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Outlet Structures T-12
Table OS-1. Orifice Sizing
Hole Dia. Hole Dia. Area per Row (in 2 ) Use one column of rectangular orifices
(in) 1 (in) n = 1 n = 2 when the needed area exceeds 3.84 in 2
1/4 0.250 0.05 -
5/16 0.313 0.08 - Rectangular Height (in) = 2
3/8 0.375 0.11 - Rectangular Width (in) = Required Area / 2 in
7/16 0.438 0.15 -
1/2 0.500 0.2 -
9/16 0.563 0.25 - Rectangular Width (in)
Steel Thickness (in)
5/8 0.625 0.31 - ≤6 ≥1/4
11/16 0.688 0.37 - ≤8 ≥5/16
3/4 0.750 0.44 - ≤10 ≥3/8
13/16 0.813 0.52 - >10 ≥1/2
7/8 0.875 0.6 -
15/16 0.938 0.69 -
1 1.000 0.79 -
1-1/16 1.063 0.89 -
1-1/8 1.125 0.99 -
1-3/16 1.188 1.11 -
1-1/4 1.250 1.23 -
1-5/16 1.313 1.35 -
1-3/8 1.375 1.48 -
1-7/16 1.438 1.62 3.24
1-1/2 1.500 1.77 3.54
1-9/16 1.563 1.92 3.84
1-5/8 1.625 2.07 -
1-11/16 1.688 2.24 -
1-3/4 1.750 2.41 -
1-13/16 1.813 2.58 -
1-7/8 1.875 2.76 -
1-15/16 1.938 2.95 -
2 2.000 3.14 -
n = Number of Columns of Orifices
Steel Thickness (Min.) 1/4” 5/16”
1 If desired, interpolate to the nearest 32"
to better match the needed area.
August 2013 Urban Drainage and Flood Control District OS-3
Urban Storm Drainage Criteria Manual Volume 3