Drainage Capacity Calculator

Calculator Inputs

This calculator uses the Manning equation for steady, uniform flow:

Design discharge applies a factor: Qd = Q × factor. Use it to account for debris, aging, or partial blockage.

1. Select a unit system and section type.
2. Enter roughness, slope, and a design factor.
3. Provide the required geometry inputs.
4. Press Calculate Capacity to view results.
5. Use CSV or PDF downloads to share outputs.

Examples assume uniform flow and do not replace local codes, inlet controls, or tailwater checks.

Drainage capacity is the maximum flow a pipe or open channel can carry without surcharging, overtopping, or causing unacceptable velocities. In construction, capacity checks support stormwater routing, foundation protection, site grading, trench dewatering layouts, and temporary diversion works. A fast estimate helps you compare options early, then refine the final design with project specifications and local standards.

This calculator uses the Manning relationship for steady, uniform flow. The method links geometry, slope, and surface roughness to average velocity and discharge. Uniform flow assumptions work best when the section is long enough for the water surface to become parallel to the channel invert, and when entrance losses and downstream backwater are not dominant. If the line discharges into a full manhole, culvert, or tidal outfall, always confirm tailwater conditions separately.

Geometry drives performance through the flow area (A) and hydraulic radius (R = A/P). A larger hydraulic radius typically reduces resistance and increases capacity. Use the Circular Pipe (Full Flow) option for gravity drains operating full, the Rectangular option for box sections or lined drains, and the Trapezoidal option for swales and earth channels. For trapezoids, the side slope (H:V) changes both area and wetted perimeter, so small grading changes can meaningfully affect capacity.

Roughness (n) represents lining resistance and surface irregularity. Smooth concrete and PVC typically use lower n values than rubble channels or grassed swales. If you are uncertain, choose a conservative n and apply a design factor below 1.00 to reflect aging, debris, minor deformation, or partial blockage. The calculator also reports velocity; very low velocities may allow sediment deposition, while very high velocities can create erosion or abrasion risks.

The example table illustrates how inputs change results. A 450 mm full-flow pipe at 0.5% slope with n = 0.013 carries about 0.2016 m³/s (≈ 201.6 L/s). A rectangular channel 0.60 m wide and 0.40 m deep at 0.2% slope with n = 0.015 produces a similar discharge, showing how different shapes can meet the same target flow. The trapezoidal example increases capacity by combining greater area with favorable geometry.

For storm drainage, pair capacity with runoff estimates and check that selected sizes maintain self-cleansing flow during typical events. Use the downloads to document your assumptions, then validate the final design using project drawings, allowable slopes, minimum cover, construction tolerances, and any required hydraulic grade line checks. These outputs are a strong starting point for estimating pipe sizes, channel sections, and comparative options during planning and budgeting.

1) What does Manning roughness (n) represent?

It reflects energy loss from surface texture, joints, vegetation, and lining condition. Lower n means smoother surfaces and higher capacity. Use values consistent with the installed material and expected wear.

2) Should I enter slope as percent or ratio?

Either is fine. Percent is common for grading (e.g., 0.5%). Ratio is dimensionless (e.g., 0.005). The calculator converts percent to ratio internally before computing velocity and discharge.

3) Does this include inlet, bend, or outlet losses?

No. Manning capacity assumes uniform flow without localized losses. For short runs, sharp bends, screens, culvert inlets, or energy dissipators, add appropriate minor-loss checks or use a full hydraulic grade line analysis.

4) Why is the design factor limited to 1.00?

The factor reduces theoretical capacity to a more practical value. Using 0.85–0.95 is common when you expect debris, sediment, roughness increase, or partial flow restrictions over time.

5) Can I size partially full pipes with this tool?

This version focuses on full-flow circular pipes and open-channel sections. For partially full pipes, the wetted area and perimeter change with depth. Use a partial-flow chart or a tool that models circular segments by depth.

6) What velocity range is generally acceptable?

Acceptable velocity depends on lining and soil protection. Low velocities can promote siltation, while high velocities can erode channels or scour outlets. Compare the reported velocity to your specifications and protective measures.

7) How can I validate results on site?

Confirm dimensions from drawings, measure actual slope, verify material and lining, and check for tailwater or backflow. If flows are critical, perform a hydraulic grade line check and review maintenance access for debris control.