How to measure for topsoil
Measure the area in square feet the same way you would for mulch: length times width for a rectangle, and for an irregular area, break it into smaller rectangles and circles you can measure separately and add together. This calculator's sketch panel lets you lay those shapes out visually and adds up the area for you, using the same math you would do by hand: length times width for a rectangle, pi times radius squared for a circle.
Topdressing depth vs. bed-prep depth, and why they differ
How much topsoil you need depends enormously on what you are doing with it, more than almost any other material on this site. Topdressing an established lawn, a thin layer worked into the existing grass to level low spots and feed the soil, only calls for about 0.5 to 1 inch, spread thin enough that the grass blades still show through. Preparing a new bed or building up a base layer under sod is a completely different job: 4 to 6 inches is typical, since new roots need real depth to establish in. Confusing the two is the most common way people either waste a truckload of soil on a lawn topdress or badly underorder for a new bed, so be clear on which project you are doing before you calculate.
Why topsoil is sold by both the cubic yard and the ton
Topsoil is bulky and moisture holds a lot of its weight, which is why suppliers price and sell it two different ways. Landscape yards and smaller orders are often priced by the cubic yard, since that matches how you actually plan a job (an area times a depth). Larger bulk deliveries and anything going by dump truck are commonly priced and weighed by the ton, since trucks have a payload weight limit that matters more than a volume limit. The conversion between the two is not fixed: a cubic yard of loose, dry topsoil weighs around 2,050 pounds, about 1 ton, but the same cubic yard fully saturated with water can weigh over 3,300 pounds, closer to 1.7 tons. If you are ordering by weight, or trying to figure out how many loads a trailer or truck can carry, plan for the wetter, heavier figure rather than the dry one, since guessing light and getting a wet delivery is how a trailer ends up overloaded.
Bags vs. bulk
For a small bed or a light topdress, bagged topsoil from a hardware store is usually the more convenient option: no delivery minimum, no truck, easy to store a partial bag of. A standard 1 cubic foot bag means 27 bags make up a full cubic yard; smaller 0.75 cubic foot bags need 36, larger 1.5 cubic foot bags need 18, and 2 cubic foot bags need about 14. Past a couple of cubic yards, bulk delivery is almost always cheaper per cubic foot and saves you from hauling a stack of bags, most suppliers price bulk soil by the cubic yard or the ton and it undercuts the equivalent bag count once you are ordering more than roughly 2 to 3 yards, though the exact break-even point depends on local pricing.
Why order about 10 percent extra
Ground settles, grading is rarely perfectly even, and some soil compacts down after the first watering, all of which eat into your measured volume. A simple, flat area with square edges needs only about 5 percent extra. A typical bed or topdress job with some unevenness needs closer to 10 percent. A project with a lot of grading, slopes, or obstacles to work around should plan for 15 percent. It is cheaper to have some soil left over for a future top-up than to run short mid-project and need a second delivery.
If part of the project is also adding mulch on top of the new soil, the mulch calculator uses the same area-and-depth approach for the finishing layer. If you are amending the soil itself rather than replacing it, the compost calculator works out the compost-to-soil ratio for tilling in, a different calculation than a straight topsoil fill.