Step by step
- Measure in feet. Convert inches by dividing by 12. A depth of 3 inches is 0.25 ft.
- Find the area in square feet using the shape formulas below.
- Multiply by depth in feet to get cubic feet.
- Divide by 27 to get cubic yards.
- Add 5 to 10% for settling, uneven ground and spills.
Area formulas
| Shape | Area | Example | Area |
|---|---|---|---|
| Rectangle | length × width | 12 ft × 8 ft | 96 sq ft |
| Circle | 3.1416 × radius² | 10 ft across (radius 5) | 78.5 sq ft |
| Triangle | base × height ÷ 2 | 10 ft base, 6 ft tall | 30 sq ft |
| Half circle | 1.5708 × radius² | radius 4 ft | 25.1 sq ft |
| Trapezoid | (top + bottom) ÷ 2 × height | 6 ft and 10 ft, 5 ft tall | 40 sq ft |
Three worked examples
A mulch bed
A bed 12 ft × 8 ft with mulch 3 inches deep: 96 sq ft × 0.25 ft = 24 cubic feet ÷ 27 = 0.89 cubic yards, or 0.98 with 10% extra. Order 1 yard.
A round fire pit area
A circle 10 ft across filled with gravel 4 inches deep: 78.5 sq ft × 0.333 ft = 26.2 cubic feet ÷ 27 = 0.97 cubic yards, or 1.07 with 10% extra.
An L-shaped patio base
Split it into two rectangles that do not overlap: 20 × 10 ft and 8 × 6 ft. That is 200 + 48 = 248 sq ft. At 4 inches deep: 248 × 0.333 = 82.7 cubic feet ÷ 27 = 3.06 cubic yards, or 3.37 with 10% extra.
Irregular areas
Break the shape into rectangles, circles and triangles, find each area and add them. Enter the total in the area box of the cubic yard calculator, which replaces length and width. If you only have the area, the square feet to cubic yards calculator does the same job.
Shapes beyond a rectangle
| Shape | Area formula | Example | Cubic yards at 3 in |
|---|---|---|---|
| Rectangle | length × width | 20 × 10 ft = 200 sq ft | 1.85 |
| Circle | 3.1416 × radius² | radius 5 ft = 78.5 sq ft | 0.73 |
| Triangle | ½ × base × height | base 12, height 8 = 48 sq ft | 0.44 |
| Half circle | ½ × 3.1416 × radius² | radius 6 ft = 56.5 sq ft | 0.52 |
Mistakes to avoid
- Leaving depth in inches. Divide by 12 first, so 4 inches is 0.333 feet.
- Mixing units, such as yards for length and feet for depth.
- Forgetting that the finished depth and the loose depth differ for fill and soil that will settle.
- Skipping the extra. A 5 to 10% allowance covers spills and uneven ground.
Frequently asked questions
What is the formula for cubic yards?
Length × width × depth, all in feet, divided by 27.
How do I calculate cubic yards of mulch?
Multiply the bed area in square feet by the depth in feet and divide by 27. A 100 sq ft bed 3 inches deep needs 0.93 cubic yards.
How do I calculate cubic yards for a circle?
Find the area with 3.1416 × radius squared, multiply by the depth in feet, then divide by 27.