Square Footage With Depth: Concrete, Mulch & Gravel Volume
Some materials sell by volume, not area. Turn square footage plus a depth into cubic feet and cubic yards for concrete, mulch, gravel and topsoil.
By Zishan Khan ·
Some materials aren’t sold by area at all. Concrete, mulch, gravel and topsoil are sold by volume, because they have depth — you’re filling a space, not covering a surface. For those, square footage is only half the calculation. You work out the area, multiply by how deep the layer is, and that gives you volume.
This guide covers turning square footage into cubic feet and cubic yards, with worked numbers for the jobs that need it. Start with the square footage calculator for the area, then bring in the depth.
Area × depth = volume
The formula is the area you already know, times the depth:
square feet × depth in feet = cubic feet
The catch is that depth is almost always given in inches, and the formula wants feet. Divide the inches by 12 first. A 4-inch layer is 4 ÷ 12 = 0.333 feet; a 3-inch layer is 0.25 feet; a 2-inch layer is 0.167 feet.
So a 100-square-foot slab poured 4 inches deep is 100 × 0.333 = 33.3 cubic feet.
Cubic feet to cubic yards
Concrete, mulch and gravel are usually ordered by the cubic yard, not the cubic foot. There are 27 cubic feet in a cubic yard (3 × 3 × 3), so divide your cubic feet by 27:
33.3 cubic feet ÷ 27 = 1.23 cubic yards of concrete for that slab. Suppliers deliver in fractions of a yard, so you’d order 1.25 and have a little spare — which you want, because concrete can’t be topped up once it’s setting.
Worked examples
The depth is what changes from job to job. Typical ones:
| Job | Area | Depth | Volume |
|---|---|---|---|
| Concrete slab | 100 sq ft | 4 in | 1.23 cu yd |
| Mulch bed | 200 sq ft | 3 in | 1.85 cu yd |
| Gravel driveway | 300 sq ft | 4 in | 3.70 cu yd |
| Topsoil / lawn | 500 sq ft | 2 in | 3.09 cu yd |
Each one is the same two steps: area × depth-in-feet for cubic feet, then ÷ 27 for cubic yards.
Bagged vs. bulk
Below about a cubic yard, bagged material is often easier; above it, bulk delivery is cheaper. The two are quoted differently, so know which you’re buying:
- Bagged material lists its coverage in cubic feet — a bag of mulch is commonly 2 cubic feet, and soil bags 0.75 or 1 cubic foot. Divide your total cubic feet by the bag size to get the number of bags.
- Bulk material is sold by the cubic yard and dumped or delivered loose. Order in quarter-yard steps.
A 2-cubic-foot mulch bag spread 3 inches deep covers 8 square feet, so that 200-square-foot bed at 3 inches needs about 25 bags — or, at 1.85 cubic yards, one bulk delivery that usually works out cheaper.
The mistakes to avoid
- Leaving depth in inches. Convert to feet first (inches ÷ 12), or the volume comes out twelve times too big.
- Forgetting to divide by 27. Cubic feet aren’t cubic yards; suppliers quote yards.
- Ordering exactly what you calculated. Add a bit for concrete (it can’t be extended) and for gravel that compacts as it settles.
- Mixing bagged and bulk units. Bags are in cubic feet, bulk in cubic yards — don’t compare them without converting.
Frequently asked questions
How do you calculate volume from square footage?
Multiply the area in square feet by the depth in feet. If the depth is in inches, divide it by 12 first. A 150-square-foot patio at 4 inches deep is 150 × 0.333 = about 50 cubic feet. Divide by 27 if you need cubic yards, which is how most bulk materials are sold.
How many cubic feet are in a cubic yard?
- A cubic yard is a cube 3 feet on each side, and 3 × 3 × 3 is 27. So to convert cubic feet to cubic yards you divide by 27, and to go back you multiply by 27. It’s the step people most often skip when ordering concrete or mulch.
How much concrete do I need for a slab?
Work out the slab’s area in square feet, multiply by its thickness in feet, then divide by 27 for cubic yards. A 10-by-10-foot slab at 4 inches thick is 100 × 0.333 ÷ 27 = about 1.23 cubic yards. Order a little extra — concrete sets fast and can’t be topped up.
How do I figure out how much mulch or topsoil to buy?
Measure the bed’s square footage, pick a depth (2–3 inches is common for mulch, 2 inches for topsoil over a lawn), and multiply for cubic feet. For bags, divide by the bag size; for bulk, divide the cubic feet by 27 for cubic yards. Deeper beds need proportionally more.
Why do some materials use volume instead of square footage?
Because they fill a space with real depth rather than coating a surface. Paint and flooring are thin enough to sell by area, but concrete, gravel, mulch and soil form a layer inches deep, so the amount you need depends on that depth as much as the area. Volume captures both.
The short version
When a material has depth, square footage alone won’t order it — you need the volume.
- Area × depth (in feet) = cubic feet; divide inches by 12 before multiplying.
- Divide cubic feet by 27 to get cubic yards, the usual bulk unit.
- Convert bag coverage from cubic feet; order bulk by the cubic yard.
- Add a margin for concrete and for gravel that compacts.
Get the area first with the square footage calculator, then multiply by your depth — that one extra measurement turns a surface into the volume you actually order.
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