How to Calculate Concrete for a Slab, Patio, or Driveway
Ordering the right amount of concrete is one of the most important steps in any slab, patio, or driveway project. Order too little and you face expensive short-load fees or cold joints. Order too much and you pay for material you cannot return. This guide walks you through the calculation process, explains when to use bagged concrete versus ready-mix, and highlights the mistakes we see most often.
The basic formula
Concrete volume is calculated the same way you would calculate the volume of a rectangular prism:
There are 27 cubic feet in one cubic yard. Ready-mix concrete is sold by the cubic yard, so converting to cubic yards is essential for ordering.
Step-by-step example: 10x10 patio at 4 inches thick
- Convert thickness to feet: 4 inches / 12 = 0.333 feet
- Calculate cubic feet: 10 x 10 x 0.333 = 33.3 cubic feet
- Calculate cubic yards: 33.3 / 27 = 1.23 cubic yards
- Add 5-10% for waste and spillage: 1.23 x 1.10 = 1.36 cubic yards
For this project, bagged concrete is practical. You would need approximately 56 bags of 80-pound concrete or 75 bags of 60-pound concrete.
Bagged concrete vs. ready-mix: which should you choose?
| Factor | Bagged Concrete | Ready-Mix |
|---|---|---|
| Best for | Projects under 1 cubic yard | Projects over 1 cubic yard |
| Cost per cubic yard | $175-$250 (material only) | $110-$165 (including delivery) |
| Labor | High — you mix each bag | Low — truck does the work |
| Consistency | Variable depending on mixing | Professional batch plant mix |
| Timing flexibility | High — mix as you go | Low — truck arrives on schedule |
The break-even point is typically around 1 cubic yard. At 2 cubic yards, ready-mix is almost always cheaper and produces better results.
How thick should your slab be?
- 4 inches: Standard for patios, walkways, and shed floors
- 5-6 inches: Driveways and areas with light vehicle traffic
- 6-8 inches: Heavy equipment, RV pads, and commercial use
Always include a gravel base of 4-6 inches beneath the slab for drainage and to prevent settling. Our calculator assumes a standard residential application; verify thickness requirements with local building codes.
Common mistakes to avoid
- Forgetting the base: Measure finished slab dimensions, not the excavated area. The excavation will be deeper and wider than the slab itself.
- Ignoring waste: Add at least 5% for a simple rectangle and 10% for irregular shapes. Complex forms with curves can need 15% extra.
- Ordering exact volume: Concrete trucks carry whole-yard increments in many markets. Ordering 3.2 yards may require paying for 4 yards anyway.
- Wrong unit math: Mixing inches and feet is the most common error. Convert all dimensions to feet before multiplying.
Use our free calculator
Save time and avoid math errors with our Concrete Slab Calculator. It instantly computes cubic yards, bag counts for 40, 60, and 80-pound bags, and estimated cost based on your local prices.