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Ideal Broad-Crested Weir Head, Width & Flow

Estimate ideal rectangular broad-crested weir discharge from upstream head, transverse crest width and gravity.

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

With g = 9.80665 m/s², head 1 m and transverse width 2 m, ideal flow is approximately 3.4092 m³/s.

C = √g × (2/3)^(3/2); Q = C L H^(3/2)

Worked example

Enter these known values and leave the other values blank.

Upstream energy head above crest
1 m
Transverse crest width
2 m
Acceleration due to gravity (g)
9.80665 m/s²
Effective discharge coefficient
1.705 m⁰˙⁵ s⁻¹
Discharge (Q)
3.41 m³ / sec

Assumptions and limitations

  • This is an ideal energy-based, free-overflow rectangular broad-crested model with unit discharge correction, not a calibrated site rating.
  • L is transverse width across the stream, not crest length in the direction of flow. H is upstream energy head above the crest; neglecting approach velocity requires justification.
  • Real installations require appropriate crest geometry, measurement location, submergence and discharge/velocity corrections. The editable coefficient can represent a supplied effective value.
  • Zero head gives zero modeled flow; width and gravity remain positive. Negative head is outside the model.

Common questions

What should I check before using this result?

This is an ideal energy-based, free-overflow rectangular broad-crested model with unit discharge correction, not a calibrated site rating.

References

Bookify accepts zero upstream head and zero ideal discharge. Feet per second squared use exactly 0.3048 meters per second squared, replacing a rounded reciprocal factor. Bookify replaces the captured rounded US gallon with the exact 3.785411784-liter definition.

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