If you are doing an overflow feed like this:






...then the overflow gallon per hour (U.S. gph) will determine how much flow you have to work with. You have to start from there, and size your screen accordingly. The maximum flow you'll get to the screen will be what's going through your overflow now. This is easy to figure out by counting how many seconds it takes your overflow to fill a one-gallon jug:

60 seconds = 60 gph
30 seconds = 120 gph
15 seconds = 240 gph
10 seconds = 360 gph
8 seconds = 450 gph
5 seconds = 720 gph
4 seconds = 900 gph
3 seconds = 1200 gph

Take this gph number that you end up with, and divide by 35, to get the number of inches wide the screen should be. For example, if your overflow was 240 gph, then divide this by 35 to get 6.8 (or just say 7) inches. So your screen should be 7 inches wide. Or you can use this chart:

Screen Width-----Gallons Per Hour (GPH)

1" 35
2" 70
3" 105
4" 140
5" 175
6" 210
7" 245
8" 280
9" 315
10" 350
11" 385
12" 420
13" 455
14" 490
15" 525
16" 560
17" 595
18" 630
19" 665
20" 700
21" 735
22" 770
23" 805
24" 840
25" 875
26" 910
27" 945
28" 980
29" 1015
30" 1050

How tall should the screen it be? That is determined by how much screen area you need, which is determined by how many gallons you have. Try to get one square inch of screen (lit both sides) for every gallon. If lit on only one side, double the screen area.

When finished, this is how you want your flow to look: