The one formula you need
Topsoil, fill, rock and mulch are sold by the cubic yard. A cubic yard is a box 3 feet long, 3 feet wide and 3 feet tall. That's 27 cubic feet.
So the math is square feet of area, times depth in feet, divided by 27. The answer is cubic yards.
Depth is where people slip up. You think about depth in inches, but the formula wants feet, so divide the inches by 12. Three inches is 0.25 feet. Four inches is about 0.33 feet. Six inches is 0.5 feet.
There's also a shortcut. One cubic yard spread 1 inch deep covers 324 square feet. At 3 inches deep it covers 108 square feet, at 4 inches about 81, and at 6 inches 54. Divide your area by the right number and you have your yards.
How to measure your yard
Grab a long tape measure and a notepad. Break the yard into simple shapes and measure each one on its own.
For a rectangle, multiply length by width. A backyard that's 40 feet by 25 feet is 1,000 square feet.
A triangle is length times width, cut in half. A corner bed that runs 12 feet along one fence and 10 feet along the other is 120 divided by 2, or 60 square feet.
Round beds take one more step. Measure from the center to the edge, which is the radius. Multiply it by itself, then by 3.14. A round bed with a 5 foot radius is 5 times 5 times 3.14, or about 79 square feet.
Add up the pieces. Subtract the patio, the driveway and anything else that won't get soil. Then sketch the yard with the numbers written on it. Any supplier or crew is going to ask for that drawing anyway.
How deep to go for sod, beds, rock and mulch
For sod or seed, plan on 4 to 6 inches of good topsoil. Grass roots need loose ground to push into. Sod laid on an inch of topsoil over hard clay can look great for a month, then go patchy because the roots never got past that thin top layer.
Flower and shrub beds want 8 to 12 inches. Trees get a wider, deeper hole of their own at planting, so don't count them in your bed math.
Landscape rock goes about 3 inches deep over fabric for most decorative rock. Bigger cobble needs more. Spread it too thin and the fabric shows through by the next summer.
Bark mulch goes 2 to 3 inches deep. Piling it thicker than that can keep water from getting down to the roots, and it shouldn't be heaped against tree trunks.
Three worked examples
A new lawn. The backyard is 40 by 25 feet, so 1,000 square feet. You want 4 inches of topsoil under the sod, and 4 inches is 0.33 feet. 1,000 times 0.33 is 330 cubic feet. 330 divided by 27 is about 12.2 cubic yards. Order 13.
A rock strip along the back fence. It's 60 feet long and 10 feet wide, so 600 square feet. Rock at 3 inches is 0.25 feet. 600 times 0.25 is 150 cubic feet. 150 divided by 27 is about 5.6. Order 6 yards.
Mulch around the shrubs. Two beds add up to 400 square feet, and you want 3 inches. 400 times 0.25 is 100 cubic feet. 100 divided by 27 is about 3.7. Order 4 yards.
Always round up. Loose soil settles once it's spread, raked and rolled, and paying for a second small delivery to cover the last corner is no fun.
Screened topsoil vs fill dirt
People mix these up all the time, and the lawn pays for it.
Fill dirt is subsoil. It's whatever came out of a hole somewhere else, so it can be clay, sand, rock, and now and then a chunk of old concrete. Fill is for raising a low spot, backing up a wall or building up grade. It packs down hard, which is what you want under a patio and the opposite of what you want under grass.
Screened topsoil has been run through a screen to pull out rocks, roots and clods. It's loose and has some organic matter in it, and that's what roots grow in. Ask where your topsoil came from, and look at it before it gets dumped. Good topsoil is dark and crumbles in your hand. If it's sticky gray clay or full of rock, it's fill with a nicer name.
A lot of yards need both. Fill to fix the grade first, then a top layer of screened topsoil to grow in.
Why new-build lots need more than you think
When a house gets built, the topsoil usually gets scraped off the lot. Digging the basement brings up subsoil, which in much of Utah County means Lake Bonneville clay or bench gravel, and that gets spread back around the house. Then trucks, skid steers and concrete crews drive over it for months. What's left when you get the keys is compacted subsoil that water runs off instead of soaking into.
Spreading 2 inches of topsoil on top of that doesn't fix it. Water soaks through the new layer, hits the hard clay and sits there. Roots stay shallow. The lawn looks fine in May and burns in July.
What works is loosening the compacted ground first by ripping or tilling it, fixing the grade so it falls away from the house, and then adding a full 4 to 6 inches of screened topsoil. Our soil also runs alkaline, so a soil test before you buy plants is worth doing.
If you're in one of the new neighborhoods around Thanksgiving Point in Lehi, check your HOA deadline for getting the yard in and order soil early. Delivery schedules fill up in spring.
We source, haul and deliver topsoil and fill for homeowners and builders across Utah County. Send us your measurements and we'll check the math with you before the truck shows up.