Manning Pipe Flow Calculator

Choose a unit system, select a mode, and enter pipe data. For partial flow, enter the water depth. For full flow, depth locks automatically.

The calculator uses the Manning relationship for gravity-driven flow: Q = (k/n) · A · R_h^2/3 · S^1/2

• Q is discharge, V is velocity, and Q = V · A.
• A is wetted area and R_h = A/P is hydraulic radius.
• P is wetted perimeter, and S is slope (energy grade approximation).
• n is Manning roughness, selected from a preset or custom input.
• k is 1.0 in SI and 1.486 in US customary units.

For partially full flow, the wetted cross-section is a circular segment. Using radius r = D/2 and depth y, the half-angle is α = arccos((r − y)/r). Then: A = r²(α − sin(2α)/2) and P = 2αr.

1. Select the unit system that matches your drawings and specs.
2. Choose full flow for closed conduit capacity checks.
3. Choose partial flow when the pipe behaves like an open channel.
4. Pick a material preset or choose custom n for your lining.
5. Enter diameter and slope to compute discharge and velocity.
6. Use solve slope to find the grade for a target discharge.
7. Use solve diameter to size a conduit for target discharge.
8. Download CSV or PDF to attach results to submittals.

These examples show typical inputs and computed flow for gravity applications. Values are illustrative only.

Design intent and typical applications

Manning-based capacity checks are common for storm drains, culverts, gravity sewers, and site conveyance where the hydraulic grade line closely follows the conduit slope. This calculator helps evaluate flow rate, velocity, and wetted geometry for smooth and rough linings. It supports full-flow sizing for capacity and partially full analysis when a pipe behaves like an open channel during normal operation.

Key inputs that control performance

Discharge is most sensitive to slope and roughness. A small increase in slope can raise flow significantly because slope enters as a square root term, while roughness scales flow inversely. Diameter affects both area and hydraulic radius, so capacity increases rapidly as diameter grows. For partially full sections, depth changes area, wetted perimeter, and hydraulic radius, shifting both velocity and conveyance.

Interpreting velocity and stability

Velocity is useful for assessing self-cleansing and scour risk. Higher velocities can reduce sedimentation but may exceed allowable limits for certain materials or bedding conditions. The displayed Froude indicator is a quick check for flow regime in open-channel conditions; values near or above one suggest supercritical tendencies and may require attention at transitions, inlets, or energy dissipation locations.

Using solve modes for engineering checks

Solve flow is ideal for “given D and S, what Q?” checks. Solve slope estimates the grade needed to carry a target flow at a selected diameter and depth. Solve diameter uses a bounded search to find the smallest diameter that meets a target discharge under the chosen slope, roughness, and depth ratio. Review computed velocity alongside local standards before finalizing.

Quality control and field alignment

Confirm that the roughness value matches lining condition, joints, and aging. Verify that slope reflects as-built grades and that depth assumptions are reasonable for the controlling storm or design event. For pressurized or surcharged behavior, a pressure-flow method is more appropriate. Exporting results to CSV or PDF supports design notes, submittals, and peer review documentation for project record retention.

1) When should I use partial flow instead of full flow?

Use partial flow when the conduit is not surcharged and the water surface is free, such as typical storm conveyance at moderate events. Use full flow for maximum capacity checks or when the pipe is expected to run full.

2) Which roughness value should I choose?

Start with the preset that matches the pipe material, then adjust based on lining condition, joints, sediment, and age. Project specifications or agency manuals often provide recommended n ranges for design and verification.

3) Does this calculator apply to pressurized pipes?

No. Manning is intended for gravity-driven, open-channel type flow. If the pipe is pressurized or consistently surcharged, use a pressure-flow method such as Hazen-Williams or Darcy-Weisbach with appropriate loss modeling.

4) Why does diameter increase capacity so quickly?

Diameter increases the wetted area and the hydraulic radius at the same time. Because conveyance scales with area and with hydraulic radius to the two-thirds power, larger diameters can carry much more flow at the same slope.

5) What does the Froude indicator tell me?

It is a quick regime check for open-channel conditions. Values below one are generally subcritical, while values near or above one can indicate rapid flow that may need attention at inlets, outlets, and transitions.

6) How accurate are the exported CSV and PDF files?

The exports capture exactly the inputs and results shown after calculation. Treat them as calculation records, not as stamped design documents. Always verify assumptions, units, and criteria before issuing final drawings or submittals.