The calculator I use is first decide how deep you want your substrate. Then measure your tank's inside footprint. Let's use a 48" x 12" for simplicity. That is 576 square inches. If you want an inch deep, then you want 576 cubic inches of substrate. 2" of substrate = 576 x 2 = 1152 cubic inches.
If you are going with pea gravel, bring a 12" x 12" x 12" box and 4" deep will give you 1" of substrate in the tank. 8" deep will give you 2" of substrate in the tank. If you don't have a 12" x 12" x 12" box, take a similar sized box and measure the foot print. Say it's 10" x 10" = 100 square inch footprint. If you want a 1" substrate, go with 6" of substrate in the box (or actually 5.76" but who's counting). If you want a 2" substrate in the tank, go with 11.5" in the box (OK.. I was counting closer that time.. lol).