Cubic yards is the unit most bulk materials are sold and delivered in, including concrete, gravel, crushed stone, mulch, topsoil and sand. Getting the calculation right is the difference between one clean delivery and an expensive second trip. The good news is that the method is identical for every one of those materials.
This guide covers the formula, the cubic feet to cubic yards conversion that trips most people up, several worked examples, a shortcut the trades use, and how the same job looks in metric.
The formula
Volume is length times width times depth. The only rule is that every measurement has to be in the same unit before you multiply. Working in feet and inches, the depth is almost always given in inches, so you convert it to feet by dividing by 12. Then you divide the total cubic feet by 27, because one cubic yard is a cube three feet on each side, which is 27 cubic feet.
Worked example one: a topsoil bed
You are filling a bed 10 feet long by 12 feet wide with 4 inches of topsoil. First convert the depth: 4 inches divided by 12 is 0.333 feet. Then 10 times 12 times 0.333 is 40 cubic feet. Divide by 27 and you get about 1.48 cubic yards. Order a yard and a half and you have a small buffer.
Worked example two: a concrete slab
A 12 by 16 foot slab at 6 inches: 12 times 16 is 192 square feet, times 0.5 feet is 96 cubic feet, divided by 27 is about 3.56 cubic yards. With a 7 percent concrete waste allowance that rounds up to about 3.8 cubic yards, which is what you would order from the ready mix supplier.
The shortcut the trades use
For a 4 inch layer there is a handy rule: divide the square footage by 81 to get cubic yards. A 4 inch pour is one third of a foot deep, and 27 times 3 is 81. So 200 square feet at 4 inches is 200 divided by 81, about 2.47 cubic yards, which matches the long method. For a 6 inch layer, divide by 54 instead, since 6 inches is half a foot and 27 times 2 is 54.
Coverage reference
| Depth | Square feet per cubic yard | Divide square footage by |
|---|---|---|
| 1 inch | 324 sq ft | 324 |
| 2 inches | 162 sq ft | 162 |
| 3 inches | 108 sq ft | 108 |
| 4 inches | 81 sq ft | 81 |
| 6 inches | 54 sq ft | 54 |
In metric
Metric is simpler because there is no 27 to remember. Multiply length, width and depth all in metres for a result in cubic metres. An area 3 metres by 4 metres at 10 cm deep is 3 times 4 times 0.1, which is 1.2 cubic metres. One cubic yard is about 0.765 cubic metres, so if a supplier quotes in metres and your figure is in yards, multiply by 0.765 to convert.
Common mistakes
- Forgetting to convert inches to feet before multiplying. A 4 inch depth is 0.333 feet, not 4.
- Dividing by 9 instead of 27. Nine converts square feet to square yards, not cubic.
- Measuring inside a form but pouring against an uneven subgrade that is deeper in places. Measure the realistic worst case, not the shallowest point.
- Ordering the bare calculated figure with no waste. Add 5 to 10 percent depending on the material.