What this calculator does
Take the peak runoff your catchment produces, then find the pipe that carries it. This calculator does exactly that with the Manning equation, and additionally checks the velocity against the self-cleansing minimum — because a pipe that is too large runs slow and silts up permanently.
How to use it
- Enter the peak flow, which should come from the Rational Method or a hydraulic model rather than an average.
- Enter the available gradient; 0.5–1 % is normal for a site storm sewer.
- Enter the Manning roughness and the minimum self-cleansing velocity, 0.75 m/s being the usual rule of thumb.
- A "does not self-clean" verdict usually means the pipe is oversized for the slope — split the run rather than accepting a flat oversized pipe.
Governing formula
Q = (1/n)·A·R^(2/3)·S^(1/2) Where:
-
n= Manning's roughness coefficient [–] -
S= Gradient [m/m]
Derived Equations:
v = (1/n)·R^(2/3)·S^(1/2) Worked example — 0.5 m³/s at a 1:200 gradient
The figures below were produced by running this page's own engine with the example inputs (Design (peak) flow (m³/s), Gradient (m/m) (m/m), Manning's n (–), Minimum self-cleansing velocity (m/s)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Required pipe size | 675 mm |
|---|---|
| Capacity at that size | 0.59 m³/s |
| Velocity when full | 1.66 m/s |
| Self-cleansing velocity met | 1 |
| Assessment | 675 mm pipe at the selected slope runs full at 0.59 m³/s (1.7 m/s). Self-cleansing velocity met. |
- A 450 mm pipe at this gradient cannot pass the flow, so the search moves up the standard sizes.
- The chosen barrel carries the design flow at a velocity above the self-cleansing minimum.
- Check the inlet capacity of the gully or catch pit, which often governs before the pipe does.
Assumptions & limitations
Engineering calculation. Selects the smallest standard pipe that passes the design flow running full, and reports whether the self-cleansing velocity is achieved.
- Assumes the pipe runs full and at uniform flow; partial-flow conditions and backwater effects are not modelled.
- No capacity reduction for inlets, gullies or manholes.
- The self-cleansing velocity criterion is empirical — very large flows can be slower and still acceptable.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS EN 752-4 — underground drainage and piping systems, requirements and construction.
- Local stormwater management manual — minimum velocity criteria.
- Chow, Open-Channel Hydraulics.