Calculating how much soil you need for a raised bed planter box is relatively easy--as long as your eyes don't roll when you are asked to do a bit of math. If the thought of doing some math on a calculator makes your head swim and your knees wobbly, don't worry. This article will take you through all of the math, step by step so it won't hurt a bit.
Measure the width of your raised bed planter box--in inches--using a tape measure. Write that number down on a sheet of paper. Your tape measure will give you an exact measurement in inches, requiring you to do no math at all so far.
Measure the length of your planter box in inches. Write this number down.
Measure how deep you want your box to be. Measure from the bottom of the box to the level at which you wish your soil to end up. Measure this in inches as well and write that number down.
Multiply the three numbers you wrote down together on your pocket calculator. Let's assume the width of your box is 48 inches and the length of your box is 96 inches and the depth of your soil is going to be 10 inches. On your calculator multiply 48 X 96 to get 4,608. Now simply multiply 4,608 by 10 for a final total of 46,080. Write that number down. That number is the number of cubic inches of soil you will need to fill your planter.
Divide 46,080 by 1,728. The reason for this is because soil is sold by the cubic foot and not by the cubic inch so we need to change your cubic inches into cubic feet. The reason you divide by 1,728 is because ,1,728 cubic inches (12 X 12 X12 = 1,728) is 1 cubic foot. 46,080 divided by 1,728 equals 26.67cubic feet. In other words, you'll need around 27 cubic feet of soil to fill a 4-by-8-foot planter 10 inches deep.