Enter the orifice flow coefficient, the opening area, and the allowable head on grate into the calculator to determine the inlet capacity.

Inlet Capacity Formula

The following equation is used to calculate the Inlet Capacity.

Q = C * A * SQRT (2*g*H)

  • Where Q is the inlet capacity (m^3/2)
  • C is the orifice flow coefficient (.67)
  • A is the inlet opening area (m^2)
  • g is the acceleration due to gravity
  • H is the allowable on on the grate (m)

To calculate inlet capacity, multiply the orifice flow coefficient by the area, then multiply the result by the square root of the product of 2 times g times the head.

What is an Inlet Capacity?


An inlet capacity is a measure of the volumetric flow rate of water the inlet to a water drain can handle.

How to Calculate Inlet Capacity?

Example Problem:

The following example outlines the steps and information needed to calculate Inlet Capacity.

First, determine the orifice coefficient. In this example, this is found to be .67.

Next, determine the clear opening area. This is measured to be 2 m^2.

Next, determine the allowable head. This drain system has an allowable head of 3m.

Finally, calculate the inlet capacity using the formula above:

Q = C * A * SQRT (2*g*H)

Q = .67 * 2 * SQRT (2*9.81*3)

Q = 10.280 m^3/s