Start with the finished slab area
This guide applies to a flat rectangular slab such as a patio, walkway, shed pad, or driveway section. Measure the finished length and width, then use the slab thickness specified for the project. If the pour includes steps, a curb, a turned-down edge, or another thickened section, calculate that extra volume separately.
Convert every dimension to the same unit
Slab dimensions are often measured in feet, while thickness is usually given in inches. Convert the thickness to feet before calculating cubic feet. For example, 4 inches is 0.33 feet, 5 inches is 0.42 feet, and 6 inches is 0.5 feet.
Use the slab volume formula
For one rectangular slab, multiply length by width by thickness. The result is cubic feet. Divide cubic feet by 27 to convert the volume to cubic yards for a ready-mix estimate.
Volume in cubic yards = cubic feet / 27
Worked slab example
Suppose a patio slab is 12 feet long, 10 feet wide, and 4 inches thick. Convert 4 inches to 0.33 feet before calculating.
39.6 / 27 = 1.47 cubic yards
With a 5% allowance, the planning estimate becomes about 1.54 cubic yards. This is a planning estimate, not a supplier quote or a construction specification.
Add a practical waste allowance
A 5% allowance is a useful starting point for a simple slab on a prepared, level base. Consider increasing it when the subgrade is uneven, the borders are irregular, forms are hand-built, or the project includes thickened edges. Avoid treating the allowance as a substitute for checking the actual site dimensions.
Compare bags and ready-mix
Bagged concrete can be practical for a small landing, repair, or compact pad. As the volume grows, ready-mix may be easier to place consistently and may reduce the amount of mixing required on site. Compare the calculator's bag count with local delivery minimums, access to the pour, material prices, and the time available for placement and finishing.
Check these items before ordering
- Confirm the finished length, width, and thickness after the forms are set.
- Calculate curbs, steps, thickened edges, and other non-uniform sections separately.
- Confirm project requirements for base preparation, reinforcement, joints, thickness, and local conditions.
- Ask the supplier about mix design, delivery minimums, small-load fees, access, and timing.
Use the slab calculator
Enter the dimensions and your local material prices in the Concrete Slab Calculator to estimate cubic yards, 60 lb bags, waste-adjusted volume, and a simple cost comparison.
Slab estimate FAQ
Can I calculate several matching slabs together?
Yes. Calculate one matching slab and multiply by the number of slabs, or enter the combined area when the thickness is the same throughout.
Does this estimate include gravel, reinforcement, or finishing?
No. This guide estimates concrete volume only. Base material, reinforcement, forms, delivery, labor, finishing, and local requirements should be planned separately.