Drain pipe calculator
Each sink, shower, WC and appliance drains to the soil stack. The calculator estimates the pipe from the number of drain points and their average distance to the stack.
Your measurements
Result
- Pipe needed (incl. 10% reserve)
- 22.0 m
Includes 10% for bends; the minimum fall is 1–2 cm per metre.
How the calculation works
- Base length = number of drain points × average distance to the stack.
- Length to order = base length + 10% for bends and branches.
Worked example
For 5 drains (basin, shower, WC, sink, washing machine) at an average of 4 m: 5 × 4 = 20 m, plus 10%, so 22 m of pipe.
What the calculator assumes
- Each drain is assumed to have its own run; in practice several can share a common branch.
When to adjust the result
- Diameters differ: about 32–40 mm for basins, 40–50 mm for sinks, showers and washing machines, 110 mm for WCs.
- The minimum fall is 1–2 cm per metre — over long runs check that it fits within the floor build-up.
- Bends, branches, traps and access fittings are not included.
Getting drains to flow
Gradient matters as much as diameter. Small branch pipes typically run at about 2% (2 cm per metre) and larger ones at 1–2%; too flat and solids settle, too steep and water outruns the solids. Keep runs as short and straight as possible and use long-radius bends or two 45° bends instead of a sharp 90°.
Every appliance needs a trap, and the system needs venting so traps are not sucked dry when a WC flushes. Fit access points (rodding eyes) at changes of direction so blockages can be cleared. Drainage connected to public sewers is usually subject to building regulations — check before altering it.
Frequently asked questions
What fall does a waste pipe need?
Commonly 1–2 cm per metre (1–2%); follow local plumbing rules.
What diameter for a WC drain?
Usually 100–110 mm.
What pipe size for a shower or basin?
Basins commonly use 32–40 mm, showers and baths 40–50 mm, and WCs 100–110 mm. Follow the appliance manufacturer and local building rules.
Why does my sink gurgle?
Usually poor venting or a partial blockage further down the pipe, which pulls air through the trap when water drains.