What are water loading units?

Loading units (LU) are a numerical value assigned to each type of water outlet to represent its probable demand on the system. BS EN 806-3 assigns standard values: WC cistern = 2 LU, wash basin = 1.5 LU, bath = 10 LU, shower = 3 LU. They allow engineers to calculate total and simultaneous demand.

What is the diversity factor?

The diversity factor accounts for the fact that not all outlets are used simultaneously. In a domestic dwelling, a factor of 0.7 is typical, meaning peak demand is about 70% of the theoretical maximum. Larger buildings may use lower diversity factors (0.4-0.6).

How do I convert loading units to flow rate?

BS EN 806 provides conversion charts. As an approximation for domestic properties, flow rate (L/s) ≈ 0.12 × √Total LU. For example, 30 total loading units gives approximately 0.12 × 5.48 = 0.66 L/s design flow rate before diversity.

What pipe size do I need for the incoming main?

Pipe size depends on the design flow rate: up to 0.3 L/s = 15mm, up to 0.6 L/s = 22mm, up to 1.2 L/s = 28mm, up to 2.0 L/s = 35mm. Water velocity should be kept below 1.5 m/s to avoid noise and erosion. Check with your local water company for their minimum connection size.

Do I need to notify the water company?

Under the Water Industry Act, you must notify your water supplier before carrying out work that may affect the water supply. New connections, extensions to existing supplies, and installations with backflow risk all require notification. Some work requires approval under the Water Supply (Water Fittings) Regulations 1999.

TradeCalculator

Water Loading Units Calculator — BS EN 806

Calculate loading units per outlet type to determine simultaneous demand, pipe sizing and meter requirements per BS EN 806.

WC Cisterns

2 LU each

Wash Basins

1.5 LU each

Baths

10 LU each

Showers

3 LU each

Kitchen Sinks

3 LU each

Washing Machines

3 LU each

Dishwashers

3 LU each

Outside Taps

5 LU each

How We Calculate This

This calculator uses the BS EN 806 loading unit method to determine the design flow rate for a domestic cold water supply. It then sizes the incoming pipe and meter based on simultaneous demand.

Method

Total LU: Sum of all outlet loading units
Design flow: 0.12 × √(Total LU) L/s
Diverse flow: Design flow × diversity factor
Pipe size: Selected to keep velocity below 1.5 m/s
Velocity: Flow rate ÷ pipe internal cross-sectional area

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.