The formula
cubic yards of concrete = length (ft) × width (ft) × thickness (ft) ÷ 27
Concrete is sold by the cubic yard, which is 27 cubic feet. Work out the volume of the forms in cubic feet, divide by 27, and add an allowance for waste.
Most slabs are measured in feet with the thickness in inches. The shortcut that skips the conversion:
cubic yards = square feet × thickness (in) ÷ 324
The concrete calculator does all of this and accepts inches, feet, yards, centimeters or meters in each box.
Step by step
- Measure the forms. Inside dimensions, length and width in feet.
- Measure the real thickness. After the base is compacted, from the top of the base to the top of the form. Check several places.
- Convert inches to feet. Divide by 12: 4 in = 0.333 ft, 5 in = 0.417 ft, 6 in = 0.5 ft.
- Multiply length × width × thickness to get cubic feet.
- Divide by 27 for cubic yards.
- Add 5 to 10 percent for waste, uneven subgrade and spillage.
- Round up to the increment your supplier sells.
Worked examples
Slab
A 20 × 24 ft garage floor, 5 in thick.
- 5 ÷ 12 = 0.417 ft
- 20 × 24 × 0.417 = 200 cubic feet
- 200 ÷ 27 = 7.41 cubic yards
- With 8 percent extra: 7.41 × 1.08 = 8.0 cubic yards
Sidewalk
A walk 40 ft long, 4 ft wide, 4 in thick: 40 × 4 = 160 sq ft; 160 × 4 ÷ 324 = 1.98 cubic yards before waste.
Continuous footing
A footing 60 ft long, 20 in wide and 10 in deep. Convert both inch figures: 20 in = 1.667 ft, 10 in = 0.833 ft.
- 60 × 1.667 × 0.833 = 83.3 cubic feet
- 83.3 ÷ 27 = 3.09 cubic yards
Round column or pier
Eight piers, each 10 in across and 4 ft deep. Radius 5 in = 0.417 ft.
- One pier: 3.1416 × 0.417 × 0.417 × 4 = 2.18 cubic feet
- Eight piers: 17.5 cubic feet = 0.65 cubic yards
The footing calculator has a table of tube sizes.
Steps
Three solid steps, 4 ft wide, each 11 in deep (tread) and 7 in high (rise). Think of them as three blocks standing side by side, the first 7 in tall, the second 14 in, the third 21 in, each 11 in deep.
- Total height of the blocks: 7 + 14 + 21 = 42 in = 3.5 ft
- Volume: 4 × (11 ÷ 12) × 3.5 = 12.8 cubic feet
- 0.47 cubic yards, plus any landing at the top as its own block
Slab with a thickened edge
A 20 × 20 ft slab at 4 in, with the outer 12 in of the edge thickened by an extra 8 in:
- Main slab: 20 × 20 × 0.333 = 133.3 cubic feet
- Edge: the perimeter is 80 ft; 80 × 1 × 0.667 = 53.3 cubic feet (the corners are counted twice, which adds a little margin)
- Total: 186.7 cubic feet = 6.91 cubic yards before waste
In the calculator, enter the slab as section A and the thickened edge as section B.
How much extra to order
| Situation | Extra to add |
|---|---|
| Rigid forms on a flat, compacted base | about 5% |
| Typical slab or walkway | about 10% |
| Uneven ground, hand-dug footings, many small pours | 10 to 15% |
These are common rules of thumb. The point is that a short load is worse than leftover concrete: a second delivery carries its own charges, and waiting for it can leave a cold joint in the pour.
Turning yards into an order
- Ready-mix: call with the cubic yards, the strength (psi) you need, the slump and any additives, the address, and whether the truck can reach the forms. Plants commonly sell in quarter or half yard increments and may charge extra for small loads.
- Bags: multiply cubic yards by 45 for 80 lb bags or by 60 for 60 lb bags, or divide cubic feet by the yield printed on the bag. See the concrete bag calculator.
- Cost: cubic yards × the price per yard plus delivery fees. See concrete cost per yard.
Concrete yardage quick reference
| Area | 4 in | 5 in | 6 in |
|---|---|---|---|
| 50 sq ft | 0.62 yd³ | 0.77 yd³ | 0.93 yd³ |
| 100 sq ft | 1.23 | 1.54 | 1.85 |
| 200 sq ft | 2.47 | 3.09 | 3.70 |
| 300 sq ft | 3.70 | 4.63 | 5.56 |
| 500 sq ft | 6.17 | 7.72 | 9.26 |
| 1,000 sq ft | 12.35 | 15.43 | 18.52 |
Measure twice before you call the plant. The quantity you order is the quantity that arrives, and returning unused ready-mix is usually not an option.
Questions people ask
- What is the formula for concrete yardage?
- Cubic yards = length (ft) × width (ft) × thickness (ft) ÷ 27. With the thickness in inches: square feet × inches ÷ 324.
- How many yards of concrete do I need for a 10 × 10 slab?
- At 4 inches thick, 10 × 10 × 4 ÷ 324 = 1.23 cubic yards. With 10 percent extra, order about 1.36, which most plants would round to 1.5.
- How do I calculate concrete for a round column?
- 3.1416 × radius × radius × height, in feet, then divide by 27. A 12 in column 8 ft tall: 3.1416 × 0.5 × 0.5 × 8 = 6.28 cubic feet, or 0.23 cubic yards.
- How do I measure concrete for steps?
- Treat each step as a block that runs from the ground to its top. Add the blocks together. Width × tread depth × (rise of step 1 + rise to step 2 + ...) gives the volume of solid steps.
- Do I need to account for rebar or mesh?
- No. Rebar and mesh take up a tiny share of the volume. The waste allowance covers far more than that.