ROCKS & WOOD

To calculate the rock needed for a freshwater aquarium, multiply the tank length by the tank width in inches, then divide that number by 25. This gives a basic baseline of about one pound of rock per gallon or standard aquascaping layouts. For a dense Iwagumi or heavy aquascape, divide by 15 instead.
Calculation Steps
Measure the length of your tank in inches.
Measure the width (depth from front to back) in inches.
Multiply length times width.
Divide the total by 25 for a light or standard look.
Divide the total by 15 for a heavy or complex rock pile.

Rock Weight Examples by Tank Size
10 Gallon (20" x 10"): About 8 pounds of rock.
20 Gallon (24" x 12"): About 11 pounds of rock.
40 Gallon (36" x 18"): About 25 pounds of rock.

NOTE: This applies to lighter rocks i.e. Dragon Rocks. Take the weight an multiply it by 1.5 or 2 to calculate the amount of heavier rock needed to setup a nice display.