What are loading units (LU)?

Loading units represent the relative demand of each outlet on the water supply. They account for both flow rate and frequency of use. A WC has 2 LU while a bath has 10 LU because a bath demands more water when running. The total LUs are converted to simultaneous demand using a probability formula.

When do I need 32mm MDPE instead of 25mm?

A 32mm pipe is typically needed when total loading units exceed about 40, or the pipe run exceeds 20 metres with high demand. Common scenarios include: 3+ bathroom houses, properties with multiple showers, or where incoming pressure is below 2 bar. Your water company can advise on available pressure.

What pressure should I have at the kitchen tap?

Building Regulations Part G requires a minimum of 1 bar (10m head) at the kitchen cold tap with a flow rate of at least 9 L/min. Most appliances need at least 0.5 bar to function. If residual pressure is below 1 bar, a larger pipe, pressure booster, or accumulator tank may be needed.

What is MDPE blue pipe?

MDPE (Medium Density Polyethylene) blue pipe is the standard material for underground cold water supply in the UK. It is colour-coded blue to distinguish it from other services. Available in 25mm and 32mm for domestic use, it is flexible, corrosion-resistant and approved for potable water to BS EN 12201.

How does height affect water pressure?

Every 1 metre of height above the water meter reduces available pressure by 0.1 bar (static head loss). A two-storey house with the meter at ground level loses about 0.3 bar to the first-floor taps. This must be accounted for alongside friction losses in the pipe.

TradeCalculator.co.uk

Water Main Sizing Calculator — Pipe Size by Loading Units

Size your incoming water main using the loading units method to determine simultaneous demand, pipe diameter and residual pressure.

Wash Basins

1.5 LU each

Baths

10 LU each

Showers

3 LU each

WCs

2 LU each

Kitchen Sinks

3 LU each

Washing Machines

3 LU each

Dishwashers

3 LU each

Outside Taps

3 LU each

Main Pipe Length

Distance from meter to property

Meter Pressure

Available pressure at water meter

Price Per Metre (optional)

Installed cost per metre of MDPE

How We Calculate This

This calculator uses the loading units method from BS EN 806 to determine the required water main pipe size based on simultaneous demand.

The formula

Simultaneous Demand (L/s) = 0.114 × LU^0.488

Residual Pressure = Meter Pressure - Friction Loss - Static Head

Loading units table

WH Basin: 1.5 LU
Bath: 10 LU
Shower: 3 LU
WC: 2 LU
Kitchen sink: 3 LU

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.