I am planning a new filter for the pond and I looked up the gravity discharge of pipes to make sure I don’t over run the filter. Knowing my pump outputs 3500 GPH on low speed and 7000+ GPH on high speed, under pressure through 2″ pipe, I need an outlet that will be greater than 8000 GPH. Below is a chart. Notice that 2 2″ pipes is less flow than 1 3″ pipe. It is simple math really, remember basic geometry? Volume of a cylinder is
. Lets look at standard US PVC pipe sizes with a height of 1 inch.
| Diameter | GPH | Volume cu in. |
| 1 | 960 | 0.79 |
| 1.5 | 2,100 | 1.77 |
| 2 | 3,300 | 3.14 |
| 3 | 8,400 | 7.07 |
| 4 | 14,400 | 12.56 |
1 cubic inches = 0.004329 US gallons
So a 2″ diameter pipe has
Add in the force of gravity, friction, and the weight of water and you can get close to the GPH value.
