The Universal Concrete Formula
Whether you're pouring a driveway, a backyard patio, a sidewalk, or a foundation slab, the formula for calculating concrete is identical. Concrete is measured and ordered in cubic yards in the United States. Here is the complete process:
The Formula
Cubic Yards = (Length × Width × Thickness in feet) ÷ 27
Since thickness is usually measured in inches, convert it first: Thickness in feet = Thickness in inches ÷ 12
Standard Concrete Thickness by Project Type
One of the most common mistakes homeowners make is pouring concrete that is too thin. The correct thickness depends on what loads the slab will carry:
| Project Type | Recommended Thickness | Notes |
|---|---|---|
| Sidewalk / walkway | 4 inches | Foot traffic only |
| Patio (foot traffic) | 4 inches | Furniture and BBQ grills |
| Residential driveway (passenger cars) | 4–5 inches | No trucks or RVs |
| Driveway (SUVs, light trucks) | 5–6 inches | Heavier personal vehicles |
| Heavy vehicle / RV pad | 6–8 inches | Delivery trucks, campers |
| Garage floor slab | 4–6 inches | Rebar required at 4 inches |
| Foundation / footing | 8–12 inches | Structural — engineer required |
Worked Examples
Example 1: Standard 2-Car Driveway
Dimensions: 20 ft wide × 40 ft long at 5 inches thick.
- Convert thickness:
5 ÷ 12 = 0.417 ft - Cubic feet:
20 × 40 × 0.417 = 333.6 cu ft - Cubic yards:
333.6 ÷ 27 = 12.36 cu yd - Add 10% waste:
12.36 × 1.10 = 13.6 cu yd → order 14 yards
Example 2: Backyard Patio
Dimensions: 16 ft × 24 ft at 4 inches thick.
- Convert thickness:
4 ÷ 12 = 0.333 ft - Cubic feet:
16 × 24 × 0.333 = 127.9 cu ft - Cubic yards:
127.9 ÷ 27 = 4.74 cu yd - Add 10% waste:
4.74 × 1.10 = 5.2 cu yd → order 5.5 yards
Example 3: Sidewalk
Dimensions: 60 ft long × 4 ft wide at 4 inches thick.
60 × 4 × 0.333 = 79.9 cu ft ÷ 27 = 2.96 cu yd- Add 10%:
2.96 × 1.10 = 3.26 → order 3.5 yards
Ready-Mix Truck vs. Pre-Mixed Bags: Which Is Better?
| Method | Best For | Cost Range | Minimum Order |
|---|---|---|---|
| Ready-Mix Truck | Projects over 1 yard | $125–$165 per cubic yard | 1 yard (short-load fees apply) |
| 80 lb Bags (0.60 cu ft each) | Projects under 1 yard | $5–$8 per bag ($225–$360/yard) | None |
For projects over 1 cubic yard, ready-mix concrete is almost always cheaper per cubic yard than mixing bags. The crossover point is approximately 30–45 bags (1 cubic yard). Above that, order a truck. Below that, bags are more convenient.
How Many Bags Do You Need?
| Project Volume | 80 lb Bags Needed | Recommendation |
|---|---|---|
| 0.5 cubic yards | ~23 bags | Use bags |
| 1 cubic yard | ~45 bags | Bags or smallest truck order |
| 3 cubic yards | ~135 bags (not practical) | Order ready-mix |
| 10+ cubic yards | Not feasible | Ready-mix only |
Control Joints: The Key to Crack-Free Concrete
No concrete slab is crack-free — but you can control where the cracks occur by cutting control joints (also called contraction joints) into the surface. These are intentional weakened planes that guide cracking to predictable, neat lines. The rule of thumb for joint spacing:
- Joint spacing (feet) = 2–3 × slab thickness (inches). For a 4-inch slab: joints every 8–12 feet. For a 5-inch slab: joints every 10–15 feet.
- Joints should be cut to a depth of at least 1/4 the slab thickness. For 4-inch slab, cut joints at least 1 inch deep.
- Cut joints within 6–12 hours of finishing while the concrete is still green (not fully cured).
Use our free Concrete Calculator to get instant cubic yard calculations, bag counts, and cost estimates for any slab, driveway, or sidewalk project.