What this calculator does
A culvert is a pipe carrying a stream or a site flow under a road or embankment. This calculator answers either direction of the question: given a barrel size, how much does it carry; or given a flow, what size barrel do you need. It assumes the barrel runs full, which is the flow condition you design against.
How to use it
- Choose whether you have a size or a flow — the two modes answer opposite questions.
- Enter the barrel gradient; 0.5–1 % is typical for a site culvert.
- Enter the Manning roughness, 0.013 for a smooth plastic pipe and 0.035–0.045 for an unlined earth channel.
- In size mode the result is the smallest standard barrel that passes the design flow.
Governing formula
Q = (1/n)·A·R^(2/3)·S^(1/2) Where:
-
n= Manning's roughness coefficient [–] -
A= Cross-sectional area [m²] -
R= Hydraulic radius = A/P [m] -
S= Bed gradient [m/m]
Derived Equations:
R = D/4 v = Q/A Worked example — 600 mm barrel at 1:200 carrying a site flow
The figures below were produced by running this page's own engine with the example inputs (Calculation mode, Barrel diameter, Gradient (m/m) (m/m), Manning's n (–), Design flow (m³/s)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Full-flow capacity | 0.43 m³/s |
|---|---|
| Full-flow velocity | 1.54 m/s |
| Required barrel size | 0 mm |
| Hydraulic radius (full) | 0.15 m |
| Assessment | Full-flow capacity 0.43 m³/s at 1.5 m/s. Inlet-control headwater must still be checked against HDS-5 nomographs for the design storm. |
- A 600 mm pipe has an area of 0.283 m² and, running full, a hydraulic radius of 0.15 m.
- With n = 0.013 and a 1:200 gradient the full-flow velocity is close to 2 m/s.
- Compare the resulting capacity with the design flow and check the headwater depth at the inlet.
Assumptions & limitations
Engineering calculation. Manning equation for a full circular barrel. Inlet and outlet control, headwater depth and submerged-inlet conditions are not modelled.
- Full-flow only. Real culverts are frequently inlet-controlled, where the headwater depth exceeds the barrel height and the true capacity is lower than this estimate.
- No sediment transport, scour protection or debris blockage allowance.
- Steep culverts may not run full at all; verify with a nomograph such as HDS-5 before finalising the size.
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
- US FHWA HDS-5 — culvert design (inlet and outlet control nomographs).
- BS EN 752-1 to 6 — underground drainage and piping systems.
- Chow, Open-Channel Hydraulics — Manning equation.