Enter the head 1 and head 2 on the weir, the width of the weir, and the discharge coefficient into the calculator to determine the broad crested weir flow rate.

Broad Crested Weir Formula

The following equation is used to calculate the Broad Crested Weir flow rate.

Q = D * h2* b * SQRT ( 2*g* (h1-h2) )

  • Where Q is the flow rate ft^3/s
  • D is the discharge coefficient
  • h2 is the head from mid-weir to the bottom of the weir (ft)
  • b is the width of the weir (ft)
  • h1 is the head from the top to mid-weir (ft)
  • g is the acceleration due to gravity (ft/s^2)

What is a Broad Crested Weir?


A broad crested weir is a hydraulic structure used for depth control in canals. It’s comprised of a wide channel and a crest large enough compared to the flow over the crest.

How to Calculate Broad Crested Weir discharge rate?

Example Problem:

The following example outlines the steps and information needed to calculate discharge rate of a Broad Crested Weir.

First, determine the discharge coefficient. In this example, the discharge coefficient is .45.

Next, determine the head from the top to mid-weir. This is measured to be 20 ft.

Next, determine the head from mid-weir to the bottom of the weir. This is found to be 10 ft.

Next, determine the width of the weir. In this case, this 50 ft.

Finally, calculate the discharge rate using the formula above:

Q = D * h2* b * SQRT ( 2*g* (h1-h2) )

Q = .45 * 10* 50 * SQRT ( 2*32.174* (20-10) )

Q = 5707.552 ft^3/s