What is a discharge unit?

A number that represents the average flow a sanitary appliance discharges to the drain, in L/s, used in BS EN 12056-2. For example a 9 litre WC is 2.5 and a washbasin 0.5 in System I. The design flow is K × √(sum of DU).

What gradient should a drain have?

It depends on size, flow and code. Many designs use about 1:40 to 1:80 for 100 mm branches with WCs and flatter slopes for larger sewers, as long as the pipe carries the flow within its filling limit and reaches a self-cleansing velocity (often taken as about 0.7 m/s).

How is peak water demand worked out?

By adding the draw-off rates of all appliances and applying a simultaneity method, such as the BS EN 806-3 loading unit chart, the DIN 1988-300 formula or the Hunter curve (fixture units) in IPC/UPC. Few appliances run at the same time, so peak flow is much lower than the sum.

How do you calculate an invert level?

Downstream invert = upstream invert − length × gradient. For 30 m at 1:80, the fall is 30 ÷ 80 = 0.375 m.

Fixture Units & Drainage Slope Calculator

Fixture Units & Drainage Slope Lab

The plumbing calculations every junior designer meets first: what size water pipe, what size drain and at what slope. Start from a fixture schedule and finish with a long section that a contractor can set out.

Fixture schedule

List the appliances served by the pipe you are sizing (for example one office toilet core). Each appliance has a water and a drainage .

Building use

Load a preset

Water supply sizing

Peak flow is far less than the sum of all taps, because they are rarely used together. The lab applies the simultaneity formula and picks the smallest pipe inside the velocity limit.

Size options

Peak flow against total draw-off

Drainage sizing

BS EN 12056-2 System I (single discharge stack, branches partly full). The waste water flow is K × √ΣDU. Choose the gradient and filling limit; the lab checks each pipe size.

Pipe capacity at this gradient

Colebrook-White with kb = 1.0 mm (BS EN 12056-2 practice for drain capacity tables), water at about 10 °C. Internal diameters are typical.

Gradient and long section

Set the run from the building to the manhole. The lab works out invert levels, depth of cover and the flow in the pipe at the design flow.

Long section

3D drain and 4D flush wave

The branch drain in 3D with its gradient exaggerated so you can see it. Press play to flush a WC and watch the wave travel to the manhole at the calculated velocity.

Drag to rotate · tap a pipe or fitting

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Engineer's notes

Lessons from real design reviews.

Quiz

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