Wow hold on everyone. No need to guestimate or use rocket science, or even a conversion website.
Hasnt anyone here done elementary geometry? Your filling a volume. Your substance is already measured in volume. All you have to do is simple conversion to get an exact amount. Measure your tank length and width and multiply by the depth you want the substrate, and then just convert cubic inches to quarts. Its a snap when they actually measure stuff in volume, and not pounds.
A standard 10G I beleive is 10"x20". Thats usually the outside dimensions, if you want a precise calculation then measure the open area inside the tank instead.
For 2" bed, type this into google:
"convert 10 x 20 x 2 cu in to quarts"
or here, I did it for you just for reference: http://www.google.com/search?hl=en&q...=Google+Search
Remember this is an exact calculation. If you buy that exact quantity and everything was measured perfectly than you'll be good. But incase things werent measured perfectly you should get some extra (always a good idea anyhow).
Its realy simple, and you dont even need to use elementary math, just let google do all the math and convert the units. LxWxH = Volume (in cubic inches). convert cu in into quarts (for your case) or liters (for something like ADA Aqua Soil). The hard part is when its not labeled in volume but pounds. Then use the susbtrate calc at top of screen it will handle the more common options.
Google will do every conversion I have ever needed to do, so you dont even have to bookmark a converions site or download any utility. You dont even have to go to google if you have a google search thingy on your web browser searchbar. You can even type it in a few different ways and google figures out what your trying to do, for the most part. To see the great features and instructions on how do use googls calculator look at http://www.google.com/help/calculator.html