What you need to measure for a concrete slab
A concrete slab calculation requires 3 measurements: length, width, and thickness (depth). All 3 measurements convert to feet before the calculation, even though slab thickness is measured in inches. Standard slab thickness depends on the load: a 4-inch slab covers general-purpose projects such as patios, walkways, and shed floors, while a 5-inch to 6-inch slab suits driveways and surfaces that carry heavy loads. Residential slab standards from the American Concrete Institute (ACI 332) and local building codes commonly set a 4-inch minimum. Length and width together give the slab area in square feet, and accurate figures matter whether you hire a contractor or pour the slab yourself.
- Length is the longer horizontal dimension of the slab, measured in feet along the ground.
- Width is the shorter horizontal dimension of the slab, measured in feet across the ground.
- Thickness is the vertical depth of the slab, measured in inches and then divided by 12 to convert to feet.
How to calculate concrete for a slab step by step
To calculate concrete for a slab, follow 5 steps: measure, convert, multiply, divide, and add a waste factor. The same 5-step process works for any home improvement project that involves a poured slab, from a backyard patio to a workshop floor.
- Measure the slab length and width in feet. Run a tape measure along both horizontal sides of the slab and record each figure in feet. For a dimension that includes part of a foot, write the remainder as a decimal, such as 10.5 feet.
- Convert the slab thickness from inches to feet. Divide the thickness in inches by 12. A 4-inch slab equals 0.33 feet (4 ÷ 12), and a 6-inch slab equals 0.5 feet (6 ÷ 12).
- Multiply length × width × thickness to get the volume in cubic feet. Multiply all 3 figures together. A 10-foot by 12-foot slab at 0.33 feet thick produces 40 cubic feet.
- Divide the cubic feet by 27 to convert to cubic yards. Ready-mix concrete is sold by the cubic yard, so 40 cubic feet divided by 27 gives 1.48 cubic yards.
- Add a 5% to 10% waste factor and round up to the next quarter or half yard. A 1.48 cubic yard result plus 10% reaches 1.63 cubic yards, which rounds up to 1.75 cubic yards. For irregular shapes or when you also want a price, enter your dimensions into our concrete slab cost calculator.
Concrete slab calculation formula
The concrete slab calculation formula is Cubic Yards = (Length ft × Width ft × Thickness ft) ÷ 27. The formula runs in two parts: Length ft × Width ft × Thickness ft gives the volume in cubic feet, and dividing that result by 27 converts the volume to cubic yards. Length and width together set the slab footprint in square feet, and the thickness turns that footprint into a 3-dimensional volume.
A 10-foot by 12-foot slab poured 4 inches thick shows the formula in action. Convert the thickness first: 4 inches ÷ 12 = 0.33 feet. Multiply the dimensions: 10 ft × 12 ft × 0.33 ft = 40 cubic feet. Divide by 27: 40 ÷ 27 = 1.48 cubic yards. Add a 10% waste factor: 1.48 × 1.10 = 1.63 cubic yards, which rounds up to a 1.75 cubic yard order. The table below applies the same formula to 3 common slab scenarios.
| Slab Dimensions | Volume (cubic feet) | Volume (cubic yards) |
|---|---|---|
| 8 ft × 8 ft × 4 in | 21.3 | 0.79 |
| 10 ft × 12 ft × 4 in | 40.0 | 1.48 |
| 12 ft × 16 ft × 6 in | 96.0 | 3.56 |
How to convert cubic feet to cubic yards of concrete
One cubic yard equals 27 cubic feet. A cubic yard is a cube that measures 3 feet on every side, so 3 ft × 3 ft × 3 ft produces 27 cubic feet of volume. Slab measurements come out in feet and cubic feet, but ready-mix concrete suppliers sell and bill concrete by the cubic yard, so every slab volume converts from cubic feet to cubic yards before the order. The conversion uses one step: cubic feet ÷ 27 = cubic yards. A slab that holds 54 cubic feet equals 2 cubic yards, because cubic feet ÷ 27 = cubic yards, and a slab that holds 13.5 cubic feet equals 0.5 cubic yards by the same step.
How much waste factor to add for a concrete slab
Add a 5% to 10% waste factor to a concrete slab order. The waste factor covers the concrete lost to spillage, an uneven or over-excavated subgrade, form deflection, and edge slump. Concrete carries a lower waste factor than roofing or flooring materials such as asphalt shingles and vinyl planks, because a slab pour produces no cutting offcuts; the surplus comes only from ground conditions and handling. The amount to add depends on how level and firm the subgrade is.
| Slab Condition | Waste Factor | When to Use |
|---|---|---|
| Standard form-ready subgrade | 5% | Compacted, level base set inside solid forms |
| Uneven subgrade | 8% | Minor dips or variable depth across the base |
| Hand-dug or rough subgrade | 10% | Hand-excavated ground with irregular depth |
Ready-mix suppliers bill in quarter-yard increments and set a minimum load, so round the final figure up to the next quarter yard rather than ordering a partial amount.
Working out concrete volume by hand stays straightforward for a simple square or rectangular slab. Once the cubic yardage is set, the next question is what the pour costs, and ready-mix pricing shifts with volume, delivery distance, and finish. Drop the slab length, width, and thickness into our concrete slab cost calculator to convert any concrete slab project into the exact concrete quantity to order and an estimated price in a single step.
How much concrete do I need for common slab sizes
A common concrete slab holds 1.23 to 7.11 cubic yards at 4 inches thick, depending on the slab size. The table below lists raw concrete volume for 6 common slab sizes, from a 10-foot by 10-foot slab to a 24-foot by 24-foot slab. These figures show raw volume before waste. Add 5% to 10% to your final order. Each row gives the area in square feet and the cubic yards at both 4-inch and 6-inch thickness.
| Slab Size | Area (square feet) | Cubic Yards at 4 in | Cubic Yards at 6 in |
|---|---|---|---|
| 10 ft × 10 ft | 100 | 1.23 | 1.85 |
| 12 ft × 12 ft | 144 | 1.78 | 2.67 |
| 12 ft × 16 ft | 192 | 2.37 | 3.56 |
| 16 ft × 20 ft | 320 | 3.95 | 5.93 |
| 20 ft × 20 ft | 400 | 4.94 | 7.41 |
| 24 ft × 24 ft | 576 | 7.11 | 10.67 |
How many bags of concrete equal a cubic yard
One cubic yard of concrete equals 90 bags of 40 lb mix, 60 bags of 60 lb mix, or 45 bags of 80 lb mix. Bagged concrete mix from brands such as Quikrete and Sakrete prints a yield on every bag, measured in cubic feet. Dividing 27 cubic feet, the volume of one cubic yard, by the yield of each bag size gives the bag count.
| Bag Size | Yield per Bag | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 cubic feet | 90 |
| 60 lb | 0.45 cubic feet | 60 |
| 80 lb | 0.60 cubic feet | 45 |
Bag yield varies by about 5% with the brand and mix, so confirm the printed yield on the label before a final order. Bagged mix suits a DIY pour on a small slab of 1 cubic yard or less. A 10-foot by 10-foot slab at 4 inches thick already reaches 1.23 cubic yards, or about 56 bags of 80 lb mix, which makes ready-mix the cheaper and lower-effort option above that point. To skip the bag math and get an exact count for your slab, use our concrete bag calculator.