantly lighter than metallic alternatives, making handling and installation easier
• Long service life — Minimum design life of 50 years when installed correctly
• Chemical resistance — Resistant to most soil contaminants and aggressive ground conditions
• WRAS approved — Suitable for potable water supply when manufactured to the appropriate standard

MDPE Pipe Size Specifications

MDPE pipe is designated by its outside diameter (OD). The internal diameter varies depending on the SDR (Standard Dimension Ratio), which defines the wall thickness relative to the outside diameter. The most common SDR for water supply MDPE in the UK is SDR 11, rated to 12.5 bar (PN12.5), and SDR 17, rated to 10 bar (PN10).

Dimensional Table — SDR 11 (PN12.5)

Outside Diameter (mm)	Wall Thickness (mm)	Internal Diameter (mm)	Weight (kg/m)	Coil Lengths
20	2.0	16.0	0.12	25m, 50m, 100m
25	2.3	20.4	0.17	25m, 50m, 100m
32	3.0	26.0	0.28	25m, 50m, 100m
50	4.6	40.8	0.68	25m, 50m, 100m
63	5.8	51.4	1.07	50m, 100m

Dimensional Table — SDR 17 (PN10)

Outside Diameter (mm)	Wall Thickness (mm)	Internal Diameter (mm)	Weight (kg/m)
20	2.0	16.0	0.11
25	2.0	21.0	0.15
32	2.0	28.0	0.19
50	3.0	44.0	0.46
63	3.8	55.4	0.73

Note: SDR 11 pipe has thicker walls and a higher pressure rating, making it the preferred choice for most mains supply connections. SDR 17 may be used where pressure requirements are lower.

Choosing the Right Pipe Size for Your Application

20mm MDPE Pipe

The smallest commonly available size. 20mm MDPE pipe has a limited flow capacity and is typically only used for:

• Short branch connections from a main supply pipe to an individual outlet
• Garden irrigation feeds
• Low-demand supply connections where the run is very short (under 10 metres)

Most water companies will not accept a 20mm connection as a main service pipe to a dwelling because the flow rate is insufficient for modern domestic demand.

25mm MDPE Pipe — The Standard Domestic Size

25mm is the default size for new domestic water supply connections in the UK. Water companies typically specify 25mm as the minimum for a single dwelling service pipe from the boundary stop tap to the internal stop tap.

Suitable for:

• Standard 3–4 bedroom houses
• Properties with up to 2 bathrooms
• Service pipe runs up to approximately 20–25 metres
• Connections to combination boilers and standard unvented cylinders

A 25mm service pipe at typical mains pressure (1–3 bar) delivers a flow rate of approximately 15–20 litres per minute, which is adequate for running a shower and kitchen tap simultaneously in most domestic situations.

32mm MDPE Pipe

32mm is specified when water demand exceeds what 25mm can comfortably deliver. This is increasingly common in modern homes with multiple bathrooms, high-flow showers, and unvented hot water systems.

Suitable for:

• Larger detached houses with 3+ bathrooms
• Properties with long service pipe runs (over 25 metres)
• Situations where mains pressure is low and a larger bore helps maintain flow
• Small commercial premises such as shops or offices
• Connections serving two or more flats via a shared supply

At typical mains pressure, 32mm pipe delivers approximately 30–35 litres per minute — nearly double the capacity of 25mm.

50mm MDPE Pipe

50mm pipe is used for larger supply mains and distribution pipework rather than individual dwelling connections. It is commonly specified for:

• Multi-occupancy residential buildings (blocks of flats)
• Small to medium commercial properties
• Agricultural and horticultural water supplies
• Shared supply pipes serving multiple properties on a private estate
• Fire hydrant supply connections (subject to water company requirements)

63mm MDPE Pipe

The largest size commonly supplied in coils. 63mm MDPE pipe is used for:

• Larger commercial and industrial supply mains
• Estate distribution pipework serving multiple dwellings
• Situations requiring high flow rates over extended distances
• Connections to larger commercial buildings

Sizes above 63mm (90mm, 110mm, 125mm, 160mm) are typically supplied in straight lengths rather than coils and are used for larger distribution mains. These are generally installed by water companies or specialist civil engineering contractors rather than general plumbing installers.

Understanding Pressure Ratings

All MDPE pipe for potable water supply must carry an appropriate pressure rating. The two most common ratings in the UK are:

• PN12.5 (SDR 11) — Rated to 12.5 bar at 20°C. This is the standard for most domestic and commercial water supply installations
• PN10 (SDR 17) — Rated to 10 bar at 20°C. Used where system pressures are known to be lower

UK mains water pressure typically ranges from 1 bar to 4 bar, with most areas falling between 1.5 and 3 bar. This means PN12.5 pipe provides a substantial safety margin above normal operating pressures, which accounts for transient pressure surges (water hammer) and long-term performance degradation.

