Culvert Size Calculator
Size a round culvert with Manning's equation. Get the full-flow capacity in cfs and velocity for a diameter, or the minimum standard pipe size for a design flow — concrete, CMP, or HDPE. Free, no sign-up.
What to calculate next
Tools commonly used alongside this calculation
Rational Method Calculator
Estimate peak stormwater runoff with the Rational Method, Q = C·i·A. Build a weighted runoff coefficient from multiple surfaces, enter the rainfall intensity and area, and get the peak flow in cfs. Free, no sign-up.
Cut and Fill Calculator
Calculate cut and fill earthwork volume with the average end area method. Enter cross-section areas along the alignment to get total cut, fill, and the net import or export in cubic yards. Free, no sign-up.
Explanation
A culvert is the pipe that carries a stream, ditch, or stormwater under a driveway, road, or embankment. Sizing it means matching the pipe’s flow capacity to the water it has to pass. This calculator uses Manning’s equation for a round pipe flowing full: enter a diameter to get the capacity, or enter a design flow to get the smallest standard pipe that carries it — in concrete, plastic, or corrugated metal.
How culvert capacity works
Manning’s equation gives the discharge of a pipe from its size, slope, and inside roughness. For a circular culvert flowing full, the cross-sectional area is π D² ⁄ 4 and the hydraulic radius is simply the diameter divided by four. Steeper slopes and smoother pipes carry more water; rougher corrugated metal carries less at the same size.
| Symbol | Meaning |
|---|---|
| Q | Full-flow discharge (cubic feet per second, cfs) |
| n | Manning’s roughness coefficient of the pipe |
| A | Full cross-sectional area (ft²) |
| R | Hydraulic radius — D ⁄ 4 for a full pipe (ft) |
| S | Slope of the culvert (ft ⁄ ft) |
Roughness and standard sizes
The roughness coefficient depends on the pipe material — a smooth plastic or concrete pipe moves more water than corrugated metal of the same diameter. Round culverts are sold in fixed diameters, so a calculated size is rounded up to the next stocked pipe.
| Material | Manning’s n |
|---|---|
| Concrete (RCP) | 0.013 |
| HDPE, smooth interior | 0.012 |
| PVC / smooth plastic | 0.01 |
| Corrugated metal (CMP 2⅔×½) | 0.024 |
| Corrugated metal, helical | 0.018 |
Standard diameters (in): 12, 15, 18, 21, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78, 84, 90, 96. Values of n are typical design figures; use the manufacturer’s value when known.
Notes and limitations
Velocity matters as well as capacity: below roughly 3 ft/s sediment can settle and clog the pipe, while above about 15 ft/s the outlet may need scour protection such as riprap. The calculator reports the full-flow velocity so you can check both.
This is a gravity full-flow estimate from Manning’s equation — a sound first pass for driveway and ditch-crossing culverts. It does not perform the full HDS-5 analysis of inlet versus outlet control, headwater depth, or tailwater that a highway or regulated crossing requires, and it does not compute the design storm flow itself. Confirm the final design and the required flow with the authority having jurisdiction.