How much water in overflow?

I have a 90gal tank with a standard overflow in the back. Anyone off the top of their head know the general water volume for those size overflows?

Here's a pic if that helps. I'm going to measure the dimensions tomorrow, just thought someone might know already. K+ to whoever is closest!

 

Tried a rough calculation and got 2.9 gallons, sound right to anyone?

 

lol. What are the dimensions of the overflow?

 

looks like 6" wide X 24" tall X (I don't know the third dimension...)

 

me either lol, I know its 24inches deep, and I believe 7 inches wide, but the last one is tough. I tried 4 inches, but it could be less or more. Not at the house right now so can't find out. I'm trying to figure out how much water I should keep in the sump, so in case of a power outage there won't be any spills =x.

 

1.3 gal? ( guessed is was 2" back)

 

hmm. Well if 4 inches is the max. of the curve then I would use 2 as the measurement, as it is a half-circle.

 

Ok. I got 1.45g, at 24" X 7" X 2"

 

good point, well if I plan for 3 gallons of space I think I'll be fine then. I need to buy a plug to test it out possibly. Would hate to have a spill.

 

Yeah, a spill would not be good. I plan to have 3.4g of overflow space in my sump for emergencies. I will have a shut off valve(using a ball valve) at the top of my plumbing from DT to sump too.