Culvert Size Calculator
Manning full-flow capacity for round culvert pipe.
What it does
A culvert is only as good as its barrel. Given diameter, slope, and wall roughness, Manning's equation caps how much water the pipe passes flowing full. That ceiling decides whether the driveway stays dry in a hard rain.
The tool above computes full-flow capacity and velocity for round pipes. Imperial mode reports cfs and ft/s; metric mode reports L/s and m/s. Pick the material to set roughness, then compare the verdict against self-cleaning bands.
FHWA culvert hydraulics guidance and state drainage manuals use this same Manning form for barrel analysis, with 1.486 in imperial units and 1.0 in metric. Rougher corrugated walls carry less than smooth concrete or plastic at equal grade, which the material menu reflects.
Use the number for planning, then check the rest of the crossing. Inlet shape, headwater depth, debris, and outlet scour all sit outside the barrel equation, so confirm with inlet control charts and local rules before buying pipe.
How to use
- Pick imperial (inches, cfs) or metric (mm, L/s).
- Enter pipe inside diameter.
- Enter slope percent, not degrees.
- Pick the wall material for roughness.
- Read capacity, velocity, and the flow verdict.
- Size for the design storm and confirm inlet control.
Formula
- D
- Pipe diameter in inches (imperial) or millimeters (metric).
- S
- Slope percent divided by 100.
- n
- Manning roughness: 0.013 concrete, 0.012 plastic, 0.022 corrugated metal.
A is full cross section, R is hydraulic radius (diameter over 4 for a round pipe), S is slope as a decimal, n is Manning roughness. Metric uses 1.0 and imperial 1.486 for unit conversion only; both give the same physical answer.
Worked example
Example: 24 inch pipe at 1 %
24 inch concrete pipe on a 1 percent grade, flowing full.
- Pipe is 24 in concrete at 1.0 % slope.
- Area: 3.1416 ft2, hydraulic radius 0.5 ft.
- Capacity: (1.486 / 0.013) x 3.1416 x 0.5^0.667 x 0.1 = 22.6 cfs.
- Velocity: 22.6 / 3.1416 = 7.2 ft/s.
- Read: Good flow, fast enough to flush sediment.
Reference table
| Wall | n | Note |
|---|---|---|
| Smooth plastic | 0.012 | Fastest of the three |
| Concrete | 0.013 | Common precast value |
| Corrugated metal | 0.022 | Slowest, check coating |
| Helical rib | 0.013 to 0.015 | Per maker tables |
| Rough masonry | 0.025 | Old stone or brick |
Limits
- Barrel only. Inlet control, debris, and tailwater can rule instead.
- Round pipes only; box culverts need their own charts.
- Steady uniform flow assumed; real storms surge.
- Local codes set the design storm and minimum cover.
Common mistakes
- Sizing for average rain instead of the design storm.
- Entering slope in degrees instead of percent.
- Using smooth n for rusted corrugated pipe.
- Skipping inlet and outlet checks.
- Burying shallow pipe with too little cover.
Try it yourself
Quick checks. Answers sit tucked under each question.
Capacity of 18 in concrete at 0.5 %?
Capacity is 7.4 cfs at 4.2 ft/s, rated Good flow.
Capacity of 450 mm concrete at 0.5 %?
Capacity is 202 L/s at 1.27 m/s, rated Good flow.
Questions
Is this the whole culvert answer or just the barrel?
Full-flow capacity, the most the barrel passes before the inlet controls. Real storms add inlet limits, debris, and tailwater, so treat this as the barrel ceiling and check inlet charts for the final answer.
Which roughness value fits my pipe?
Concrete near 0.013, smooth plastic near 0.012, corrugated metal near 0.022. Rougher walls slow water and cut capacity at the same slope. Match the value to the actual pipe material.
What velocity keeps a culvert clean?
About 2 ft/s (0.6 m/s) at low flow. Below that, sand and silt settle and shrink the barrel over time. Above about 10 ft/s (3 m/s), protect the outlet against scour.
Can I use this for a driveway pipe?
Yes, within the tool range. Small pipes clog faster, so many owners round up a size and add a trash rack. Recompute capacity after any change in diameter or slope.
Is slope the same as the ditch grade in degrees?
No. Slope here is fall over length (a 1 ft drop in 100 ft is 1 %). Steeper grades raise capacity and velocity together, which is why flat pipes silt and steep pipes scour.
What else limits a culvert besides the barrel?
Headwater depth, inlet shape, and outlet tailwater. When the pond gets deep, inlet control rules and the barrel equation reads high. FHWA charts cover both regimes.