Learn the simple formula for calculating cubic yards for concrete, mulch, gravel, or dirt — plus real examples so you don’t over-order or run short.
A cubic yard is a 3-foot cube — 27 cubic feet, to be exact. Almost every material you’d order by the yard (concrete, mulch, topsoil, gravel, sand) gets sold that way because it’s a manageable unit for trucks and wheelbarrows alike. The formula never changes, no matter what you’re pouring or spreading:
Length (ft) × Width (ft) × Depth (ft) ÷ 27 = cubic yards
The part people get wrong isn’t the math — it’s the depth. Depth gets measured in inches on the ground and feet in the formula, so you have to convert first. A 4-inch slab is 0.33 feet, not 4.
Step-by-step
- Measure length and width in feet. For an irregular area, break it into rectangles and add the areas together.
- Convert your depth to feet. Divide inches by 12. So 3 inches = 0.25 ft, 6 inches = 0.5 ft.
- Multiply length × width × depth. This gives you cubic feet.
- Divide by 27. That’s your cubic yards.
- Round up, always. Suppliers sell in fractions of a yard but delivery trucks don’t like exact math — add 10% for compaction and spillage, especially with gravel or mulch.
Worked example: A patio slab 12 ft long, 10 ft wide, poured 4 inches thick.
12 × 10 × 0.33 = 39.6 cubic feet
39.6 ÷ 27 = 1.47 cubic yards
Add 10% waste: order about 1.6 yards, and most yards will round that to 1.5 or 2.
Where people actually mess this up
The single biggest error is forgetting to convert inches to feet before multiplying — it inflates your number by a factor of 12, and you’ll end up ordering way more than you need. The second most common mistake is measuring depth at one spot on a sloped or uneven yard instead of averaging a few points.
If you’d rather skip the manual conversion, plug your dimensions straight into the cubic yard calculator — it handles the inch-to-feet conversion and rounds for waste automatically. Planning a bigger project? See how much flooring you need, how much paint you need, or browse all Home & DIY calculators.
FAQ
How many bags of concrete equal a cubic yard?
An 80 lb bag makes about 0.6 cubic feet of concrete, so it takes roughly 45 bags to make one cubic yard.
Do I need to add extra for waste?
Yes — 10% is standard for concrete and gravel; go closer to 15% for mulch since it settles and compacts after spreading.
What if my area isn’t a rectangle?
Split it into rectangles or triangles, calculate each separately, then add the volumes together before dividing by 27.