How Large should my Sump be?

[ReefDreamz]

I am probably making this more complicated than it should be but how do you generally calculate how large your sump should be? I know it needs to be large enough to contain the water in it plus the water in the tank that will flow into the sump when the return pump is turned off. My display is 49.5x31x23 with two 1.5" drains and two 1" returns. The very bottom of the return hole is just under 5" from the top of the tank. Assuming the tank was filled to the brim (it won't be) the water volume that would travel down the returns would be 33.21 gallons (49.5x31x5) plus the water in the return and drain pipes. My sump will be in the basement and my display on the first floor so there will be at least a few gallons in the plumbing.

How do I know what size sump to get without knowing the volume of water that would already be running through the sump?

Tank dimensions:

 

[Quietman]

Well sounds you like already know the volume that the DT will add when shutting off the return pump (btw - you can drill a small hole in your return pipe just below water line to break siphon without impacting flow much - just clean it once in a while). Also, you really only need to calculate from the height of the weir.

The amount in the sump is calculated by the height of the baffle areas. The amount if can take without overflowing is the total sump tank volume - sum of the baffled area volumes. I'd add a couple gallons buffer as a safety factor.