I think I remember that you take the width and length in inches, multiply them together then divide by 10. This will give you an approximate pounds for 2" of gravel.
I would agree with stingray. From what I know, it is best to go with 1 pound per gallon. You don't want to overflow your tank with gravel. Too little might not look good either so. It's up to you.
a cubic yard of gravel weighs about 3500 pounds.....go from there ....figure you cubic inches of gravel wanted and what fraction that is of 36x36x36 times 3500 pounds = pounds you need
I always went by the lbs. of gravel per gallon of water, for standard size tanks. I don't want that much, im looking for more around 1" to 1.5" of gravel. Not that big of a deal, I'll just by what I will know is enough and have some left over.
The width by length divide by 10 formula will give you 2" pretty darned close. If you want 1" then take that number and divide again by 2. If you want 1.5" then divide by 1.34.
I used it for my 300. 96" x 30" = 2880/10 = 288 lbs. I bought 300 lbs of gravel and have just a hair over 2" in my tank.