Some gardening issues are best addressed by adding more dirt. More dirt smooths out the bumpy places for the new lawn. More dirt builds the berm to hold the weeping cherry tree. Any gardener who has worked in the same flower and vegetable beds for years will assure you that cultivated dirt wears out. A mild challenge exists: calculating how much new dirt your garden needs. To measure topsoil needed for a large or small project, follow a simple formula and take advantage of online help to verify your calculations.
Measure both the length and width of areas to be covered with new topsoil in feet and decimal fractions of feet. Remember that decimals express amounts in 1/10ths, not in inches; an area that measures 3 feet 6 inches in length translates into a decimal figure of 3.5 feet, not 3.6. Multiply the length of your area by the width to determine the square feet of topsoil you will need. An area 30 feet in length and 12 feet wide totals 360 square feet.
Multiply square footage by the amount of topsoil you need to add to determine the cubic feet you will need. For example, you plan to put 6 inches of topsoil on that 360-square-foot strip bordering the driveway. Six inches lets you plant ornamental grass, to stop the annual erosion of that bed. Six inches equals .5 foot. Therefore, 360 square feet x .5 foot (deep) = 180 cubic feet of soil.
Convert your cubic-feet measurements into cubic yards. Topsoil is customarily sold in cubic yards. To convert cubic feet to cubic yards, divide the cubic feet (180) by 27, for a result of 6.04 cubic yards. (The number 27 divides each of the cubic feet measurements, height, width and depth by 3, to convert them to yards; 3x3x3=27.)
Round your cubic yard figure up (or down) to the nearest whole number. Since your calculation shows only 4/100 over 6 cubic yards, round down to 6. If your calculation had shown 6.5 or even 6.1 cubic yards, rounding up to 7 cubic yards assures that you have enough topsoil for your job.
Confirm your calculations with an online topsoil calculator. The only difference between the calculations you made on your own and those done by the online formula is that online you may be asked for some information in feet and some in inches. Just follow the directions; your concern is that your figures and the online figures come out the same or nearly so.