Storm Sewer Pipe Sizing Calculator

Overview: Storm sewer pipe sizing with the Manning equation

Underground storm sewer systems collect runoff from streets, parking lots, and roofs and convey it safely to outfalls, detention basins, or treatment systems. Correct pipe sizing is critical: undersized pipes can cause surface flooding, while oversized pipes are costly and may promote sediment deposition and maintenance issues.

This calculator estimates the internal diameter of a circular storm sewer flowing full using the Manning equation. It is intended for preliminary design and educational use, not for final construction documents. The calculations assume uniform, steady flow in a full circular pipe, with the pipe slope approximating the energy grade line slope and no allowance for localized head losses at inlets, manholes, bends, or transitions.

All inputs and outputs on this page use SI units:

• Design flow, Q: cubic metres per second (m³/s)
• Pipe slope: percent (%) along the pipe centreline
• Manning roughness, n: dimensionless coefficient
• Pipe diameter, D: metres (m)
• Average velocity, V: metres per second (m/s)

Key formulas used in this calculator

Manning equation for discharge

For steady, uniform open-channel flow or pipe flow, the Manning equation in SI units is commonly written as:

where:

• Q = discharge (m³/s)
• n = Manning roughness coefficient (dimensionless)
• A = flow area (m²)
• R = hydraulic radius = A / P (m), with P the wetted perimeter
• S = slope of the energy grade line (approximated by pipe slope for uniform full-flow)

Full circular pipe relationships

For a circular pipe flowing completely full, geometry simplifies to:

• Area:
• Hydraulic radius:

Substituting these into the Manning equation and rearranging for diameter gives a closed-form expression of the form:

where C is a constant resulting from the geometric substitutions and unit system. The calculator uses this exact relationship internally to compute the required full-flow diameter for the specified flow, slope, and roughness.

Velocity check

Once the required diameter is known, the average full-flow velocity is evaluated as:

Design guidelines for storm sewers often recommend keeping velocities within a range that avoids both excessive scour and long-term sedimentation. A commonly cited target band is roughly 0.6 to 3.0 m/s, but you must check the standards that apply in your jurisdiction.

How to use this storm sewer pipe sizing calculator

1. Determine design flow Q (m³/s)
   Use a hydrologic method (such as the Rational Method or a rainfall–runoff model) to estimate peak runoff at the location of interest. Convert the result to cubic metres per second. The default value of 0.5 m³/s is roughly representative of a small urban catchment; always replace it with a value derived for your project.
2. Select an allowable pipe slope (%)
   Pipe slope is usually constrained by ground elevations, cover requirements, and connection to existing infrastructure. Enter the longitudinal slope as a percentage, for example:
   • 0.5% slope → enter 0.5
   • 2% slope → enter 2.0
   The default slope of 0.5% is typical of a relatively flat storm sewer run.
3. Choose Manning roughness n
   Roughness depends mainly on pipe material and interior condition. A commonly used value for new, smooth concrete storm sewer is n = 0.013, which appears as the default. Adjust this value for your selected material using the reference table below.
4. Run the calculator
   After entering Q, slope, and n, run the calculation. The tool will report the required internal diameter (assuming full flow), the corresponding cross-sectional area, and the resulting average flow velocity.
5. Interpret the results
   Compare the computed velocity with local design criteria. If the velocity is too high, consider reducing the slope, increasing the diameter, or selecting a smoother material. If it is too low, consider a steeper slope (if available) or a smaller diameter, while respecting minimum allowable sizes in your standards.

Typical Manning roughness values for storm sewer materials

The table below summarises typical design Manning roughness coefficients for common storm sewer materials under reasonably clean conditions. Values are indicative only; consult local design manuals or material specifications for authoritative ranges.

When in doubt, using a slightly higher n (rougher pipe) is conservative for sizing because it results in a larger required diameter for the same flow and slope, providing additional capacity and robustness against future deterioration.

Worked example

Consider a small urban catchment draining to a proposed storm sewer. A hydrologic analysis indicates a design peak flow of Q = 0.50 m³/s at a particular reach. The available ground profile allows a pipe slope of approximately 0.5%, and the designer intends to use smooth concrete pipe with n = 0.013.

1. Input values
   • Q = 0.50 m³/s
   • Slope = 0.5% (enter 0.5)
   • Manning n = 0.013
2. Computed diameter
   The calculator applies the full-pipe Manning relationship and returns an internal diameter on the order of several hundred millimetres (exact value depends on the precise constants implemented). In practice, you would round up to the next available standard pipe size, for example from 0.46 m to 0.50 m.
3. Velocity check
   Using the computed diameter, the tool calculates the corresponding area and velocity. Suppose it reports a velocity of around 1.5 m/s. This lies within the typical 0.6–3.0 m/s band, suggesting that the design is hydraulically reasonable for preliminary sizing.
4. Refinement
   If the velocity had been below 0.6 m/s, you would consider a smaller diameter (if above minimum code size) or a slightly steeper slope, if feasible. If it had been above 3.0 m/s, you might increase diameter, reduce slope, or use a rougher lining material, and then reassess.

Interpreting results: comparison of design scenarios

The following qualitative comparison illustrates how changing slope and roughness affects the required diameter and velocity for a fixed design flow.

For a fixed Q, using a smoother material or steeper slope reduces the required diameter but increases flow velocity. Conversely, using a rougher material or flatter slope increases the required diameter and generally reduces velocity. Proper design balances constructability, cost, allowable cover, erosion risk, and self-cleansing performance.

Assumptions and limitations

This tool simplifies storm sewer hydraulics to provide quick, order-of-magnitude sizing. Before adopting any result, review the assumptions below and confirm that they are acceptable for your design stage.

• Full-flow assumption: The pipe is assumed to flow full at the design discharge. Partially full or surcharged conditions are not explicitly modelled.
• Uniform, steady flow: The Manning equation is applied under uniform, steady flow conditions. Rapidly varied flow, backwater effects, and transient surges are not considered.
• Slope approximates energy grade line: The input pipe slope is taken as a reasonable approximation of the energy grade line slope. Systems with significant minor losses or backwater may violate this assumption.
• No local head losses: Entrance losses, manhole losses, junctions, bends, transitions, and outlet conditions are not included. In detailed design, these effects can materially change required diameters and slopes.
• Single reach analysis: Each calculation represents a single uniform reach. Network interactions and upstream–downstream dependencies are not captured.
• Idealised roughness: Manning n values are treated as constants. In reality, roughness can change over time due to ageing, corrosion, joint defects, or sediment build-up.
• Metric units only: Inputs and outputs are in SI units. If your hydrologic analysis is in imperial units, convert to m³/s and metres before using this tool.
• Preliminary design only: Results are intended for preliminary sizing, educational examples, and sensitivity checks. Final designs must comply with local codes, design manuals, and may require more sophisticated hydraulic modelling (e.g., gradually varied flow, unsteady routing).
• Professional judgement required: This calculator does not replace the judgement of a qualified engineer. Always confirm that chosen pipe sizes are compatible with structural requirements, minimum cover, constructability, and maintenance access.

Typical reference sources for Manning roughness values and velocity criteria include national stormwater manuals, transportation drainage guidelines, and hydraulic design textbooks. Where local standards differ from the indicative values used here, local requirements should govern.