water displacement calculator

Here are my thoughts

Basic info:

There are 231 cubic inches in one US gallon. Since a cubic foot is 12 by 12 by 12 inches, a cubic foot has 1728 cubic inches. Thus one cubic foot = 1728 cubic inches = (1728/231) gallons = 7.48 gallons

So you can measure the bag that your material came in with it full. adjusting for the density (ie. Lava Rock is porous so you may want to deduct a little for that and space between rock). Make an educated guess on that. For every 231 cubic inches of material you put in the tank you used up 1 gallon of water space.

If you want to get more specific. Purchase a large liquid measuring container. Weigh your material then take a portion (ie. 1 tenth to make it easy) fill the container with water to a marked amount, add the sample of material. Figure out how much "space" it took up (probably in oz.) then multiply it out. (1 tenth = x oz.)

Either way you have more information than just guesstimating.

Hope this was helpful .... let me know how it went!