The garden art forum dot com has that info on their site.
This site will not permit a link to the specific page, but look under the knowledge pages. Some interesting facts..including answering your question
Marcia

I would figure the approx. total volume of material in the finished piece.
11x4x4x2=352cu.in. for the sides.
11x11x2=242cu.in. for bottom.
Total 594cu.in. This is only approximate assuming a two inch wall thickness. These measurements are larger than actual, but you would rather have too much than too little mix.
I have selected a large coffee can as my unit of measure for all pieces I make. I know from experience that ten units will make a piece of 900cu.in volume, so I would make dry mix of 6 1/2 units of dry mix.--If mix is equal parts cement perlite and peatmoss, I probably would use two over flowing units of each.
Keep in mind that this is only an approximation that is a good starting amount. Experience will tell what adjustments to make in the amount of mix actually used. When it comes to bowl shapes I assume a cylinder of the average diameter x thickness plus a circle of the same size x thickness for the bottom. My method is seldom perfect,but it is usually close.

thanks so much for the info - both of you! I'm anxious to get started! I just picked up an old (free) cement mixer and plan to use it for my start in hypertufa. I'll start with a few troughs for my plants, but then I have a big project in mind. I have a pond, and I want to make rocks to hide the pipe coming from my biofilter to the waterfall - kind of like a tunnel - to put over the pvc piping. Now I just have to search out the material components in my neighborhood. Thanks for the help.

I mix all but the portland and water, say 1/2 sand and 1/2 peat, even alternating scoops until the form walls are full. Count as you go. If it's 20 sand, 20 peat, add 20 Portland and water and it will all work out. Portland and water flow around the other components and don't add much volume.

Mercedes

Mercedes your measuring process sounds really simple - and minimal math involved.