Can anyone calculate overflow size?


Reef enthusiast
Hi, I need some help. Math and engineering is definitely NOT my forte. ;)

I'm having a rimless cube built and I need to figure out what size overflow and what size "teeth" cut into the top. It's a center column overflow in a 36" x 36" x 24" and I need to run a 1200gph pump. 3 holes will be drilled in the base of the tank inside the overflow for two 1" pipes and one 1.5" pipe. Just to have enough room for the bulkhead holes I'm figuring I need at least 6", but I have no idea how to calculate size for flow.

I want to double check vs what whomever builds my tank thinks.
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1in pipe will drain 600gph i would do a 2in drain for about 1200gph. call they can tell you what you need for shure
use a compass and a sheet of paper and draw out the drains and returns than make a box around them. this will allow you to play with how big it is going to be on paper.
Don't forget to allow for the flanges on the bulkheads. Just because it's a 1" pipe does not mean you need a 1" hole for that bulkhead. More like 1.5" or 2" hole for the right bulkhead to fit in there and tighten up.

So when you get the compass and paper out, make sure you aren't just using 1" as the size of your holes. They'll be much larger than that to accommodate the bulkheads.

If you have a square tower coming up in the center of the tank, cut slots on all 4 sides. You can never have too many slots or too many overflows. The tank and overflows will only flow as much as the pump will push into the tank. Basically, you can't really OVERSIZE an overflow. You can UNDER SIZE them though.

If you want to push 1200GPH:
We know a 1" ID (internal diameter) pipe will flow 600GPH. If I was you, I'd put in (2) 1.5" or 2" overflows. That will more than handle your pump volume of 1200GPH.

Don't worry about them overflowing faster than the pump can put the water back in the tank. You will be cutting slots in the square tower. As soon as the water drains down so far -- it will not be able to flow anymore because it will fall below the level of the slots in the tower. Water flow stops.

It's only going to overflow as fast as the pump will actually pump the water from your sump back into the tank. Just make sure the overflows are BIG enough to handle the pump or your sump will get sucked dry all the time.

Like I said, (2) 1.5" or 2" overflows will work for your 1200GPH pump and give you a nice safety margin to work with.