Drainage Grate Flow Calculators

Calculate the flow of water through a drainage grate.

Orifice Flow Calculator

Use Orifice calculation to determine flow through a

grate if water depth is 4" or more.

Weir Flow Calculator

Use Weir calculation to determine how much water

flows to the grate if water depth is less than 4".

About Our Calculations

Weir and Orifice calculators are used to determine the inlet capacity in sag (ponding) condition to help choose the proper grate for your project. Below outlines the EJ equations used in our calculations.

Orifice Equation
Weir 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

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

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 equation

Our Orifice Equation is originally derived from the following equation:

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)

Using an Orifice coefficient of 0.67 and correcting open area from square feet to square inches and depth of water from feet to inches, this equation simplifies to: THE ORIFICE EQUATION.

The provided flow capacity calculators are theoretical calculations and are provided for your guidance. The calculators do not take into account the many variables that occur in the field. For any questions, please contact us.