DrainageCalculators

Culvert Outlet Control Calculator

Calculate outlet control hydraulics for culverts including headwater depth, outlet velocity, critical depth, normal depth, and head loss breakdown. Uses FHWA HDS-5 methodology for circular and box culverts.

What This Solves

Calculates headwater elevation and outlet velocity for a culvert under outlet control conditions (when downstream water level affects upstream hydraulics).

Best Used When

  • You need to determine the upstream water level for a culvert with known tailwater elevation
  • You are checking whether a culvert operates under inlet or outlet control
  • You need to calculate outlet velocity for erosion protection design

Do NOT Use When

Key Assumptions

  • Outlet control conditions exist (tailwater or barrel friction controls the headwater)
  • The culvert barrel has uniform cross-section and roughness along its length
  • Standard entrance loss coefficients apply to the inlet geometry
  • Flow is steady (not changing over time)
  • The culvert is flowing full or partially full depending on tailwater depth

Input Quality Notes

Accurate tailwater elevation is critical for outlet control calculations. Use field measurements or routing calculations to estimate tailwater, not just normal depth in the downstream channel.

Inlet Loss Coefficients (Ke)

Inlet TypeKe
Square edge with headwall0.50
Groove end with headwall0.20
Groove end projecting0.25
Mitered to slope0.70
Beveled edges0.25
Side-tapered inlet0.20
Slope-tapered inlet0.15

Source: FHWA HDS-5, Table 4-1

Manning's n Values for Culverts

Materialn
Concrete0.012
Corrugated Metal (annular)0.024
HDPE (smooth)0.012
PVC (smooth)0.010
Corrugated Metal (helical)0.012-0.024
Stone Masonry0.025-0.030

Source: FHWA HDS-5, Table 3-3

Input Parameters

Flow Parameters

cfs

Total flow through culvert

ft

Water depth above outlet invert

Culvert Geometry

ft

Inside diameter

ft

Total barrel length

ft/ft

Longitudinal slope (rise/run)

Elevations

ft

Invert elevation at culvert entrance

Hydraulic Properties

Roughness coefficient

Entrance loss coefficient

Typically 1.0 for sudden expansion

Ready to Calculate

Enter culvert parameters and click Calculate to analyze outlet control hydraulics.

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

This disclaimer applies to the Culvert Outlet Control Calculator calculator and all associated calculations.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.