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Engineering Calculators

Drainage Grate Flow Calculators

Our Orifice and Weir calculators are be used to calculate the flow of water through a drainage grate. They will help you determine the inlet capacity in sag (ponding) condition so you can choose the proper grate for your project.

Orifice Flow Calculator

Use this Orifice calculator to determine flow through a grate if water depth is 4" or more.

Launch calculator

Weir Flow Calculator

Use this Weir calculator to determine how much water flows to the grate if water depth is less than 4".

Launch calculator

About our calculations

These are the equations used in our calculations.

Orifice Equation

Q = 0.0108A√d

  • Q = Flow (CFS, cubic feet per second)
  • A = Open area in grate (inches2  )
  • d = Depth of water over grate (inches)
  • Coefficient = 0.0108

Weir Equation

Q = 3.33L(H)3/2

  • Q = Flow (CFS, cubic feet per second)
  • L = Perimeter of grate where flow is present (in feet)
  • H = Depth of water (in feet)
  • Coefficient = 3.33

More about our Orifice equations

The Orifice Equation is originally derived from the following equation: Q = CA√(2gd)

To simplify the Orifice equation, use a coefficient of 0.67 and correct open area from square feet to square inches and depth of water from feet to inches.

  • Q = Flow capacity (in cubic feet per second)
  • C = Orifice coefficient
  • A = Open area of grate (ft2  )
  • g = Acceleration due to gravity (32.2ft/sec2  )
  • d = Depth of water over grate (in feet)

We provide the flow capacity calculators to be used in theoretical calculations and are provided as a guidance only. There are many variables that occur in the field that are not taken into account. Please contact one of our Engineers with any questions you may have.

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