Culvert Calculator
A culvert calculator using Manning's equation, Q = (1.49/n) × A × R^(2/3) × S^(1/2). An 18 in concrete pipe at 1% slope carries about 10.53 cfs flowing full.
Culvert calculator: full-pipe capacity from Manning’s equation
This culvert calculator estimates how much water a round pipe can carry when it flows full, using Manning’s equation: Q = (1.49 ÷ n) × A × R^(2/3) × S^(1/2). Q is flow in cubic feet per second (cfs), n is the pipe’s roughness, A is the pipe area in square feet, R is the hydraulic radius (diameter ÷ 4 for a full round pipe), and S is the slope as a decimal. The result is also shown in gallons per minute (cfs × 448.831).
Enter the pipe diameter in inches, Manning’s n, and the slope in percent.
What each part of the formula does
- Area (A): a bigger pipe has more room for water. Area grows with diameter squared.
- Hydraulic radius (R): area divided by the wetted wall length. For a full circle that is D ÷ 4. Larger pipes lose less to wall friction for each unit of flow.
- Slope (S): steeper pipes push water faster. Capacity rises with the square root of slope.
- Roughness (n): corrugations slow water near the wall. A higher n means less flow.
Capacity grows with roughly the diameter to the power 8/3, so going up one pipe size often adds a lot of capacity.
Worked examples
- The default: 18 in concrete pipe, n = 0.013, 1% slope
- Diameter: 18 ÷ 12 = 1.5 ft
- Area: 3.1416 × 0.75² = 1.767 sq ft
- Hydraulic radius: 1.5 ÷ 4 = 0.375 ft
- Q = (1.49 ÷ 0.013) × 1.767 × 0.375^(2/3) × 0.01^(1/2) = 10.53 cfs
- In GPM: 10.53 × 448.831 = 4,727 GPM
- Same flow, corrugated metal pipe (n = 0.024)
- 18 in CMP at 1%: 5.71 cfs, about half the concrete pipe
- 24 in CMP at 1%: 12.29 cfs, which beats the 18 in concrete pipe
Corrugated pipe is often cheaper and lighter to handle, but it usually needs to be one size larger to match smooth pipe.
Full-flow capacity table, 1% slope
| Diameter | Concrete (n = 0.013) | Corrugated metal (n = 0.024) |
|---|---|---|
| 12 in | 3.57 cfs | — |
| 15 in | 6.48 cfs | — |
| 18 in | 10.53 cfs | 5.71 cfs |
| 24 in | 22.68 cfs | 12.29 cfs |
| 30 in | — | 22.28 cfs |
| 36 in | — | 36.23 cfs |
Effect of slope (18 in concrete pipe)
| Slope | Capacity |
|---|---|
| 0.5% | 7.45 cfs |
| 1% | 10.53 cfs |
| 2% | 14.90 cfs |
How to measure
- Diameter: use the inside diameter. Corrugated pipe is sold by nominal inside size.
- Slope: subtract the outlet invert (the inside bottom) from the inlet invert, divide by the pipe length, and multiply by 100. A 0.4 ft drop over 40 ft is 1%.
- n value: match the pipe material. If an old pipe is rusted or partly blocked, real roughness is higher.
- Design flow: estimate the peak runoff the culvert must carry. The rainfall calculator can help with volumes, but peak-flow methods for drainage design need local rainfall data.
Common mistakes
- Sizing by full-pipe flow alone. Many culverts are limited by the inlet, not the barrel. A square-edged inlet or a short pipe can pass less than this figure until water ponds upstream.
- Using slope in percent as a decimal. Enter 1 for 1%, not 0.01. The calculator converts it.
- Ignoring sediment. A pipe one third full of silt has far less capacity. Set the bottom slightly below the stream bed only where your design calls for it.
- Skipping outlet protection. Fast water leaving a steep pipe erodes the ditch. Riprap aprons are common; see the rip rap calculator.
For open channels and ditches rather than pipes, use the Manning equation calculator. To find flow from a known velocity, try the flow rate calculator.
A planning estimate for full-pipe flow. Culverts in roads, streams, or regulated drainage should be designed by a licensed civil engineer and permitted under local rules.
Frequently asked questions
What is Manning's n and which value should I use?
Manning's n is a roughness number for the pipe's inside wall. Smooth concrete pipe is commonly taken around 0.013, and standard corrugated metal pipe around 0.024. Smooth-wall plastic is often near concrete or a little lower. Using 0.024 instead of 0.013 for an 18 inch pipe drops full-flow capacity from 10.53 to 5.71 cfs.
What does 'flowing full' mean?
It means water fills the entire pipe cross-section without pressure building behind it. The calculator gives that full-pipe figure, which is a common way to compare pipe sizes. A real culvert's capacity also depends on how water enters the inlet and how deep it can pond upstream.
How do I choose a culvert size?
Find the peak flow your site must pass, often from a design storm and the drainage area, then pick the smallest pipe whose capacity exceeds it with margin. Road and highway culverts are usually designed with inlet and outlet control methods, so treat this calculator as a first check, not the final design.
What happens if I double the slope?
Flow rises with the square root of slope, so doubling the slope increases capacity by about 41%. An 18 inch concrete pipe goes from 10.53 cfs at 1% to 14.90 cfs at 2%. Very steep pipes can cause fast outlet velocities that scour the ditch, so outlet protection may be needed.
Is this enough to size a culvert on a real project?
No. It is a planning-level estimate. Culverts under public roads or in regulated streams typically need permits and a design by a licensed civil engineer that accounts for the design storm, headwater limits, inlet type, and fish passage where it applies.
Estimates only. Verify quantities with your supplier before purchasing.