Rectangular Weirs

Head
(H)
(in.)		 Length (L) of Weir
(ft.)
1 3 5 additional g.p.m for
each ft. over 5ft.
   1/4
4.50 13.4 22.4 4.50
   1/2
12.8 38.2 63.8 12.8
  3/4
23.4 70.2 117 23.4
 1  
35.4 108 180 36.1
 1 1/4
49.5 150 250 50.4
 1 1/2
64.9 197 330 66.2
 1 3/4
81.0 248 415 83.5
2  
98.5 302 506 102
2 1/4
117 361 605 122
2 1/2
136 422 706 143
2 3/4
157 485 815 165
3  
178 552 926 187
3 1/4
200 624 1,047 211
3 1/2
222 695 1,167 236
3 3/4
245 769 1,292 261
4  
269 846 1,424 288
4 1/4
294 925 1,559 316
4 1/2
318 1,006 1,696 345
4 3/4
344 1,091 1,835 374
5  
370 1,175 1,985 405
5 1/4
396 1,262 2,130 434
5 1/2
422 1,352 1,696 465
5 3/4
449 1,442 2,440 495
6  
477 1,535 2,600 538
6 1/4
  1,632 2,760 560
6 1/2
  1,742 2,920 596
6 3/4
  1,826 3,094 630
7  
  1,928 3,260 668
 

Formula:   Q = 3.33 ( L – 0.2H) H 1.5
Where:   Q = flow of water in cu. ft. per sec.
L = width of weir opening in ft. (4 to 8 times H)
H = head of weir in ft. (measured at least 6 ft. back of weir opening)
a = at least 3H