Water Loading Units Calculator — BS 8558

Loading units (LU) are a numerical value assigned to each type of water outlet to represent its probable demand on the system, taking account of the flow rate, how long the outlet is used and how often. In the BS 8558:2015 / CIPHE loading-unit set, 1 LU = a draw-off flow rate of 0.1 L/s, and typical domestic values are: WC flushing cistern = 2 LU, wash basin = 1.5 LU, bath = 10 LU, shower head = 3 LU, sink = 3 LU, washing machine = 3 LU, dishwasher = 3 LU. (BS EN 806-3 uses a different, lower set — WC 1, basin 1, bath 4, shower 2 — so do not mix the two.)

No, and you should not add one. The BS 8558:2015 design curve that converts loading units to a flow rate (the CIPHE loading-unit conversion graph) already incorporates the probability of simultaneous use, so multiplying the result by a further diversity factor would apply diversity twice and under-size the supply. A separate diversity allowance only applies when you sum actual measured outlet flow rates instead of using the loading-unit curve.

Read the total loading units across the BS 8558:2015 design curve (the CIPHE loading-unit conversion graph used to convert loading units to a simultaneous design flow). Published points on this curve include 24 LU ≈ 0.49 L/s, 40 LU ≈ 0.67 L/s and 100 LU ≈ 1.25 L/s. For example, a typical 3-bed dwelling at about 29 LU gives a peak design flow of roughly 0.54 L/s. This calculator interpolates that published curve and applies no separate diversity factor.

Choose the smallest pipe that keeps the design velocity within about 2 m/s for cold water (erosion-corrosion becomes a risk above roughly 2.5 m/s). Using true copper internal bores to BS EN 1057, a peak flow up to about 0.29 L/s suits 15 mm, up to about 0.64 L/s suits 22 mm, up to about 1.08 L/s suits 28 mm and up to about 1.67 L/s suits 35 mm. Aim for under 1.5 m/s for quiet running, and always confirm the minimum connection size with your local water undertaker. Use our Pipe Sizing Calculator for detailed pipe sizing throughout the property.

Under the Water Supply (Water Fittings) Regulations 1999 you must give your water undertaker advance notice of certain work, including a new connection, a material extension or alteration to the supply, and any installation with a backflow risk above fluid category 3. Notifiable work generally needs consent before it starts, and your supplier also sets the minimum connection size for the incoming main.