You can easily figure how many yards of topsoil you'll need to adequately supply your garden without ending up with too much or coming up short. And you won't have to just take a wild guess at it, embarrass yourself at the garden center, or ask your third-grader for help. You'll be calculating units of volume, which is a simple math function. Have some fun by practicing this skill until you can do it comfortably, then impress your kids with your mathematical prowess.
Make two notes at the top of a piece of scrap paper. This is an easy calculation for a layer of topsoil 1 inch deep. Write "1 cubic yard = 324 square feet." Then note that "1 cubic yard is 27 cubic feet." These are the basic facts you need to know for covering any area with any amount of material.
Establish the square footage of a rectangular garden you want to topsoil. Measure the area's length and its width in feet, and write those measurements down. If the section is 25 feet long and 35 feet wide, that's 23 x 35 = 805 square feet.
Decide how deep you want the layer of topsoil to be, and multiply that number by the section's square feet. If 1 inch is the desired depth, then 1 x 805 = 805 square feet.
Change the square feet to cubic yards, which is how topsoil is typically sold. Divide 805 square feet by 324, which is the number of square feet in 1 cubic yard. So 805 divided by 324 = 2.4845679. That scary-looking number is easily managed. Just round it up by dropping all the excess figures after the decimal, leaving only the first two figures. So now you have 2.48, which you round up to the nearest tenth---2.5. You're going to need 2.5, or 2½ cubic yards of topsoil for your garden.
Measure the three sides of a triangular area, two of which will be long and one of which will be short. The short side is called the base. The longest side is called the height. Don't worry about the other long side, since it doesn't figure into the equation. Multiply the length of the base by the length of the height, and then divide that figure by 2. So if the base is 10 feet long and the height is 20 feet, 10 x 20 = 200, and 200 divided by 2 is 100 square feet. Change that square footage to cubic yards just like you did for the rectangular garden.
Measure the radius of a circular area. This is the length from the center of the circle to any point on its outer edge. Multiply that figure by itself. If the radius is 10 feet, then 10 x 10 is 100. Now multiply that by pi, which is no more than a silly name for 3.14. This gives you the circle's square footage. So 100 x 3.14 = 314 square feet. Convert this to cubic yards, and you're ready to buy the exact amount of topsoil you need.