(Determines how much water flow to the grate)

One of the most accurate methods of measuring water is by use of a weir. Many weir configurations are around with just about as many equations. The are all adaptations of the general weir equation Q = 3.0 L (h1.5) where Q = flow in cubic feet per second, L = length of crest of weir in feet, and h = feet of head observed where there is negligible flow velocity.

Using the Neenah Equations, a slightly modified form is employed, namely: Q = 3.3P(h)1.5.


  • 3.3 = coefficient of discharge (increased by 10%)
  • Q = Capacity CFS
  • P = Perimeter of grate in feet
  • h = Head in feet

When determining inlet perimeter, add only the footage of the sides subject to flow.

  • For a circular grate π D (where D = diameter)
  • For a circular grate next to a curb: 1/2 (π D) + D
  • For a square grate: 4 x length of one side
  • For a square grate against a curb: 3 x length of one side
  • For a rectangular grate: 2 (width + length)

For a rectangular grate with one length up against a curb: (2 x width) + length. To maximize effective weir perimeter with two grates against a curb at a sag, place the grates (2 x width) apart.

It's always wise to compare the weir flow expected with the orifice equation results. The equation with the lower flow value predominates. A graphic illustration of both equations follows. When both flow rates are approximately the same, physically a vortex appears over the grate. This type of flow, due to its rotation, is not as efficient as either the weir or orifice equations imply. Using 80% of the expected flow in this situation should be a conservative estimate.

This example shows the relationship between the weir and orifice equations showing the expected flow through a grate under varying head conditions.

Given: R-4990-HA, Type A Grate, 2 feet long
Free open area: A=1.8 sq.ft. -- Q Orifice = 0.6 (1.6) 
Weir Perimeter: P=8.3 feet -- Q Weir - 3.3 (8.3) H3/2

Depth Q Orifice Q Weir
0.01 0.87  
0.04 1.73  
0.08 2.45 0.62
0.12 3.00 1.14
0.16 3.47 1.75
0.20 3.88 2.45
0.24 4.25 3.22
0.28 4.59 4.06
0.32 4.90 4.96
0.36 5.20 5.92
0.40 5.48 6.93
0.44 5.75 7.99
0.48 6.00 9.11
0.52 6.25  
0.56 6.49  

For additional information regarding Neenah Inlet Grate Capacities, please contact Ryan Bonell P.E., at (920) 729.3463 or email at ryan.bonell@neenahenterprises.com, or Joe Falle P.E., at (920)729.3653 or email at joseph.falle@neenahenterprises.com.