Culvert Sizing Calculator

Size circular culverts by checking inlet and outlet control headwater depths. Determines if a selected pipe size is adequate for the design flow.

Design Flow

cfs

Allowable Headwater Depth

Tailwater Depth

Culvert Length

Culvert Slope

ft/ft

Tab to move between fields

Selected Pipe Diameter

36 in

≈ 6 smartphones

Design Adequate

Inlet Control HW8 ft

Outlet Control HW2.74 ft

Outlet Velocity7.07 ft/s

How to Use This Calculator

Enter design flow (cfs), allowable headwater depth (ft), and tailwater depth (ft).
Set culvert length (ft) and slope (%).
Review Required Diameter (inches) and inlet control headwater depth.
Round up to the next standard pipe size and verify both inlet and outlet control conditions.

What each input means

Design Flow: Peak design discharge, typically from hydrologic analysis (Rational Method or TR-55) for the design storm.
Allowable Headwater Depth: Maximum headwater depth above the culvert invert. Often limited to 1.2D or road subgrade elevation.
Tailwater Depth: Water depth at the culvert outlet, from downstream channel analysis or normal depth calculation.
Culvert Length: Total length of the culvert barrel from inlet to outlet.
Culvert Slope: Slope of the culvert barrel. Should generally follow the natural stream grade.

Identify Input Parameters
4 parameters
Design Flow = 50, Allowable Headwater Depth = 4, Tailwater Depth = 1, Culvert Length = 60 = 5 input(s) provided
Calculate Selected Pipe Diameter
36 = 36
Calculate Design Adequate
Design Adequate
0 = 0
Calculate Inlet Control HW
Inlet Control HW = max(0
8 = 8
Calculate Outlet Control HW
Outlet Control HW = max(0
2.74 = 2.74

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