Material Summary
- Concrete—
- Gravel base—
- Reinforcement—
- Recommended strength—
- Estimated concrete cost—
Slab Thickness Guide
| Project | Typical thickness |
|---|---|
| Sidewalk | 4 in |
| Patio | 4 in |
| Driveway | 5 – 6 in |
| Garage floor | 4 – 6 in |
| Shop floor | 5 – 6 in |
| RV / heavy pad | 6 in+ |
Concrete Strength Guide
| Use | PSI |
|---|---|
| Patio / sidewalk | 3,000 |
| Driveway | 4,000 |
| Garage / shop floor | 4,000 |
| Heavy vehicle / RV | 4,500+ |
How Much Concrete Do I Need for a Slab?
Concrete for a slab is volume: length times width times thickness. Multiply the length and width in feet, multiply by the thickness in feet (inches divided by 12), and you have cubic feet; divide by 27 for cubic yards, the unit ready-mix is sold by. A 20 by 20 foot patio at 4 inches is about 133 cubic feet, or roughly 5 cubic yards. This calculator returns the volume in yards, cubic feet, and 80 pound bags at once, then adds gravel base, reinforcement, a bagged-versus-ready-mix recommendation, and cost, so you plan the whole pour rather than just the concrete.
How Thick Should a Slab Be?
Thickness follows the load. Sidewalks and patios are typically 4 inches. Driveways for cars run 4 to 5 inches, and 5 to 6 inches if trucks or RVs park on them. Garage and shop floors are usually 4 to 6 inches depending on equipment. Thicker is not automatically better; the bigger gains come from a well-compacted gravel base, proper reinforcement, and control joints to manage cracking. Pick the project type here and the calculator sets a sensible default thickness and strength you can adjust.
Bagged Concrete vs Ready-Mix for a Slab
Almost any real slab tips into ready-mix territory. Bagged concrete works for very small pads, footings, and repairs under about a cubic yard, but a 4 inch patio passes that quickly. Mixing dozens of bags by hand is exhausting and risks cold joints between batches, while a ready-mix truck delivers a uniform load fast. Factor in short-load fees, but for a yard or more, ready-mix is usually cheaper and far less work than the bag equivalent.
Frequently Asked Questions
How much concrete do I need? Length times width times thickness in feet, divided by 27 for cubic yards; add 10 percent waste.
How thick should my slab be? 4 inches for patios and sidewalks, 5 to 6 for driveways and shop floors.
How much waste should I add? About 10 percent for uneven subgrade and spillage; more on rough ground.
Do I need gravel under a slab? Yes, about 4 inches of compacted gravel gives drainage and a stable base.