The formula
Topsoil is sold by volume, and the standard bulk unit is the cubic yard. The formula is: length times width times depth, all in feet, gives cubic feet. Divide cubic feet by 27 to get cubic yards. Depth is usually measured in inches, so convert it first by dividing by 12.
For a rectangular bed 20 feet long, 10 feet wide, and 4 inches deep: 20 times 10 times 4 divided by 12 equals 66.7 cubic feet. Divide by 27 and you get 2.47 cubic yards, so you would order 2.5 cubic yards. The full example with bag counts is on the topsoil calculator page.
For round beds, use the area of a circle: 3.14 times the radius squared times the depth. A round bed 6 feet across (3 foot radius) at 8 inches deep is 3.14 times 9 times 0.667, which is 18.8 cubic feet, or about 0.7 cubic yards.
Worked example 1: a vegetable garden bed
Say you are building an in-ground vegetable bed that is 12 feet long and 8 feet wide, and you want 10 inches of topsoil. Convert depth: 10 divided by 12 is 0.83 feet. Multiply: 12 times 8 times 0.83 equals 80 cubic feet. Divide by 27: 80 divided by 27 is 2.96 cubic yards, so 3 cubic yards is the right order, plus a little extra.
If you prefer bags, divide 80 cubic feet by the bag size. With 2 cubic foot bags, 80 divided by 2 is exactly 40 bags, and rounding up gives you a buffer. With 0.75 cubic foot bags, 80 divided by 0.75 is 106.7, so 107 bags. That is a lot of bags, which is why most people buy 3 cubic yards by the truckload for a bed this size.
Worked example 2: topdressing a lawn
Topdressing uses a thin layer, so the volume is much smaller. A 2,000 square foot lawn topdressed at 1/4 inch: 2,000 times 0.25 divided by 12 equals 41.7 cubic feet. Divide by 27 to get 1.54 cubic yards, so 1.5 cubic yards covers it. In bags, 41.7 cubic feet divided by 1.5 cubic foot bags is 27.8, so 28 bags.
This is a case where bag math is useful for comparison shopping, but bulk is still cheaper if you have a delivery source. One and a half cubic yards is roughly 14 to 15 wheelbarrow loads, so plan a half day for spreading it and raking it in.
Worked example 3: a brand new lawn
A new lawn seeded from scratch needs 4 to 6 inches of topsoil. Take a 1,500 square foot yard at 5 inches deep: 1,500 times 5 divided by 12 equals 625 cubic feet. Divide by 27 for 23.1 cubic yards. That is a full truckload from a landscape supplier, likely with a delivery fee.
Notice how depth dominates the math. Doubling the depth doubles the soil. This is why the depth question matters more than getting the square footage exactly right. Measure your existing soil first: if you already have 2 inches of decent soil and need 6 total, you only order 4 inches worth.
The settling rule: order 5 to 10 percent extra
Every volume you calculate is the compacted, settled volume. Fresh topsoil fluffs up when dumped and settles after rain and foot traffic. It also spills during spreading. Add 5 to 10 percent to your calculated number. For the 2.5 cubic yard garden bed, that means ordering about 2.7 cubic yards, which a supplier will round to 3 anyway. Suppliers round up to whole or half yards, so the extra usually costs nothing.
Bags vs bulk: the break-even question
Bagged topsoil makes sense up to about 1 cubic yard. Beyond that, bulk wins on price and convenience. A 2 cubic foot bag is one way to think of it: 27 cubic feet per yard divided by 2 means a yard equals 13.5 two-cubic-foot bags. Compare the bulk price per yard against the bag price times 13.5. Almost always, the bulk price is a fraction of the bagged equivalent.
There is one exception: small city yards with no driveway access. If a delivery truck cannot reach your yard, hauling 36 small bags (the 40 pound, 0.75 cubic foot kind) in your car may be the only option. A 0.75 cubic foot bag at 3 inches deep covers about 3 square feet, so multiply that by your area to sanity-check the bag count.
Common mistakes
The most expensive mistake is measuring depth in inches and forgetting to convert. Using 4 instead of 0.33 in the formula gives you 12 times too much soil. The second is forgetting that 27 cubic feet equals one yard: the number in cubic feet looks huge and scares people into ordering less than they need.
The third mistake is skipping the settling allowance on grading projects. Filled low spots settle back into dips within a season if you order the exact math number. Order proud, then let the rain and rolling do the final leveling before you seed or plant.