How to Calculate Concrete for a Slab
Calculating the amount of concrete needed for a slab, patio, or driveway requires finding the total cubic volume of the poured area. Ready-mix concrete companies universally deliver and price wet concrete by the cubic yard, while home improvement stores sell dry concrete mix in bags scaled to cubic feet fractions.
- L = Slab Length (in feet)
- W = Slab Width (in feet)
- T = Desired Thickness (in inches)
Note: Dividing the thickness in inches by 12 converts it to feet. Dividing the total cubic feet by 27 converts the final volume to cubic yards.
If you are planning to mix your own concrete using pre-mixed dry bags, keep in mind that one cubic yard equals 27 cubic feet. A standard 80-pound bag of concrete mix yields approximately 0.60 cubic feet when mixed with water.
| Slab Thickness | Coverage Area (Square Feet) | Common Application |
|---|---|---|
| 4 inches | 81.0 sq ft | Standard sidewalks and residential patios. |
| 5 inches | 64.8 sq ft | Standard residential driveways and garage floors. |
| 6 inches | 54.0 sq ft | Heavy vehicle driveways and reinforced slabs. |
| 8 inches | 40.5 sq ft | Commercial slabs and heavy equipment pads. |
Frequently Asked Questions
How many 80lb bags of concrete are in a cubic yard?
It takes exactly 45 standard 80-pound bags to yield one cubic yard of concrete. A cubic yard is 27 cubic feet, and each 80-pound bag yields roughly 0.60 cubic feet of wet concrete (27 ÷ 0.60 = 45 bags). Alternatively, it takes 60 bags of the lighter 60-pound mix to equal a yard.
Should I order extra concrete for waste?
Yes. Industry standard practice dictates adding a 5% to 10% "waste factor" to your final calculation before placing an order. This margin accounts for uneven subgrades, forms bowing slightly under pressure, spillage during pouring, and minor miscalculations in measurement depth.
How thick should a standard concrete slab be?
The standard thickness for residential concrete slabs, including patios, walkways, and light-traffic floors, is 4 inches. If the slab will bear heavier loads, such as an RV pad or a busy residential driveway, upgrading to a 5-inch or 6-inch reinforced thickness is strongly recommended.