What Does SDR Mean?

SDR (Standard Dimension Ratio) is the ratio of the pipe’s outside diameter to its wall thickness. A lower SDR number means a thicker wall relative to the diameter, and therefore a higher pressure rating:

• SDR 11 = OD ÷ Wall Thickness = 11. Higher pressure rating (PN12.5)
• SDR 17 = OD ÷ Wall Thickness = 17. Lower pressure rating (PN10)

WRAS Approval and Compliance

All MDPE pipe used for potable water supply in England and Wales must comply with the Water Supply (Water Fittings) Regulations 1999, commonly referred to as the Water Fittings Regulations. In Scotland, the equivalent is the Water Byelaws 2014.

WRAS (Water Regulations Advisory Scheme) approval provides verification that a product is suitable for use in contact with drinking water. When specifying MDPE pipe, always ensure it carries:

• WRAS approval for potable water use
• Compliance with BS EN 12201 (Polyethylene pipes for water supply)
• The appropriate PE80 or PE100 material designation

PE80 is the traditional material grade for MDPE pipe in the UK. PE100 is a higher-performance grade (technically HDPE) that offers improved pressure ratings for the same wall thickness, but PE80 remains the standard for most domestic MDPE installations.

Installation Considerations by Pipe Size

Minimum Cover Depth

All sizes of MDPE pipe must be buried at a minimum depth of 750mm below finished ground level, or 900mm under roads and driveways, to protect against frost damage and mechanical loads. This applies regardless of pipe diameter.

Bending Radius

One of MDPE’s key advantages is its flexibility. The minimum cold bend radius varies by pipe size:

Pipe Size (mm)	Minimum Bend Radius
20	500mm (25 x OD)
25	625mm (25 x OD)
32	800mm (25 x OD)
50	1,250mm (25 x OD)
63	1,575mm (25 x OD)

The general rule is a minimum bend radius of 25 times the outside diameter for cold bending. Tighter bends require fittings.

Pipe Bedding and Backfill

MDPE pipe should be laid on a bed of granular material (sharp sand or pea gravel) at least 100mm deep, with a further 100mm of the same material covering the pipe before general backfill is placed. This protects the pipe from sharp stones and provides uniform support.

Flow Rate Comparison

To help visualise the practical difference between pipe sizes, here are approximate flow rates at 2 bar supply pressure over a 15-metre run:

Pipe Size	Approximate Flow Rate	Typical Application
20mm	8–12 L/min	Single outlet feed
25mm	15–22 L/min	Single dwelling
32mm	30–40 L/min	Large dwelling / 2 flats
50mm	70–90 L/min	Multi-occupancy block
63mm	110–140 L/min	Commercial / estate main

Note: Actual flow rates depend on supply pressure, pipe length, number of fittings, and elevation changes. These figures are indicative only. For precise sizing, consult your water company’s requirements or use a hydraulic calculation tool.

Frequently Asked Questions

What size MDPE pipe does a water company supply to a house?

Most UK water companies install a 25mm communication pipe from the water main to the property boundary. The homeowner or developer is then responsible for the service pipe from the boundary stop tap to the internal stop tap, which should match or exceed this diameter — typically 25mm for a standard dwelling.

Can I reduce the pipe size from the boundary to the house?

It is not recommended to reduce the pipe diameter between the boundary stop tap and your internal stop tap. Reducing the bore restricts flow and can cause problems with pressure and flow rate, particularly if multiple outlets are in use simultaneously. Always maintain the same diameter or increase it.

Is 25mm MDPE pipe big enough for a 4-bedroom house?

For most standard 4-bedroom houses with up to 2 bathrooms and a run length under 25 metres, 25mm MDPE is adequate. However, if the property has 3 or more bathrooms, a high-flow unvented cylinder, or a long service pipe run, upgrading to 32mm is advisable.

What is the difference between blue and black MDPE pipe?

Blue MDPE pipe is colour-coded specifically for below-ground potable water supply. Black MDPE pipe with a blue stripe serves the same purpose. Black MDPE without any stripe markings may be designated for non-potable applications such as gas or industrial use. Always check the pipe markings and ensure WRAS approval for drinking water installations.

Do I need WRAS-approved MDPE pipe?

Yes. The Water Supply (Water Fittings) Regulations 1999 require that all fittings and materials in contact with potable water are of an appropriate quality. WRAS approval provides the simplest way to demonstrate compliance. Using non-approved products is a regulatory offence and may result in your water company refusing to connect the supply.

Related Kalsi Products

• MDPE Pipe Range — Blue MDPE pipe available in 20mm to 63mm sizes
• MDPE Fittings — Compression fittings for all MDPE pipe sizes
• Underground Drainage Systems — Complete below-ground drainage solutions
• Installation Guides — Step-by-step guides for MDPE and drainage installations
• Technical Resources — Datasheets and specifications for all Kalsi products