Short answer
A 10 ft × 10 ft slab, 4 inches thick, needs 1.23 cubic yards of concrete, and 1.36 cubic yards with 10% extra for waste. Use the formula below with your own slab size and thickness, or try the Concrete Slab Calculator for an instant result.
The formula
Volume in cubic feet = length × width × thickness, with all three in feet. Cubic yards = cubic feet ÷ 27, because one cubic yard is 27 cubic feet. Add 10% for waste and round up, because spillage, uneven ground and formwork always take a little extra.
Worked example: 10 ft × 10 ft slab, 4 inches thick
Step 1: convert the thickness to feet: 4 inches ÷ 12 = 0.333 feet. Step 2: the volume is 10 × 10 × 0.333 = 33.33 cubic feet. Step 3: convert to cubic yards: 33.33 ÷ 27 = 1.2346 cubic yards (about 1.23). Step 4: add 10% for waste: 1.2346 × 1.10 = 1.36 cubic yards to order. That is the same result the Concrete Slab Calculator gives for these numbers.
Concrete for common slab sizes, 4 inches thick
Order the "with waste" column. If your slab is a different thickness, scale the result: a 6-inch slab needs 1.5 times the 4-inch volume.
| Slab (length × width) | Cubic yards | With 10% waste |
|---|---|---|
| 10 ft × 10 ft | 1.23 cubic yards | 1.36 cubic yards |
| 12 ft × 12 ft | 1.78 cubic yards | 1.96 cubic yards |
| 10 ft × 20 ft | 2.47 cubic yards | 2.72 cubic yards |
| 20 ft × 20 ft | 4.94 cubic yards | 5.43 cubic yards |
How to measure your slab
Measure the length and width along the inside of the formwork, and measure the thickness from the base to the top of the form. If the slab has more than one section, measure each section separately and add the volumes together. Always convert inches to feet before multiplying: divide inches by 12.
What changes the number
Thickness changes it fastest, because volume is directly proportional to thickness: doubling the thickness doubles the concrete. Waste matters too, because uneven ground, spillage and formwork gaps all consume extra mix. For footings, driveways and walls, the shape is different, so use the calculator made for that shape.
Common mistakes
The most common mistake is multiplying inches directly with feet, which inflates the answer by 12 times. The next is ordering with no waste allowance and running short mid-pour. The third is measuring only one section of a multi-section slab and forgetting the rest.
Try it with your own numbers
Open the Concrete Slab Calculator, enter your length, width and thickness, and it gives the cubic yards and an estimated cost. For a driveway, see the Concrete Driveway Calculator, and for cost only, see the Concrete Cost Calculator. More guides are on the BuildCalc Blog and in Construction calculators.
Results are estimates and general information only. Always confirm final quantities with your supplier or a qualified professional.