Concrete Slab Calculator
Calculate exactly how much concrete you need for any slab project -- patios, driveways, sidewalks, garage floors, and more.
Average ready-mix concrete costs $120-$200 per cubic yard.
Enter dimensions above to see your concrete estimate.
How to Calculate Concrete for a Slab
Calculating concrete for a slab is straightforward once you know the three key measurements: length, width, and thickness. Our concrete slab calculator takes these inputs and instantly computes the total volume in cubic yards (the standard ordering unit in the United States) as well as cubic meters for international projects.
The basic formula is simple: Volume = Length x Width x Thickness. However, the critical detail most people miss is unit conversion. Length and width are typically measured in feet, while thickness is measured in inches. You must convert inches to feet (divide by 12) before multiplying, and then divide the total cubic feet by 27 to arrive at cubic yards.
Standard Slab Thicknesses
Choosing the correct thickness is essential for structural integrity. Here are the most common residential and light commercial applications:
- Patios and walkways: 4 inches -- the minimum recommended thickness for pedestrian traffic. A well-prepared base and proper curing make 4 inches perfectly adequate for foot traffic.
- Driveways: 5 to 6 inches -- standard for passenger vehicles. If you expect occasional heavy trucks, go with 6 inches.
- Garage floors: 5 to 6 inches -- similar to driveways. Many building codes require a minimum of 4 inches, but 5-6 inches provides better long-term durability.
- Shed foundations: 4 inches is sufficient for most sheds. Larger storage buildings may need 5 inches.
- Commercial slabs: 6 to 8 inches or more, depending on load requirements and soil conditions.
Concrete Volume by Common Slab Sizes
Below are pre-calculated volumes for common slab dimensions at 4-inch thickness. Use these as quick reference points, or enter your exact dimensions in the calculator above.
| Slab Size | Thickness | Cubic Yards | 80-lb Bags |
|---|---|---|---|
| 8 x 10 ft | 4 in | 0.99 | 45 |
| 10 x 10 ft | 4 in | 1.23 | 56 |
| 10 x 12 ft | 4 in | 1.48 | 67 |
| 12 x 12 ft | 4 in | 1.78 | 81 |
| 10 x 20 ft | 4 in | 2.47 | 112 |
| 20 x 20 ft | 4 in | 4.94 | 225 |
| 20 x 24 ft | 5 in | 7.41 | 337 |
Tips for Ordering Concrete
Always order 5-10% more than your calculated amount. Real-world conditions like uneven ground, form imperfections, and spillage mean you'll almost always use slightly more than the mathematical estimate. Running short during a pour is the worst-case scenario -- it creates cold joints that weaken the slab.
For projects requiring less than 1 cubic yard, buying bags from a home improvement store is usually most practical. For anything over 1 to 2 cubic yards, ordering ready-mix concrete delivered by truck is more economical and results in a better, more consistent pour.
When ordering ready-mix, tell the dispatcher: the total cubic yards you need, the PSI strength (3000 PSI is standard for slabs, 4000 PSI for driveways), any admixtures you want (fiber reinforcement, accelerator for cold weather, retarder for hot weather), and the slump you prefer (typically 4-5 inches for slabs).
Slab Preparation Checklist
Before pouring your concrete slab, make sure you have completed these steps:
- Excavate to the required depth (slab thickness + 4 inches for base material)
- Compact the subgrade with a plate compactor or hand tamper
- Add 4 inches of compacted gravel base for drainage
- Install forms using 2x4 or 2x6 lumber, staked every 2-3 feet
- Add reinforcement -- welded wire mesh or rebar on chairs
- Place a vapor barrier (6-mil polyethylene) for interior slabs
- Confirm dimensions and recheck your concrete quantity calculation