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Culvert Sizing Calculator

Work out culvert sizing instantly with clear inputs, formula shown and shareable results.

About Culvert Sizing Calculator

Sizing a circular culvert inverts Manning's equation for full-bore flow, giving diameter proportional to (Qn/sqrt S)^(3/8). A 0.85 m3/s design flow at 1% slope in concrete needs about 0.66 m, so a 750 mm pipe would be specified. Velocity should stay between about 0.9 m/s to avoid silting and 4 m/s to avoid scour.

Formulas

Full-bore diameter

D = (3.208 Q n / sqrt(S))^(3/8)

Velocity check

v = Q / (pi D^2 / 4)

Worked example

With the default values below, Culvert Sizing Calculator returns Required Culvert Diameter of 0.678. Change any input above and every figure here is recalculated in your browser.

Inputs used

Design flowminimum 0 m3/s
0.85 m3/s
Culvert slopeminimum 0
0.01
Manning roughness nminimum 0
0.013

Results

Required Culvert Diameter
0.678 m
Same Diameter In Millimetres
678
Velocity At That Diameter
2.356 m/s
Next Standard Pipe Size Up
750 mm

Step by step

  1. 1Values used
  2. 2Full-bore diameter
  3. 3Velocity check
  4. 4Required Culvert Diameter= 0.678 m
  5. 5Same Diameter In Millimetres= 678
  6. 6Velocity At That Diameter= 2.356 m/s
  7. 7Next Standard Pipe Size Up= 750 mm

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why round up to a standard size?

Because pipes are manufactured in fixed diameters, and undersizing risks surcharge and upstream flooding.

Should culverts run full?

Design usually targets part-full flow at the design storm, leaving freeboard for blockage and larger events.

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