Broad Crested Weir Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Fluid calculations are especially sensitive to density, viscosity, geometry, pressure conventions, and unit systems. Broad Crested Weir Calculator narrows that problem to the relationship used on this page, making the displayed discharge easier to audit against the inputs and the governing equation.

What this calculator does

The Broad Crested Weir Calculator uses Height of stream above the weir crest (H), Weir length (L), Coefficient (C) to estimate the page’s Discharge from the fluid-mechanics relationship below. It is meant for a defined geometry and property set, so the useful part is not just the headline number but also whether your density, viscosity, dimensions, pressure reference, and flow convention match the model.

How to use it

Start with the fields that actually drive this result: Height of stream above the weir crest (H), Weir length (L), Coefficient (C). Keep units consistent with the menus beside the fields and avoid mixing values measured under different conditions. After calculating, change one input at a time if you are comparing scenarios; that makes cause-and-effect much easier to see.

How the calculation works

The current broad-crested-weir model uses Q = C·L·H^(3/2), where C is the entered discharge coefficient, L is crest length, and H is head over the crest.

Example

Using the page’s default example (Height of stream above the weir crest (H) = 0.5 m; Weir length (L) = 1 m; Coefficient (C) = 1.705), the calculator reports Discharge of 0.602809 m³/s. Change one driving input at a time and confirm the result moves in the direction predicted by the equation; that is a quick way to catch a unit or mode mistake.

How to interpret the result

Interpret the Discharge within the fluid, geometry, pressure reference, and property values you entered. A numerically plausible answer can still be physically wrong if gauge/absolute pressure, diameter/radius, viscosity type, or unit convention is mismatched, so compare the result with the assumptions as well as the formula. For the Broad Crested Weir case on this page, keep that check tied to the displayed inputs rather than carrying the same assumption over from a different calculator.

Limitations and notes

Gravity is visible but is not explicitly used in the current Q = C·L·H^(3/2) form, so the entered coefficient must be compatible with that equation and unit convention. Real weirs need approach-velocity, submergence, geometry, and calibration checks.