Making the decision whether or not to mulch your yard is the easy part. Natural mulches benefit lawns by adding nutrients back into the soil, help retain moisture and aid in weed control. Decorative mulches dress up bare areas of the yard where grass can stubbornly refuse to grow, and prevent erosion of these spots. But not knowing how to calculate how much you'll need for the job is the hard part. You don't want to guess at the amount and end up making trip after trip to the supply store because you don't have enough. On the other hand, you don't want to have to an excess truckload of the stuff. Here's the secret, and it's simple--even if you're mathematically challenged. All you need is a pencil and a piece of paper.
Write this formula on a sheet of paper. At the upper left, write "1 cubic yard equals 27 cubic feet." At the upper right, jot down, "1 cubic yard covers 324 square feet, 1 inch deep." Those will be the base facts for determining how much material you'll need, no matter what shape or size the area may be.
Measure the area you wish to cover with mulch to determine the square footage. That figure will be the length multiplied by the width, in feet. Let's use this easy example for a rectangular or square area, and writing it down will be helpful for you. The section of lawn is 30 feet long and 20 feet wide, so that's 30 multiplied by 20 equals 600 square feet.
Multiply that 600 square feet by how many inches deep you want the layer of mulch to be. Let's call it 2 inches deep, so that's 600 multiplied by 2 equals 1,200 square feet.
Convert that 1,200 square feet to cubic yards by dividing it by 324 (the number of square feet in a cubic yard). So 1,200 divided by 324 equals 3.7037037. You can round that up to an even 4 cubic yards.
Obtain the square footage of a triangular area, which has one short side and two longer ones. Multiply the base, the short side by the height, or the longest of the two other sides, and then divide by 2. If the triangle's base is 9 feet wide and the sides are 12 and 15 feet long, multiply 9 by 15, which is 135, and then divide that by 2, which gives you 67.5 square feet. Refer to Steps 3 and 4 to complete your conversion to cubic yards.
Determine the square footage of a circular area by measuring from its center to the outside of it to get the radius. Multiply the radius by itself. If the circle's radius is 9, multiply 9 by 9 to get 81. Multiply that by 3.14, or pi, for a square footage of 254.34. Refer to Steps 3 and 4 to complete your conversion to cubic yards.