Adding a layer of mulch to your garden helps to conserve water, maintain soil temperature and suppress weeds. Some mulches add aesthetic value to the garden by highlighting the plants and flowers. Whether you choose organic mulch, such as leaf mold, wood chips and grass clippings, or inorganic mulch such as plastic or pebbles, depends on your specific goals. Either way, calculating the amount needed requires a little math.
Determine the desired depth of the mulch. For effective weed control, a 3- to 4-inch layer of organic mulch may be effective, but the depth depends on the material used and your desired results. If using crushed stone or other inorganic mulch, a depth of 2 inches may be adequate.
Measure the length and width of the gardening area in feet. Note partial feet as decimals. Use .25 for 3 inches, .50 for 6 inches and .75 for 9 inches. Round up to the nearest 1/4 foot (3 inches). For example, if your bed measures 4 feet 4 inches, round the inches up to 6 inches (.50) and convert it to 4.5 feet.
Multiply the width times the length. This number indicates the surface area of the garden, noted in square feet. For example, in a garden with a width of 4.5 feet and a length or 6.25, the surface area equals 28.125 square feet.
Multiply the square footage of the area, in this case 28.125 square feet, by the depth of the mulch required for your garden. For example, if you desire a 3-inch layer of mulch, convert it to a decimal. (Three inches equals .25 feet.)
Multiply this number by the area of the garden. Using the previous example, multiply 28.125 by .25 to get 7.03 cubic feet of mulch.
Purchase the appropriate mulch using 7.03 cubic feet as your guide. Check the cubic feet of mulch in the container to ensure that you will have enough mulch. The cubic feet of mulch is typically prominently labeled on the front or side of the package.