Figuring out how much water excluding sand and rock

[Bob Escher]

so I'm trying to figure out how much water I have to do a proper water change. I know that water is approx 8 lbs. I have 35lbs of rock ( approx) 60 Lbs of Fiji pink live sand in a 45 gallon JBJ nano,
How to I figure out how much the rock and sand is displacing water and how much water I actually have.
Thanks

 

[dbl]

Try this site. I think it will get you pretty close.

http://reef.diesyst.com/volcalc/volcalc.html

 

[Bob Escher]

Thanks saving as a bookmark close to what we though it would be

 

[Reef Junky150]

Wow I was only twelve gallons off from my uneducated guess. And ten of those gallons where from not remembering my refugium size. Nice calculator.

 

[Bob Escher]

We were under by a gallon and a half