Concrete Calculator
Find out how much concrete a slab, patio or shed base needs — in cubic yards for ready-mix, or in bags if you're mixing it yourself.
Concrete volume formula
Cubic yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27
A 12 × 10 ft slab that is 4 inches thick holds 40 cubic feet, or about 1.48 cubic yards. With 10% extra that is 44 cubic feet. To fill it with bags you would need about 74 bags of 80 lb mix.
How much does a bag of concrete make?
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Yields vary slightly by brand — check the bag.
Bags or ready-mix truck?
Bags make sense for small projects such as fence posts, steps or a slab under about one cubic yard. Beyond that, mixing dozens of bags by hand is slow and hard to finish evenly, and ready-mix delivery is usually easier. Many suppliers charge a short-load fee for orders under a few yards.
Recommended thickness
- Sidewalks and patios: 4 inches
- Driveways: 4 – 6 inches (6 for heavy vehicles)
- Shed or garage floor: 4 – 6 inches
Local building codes may require specific thicknesses, reinforcement or footings — check before you pour.
Frequently asked questions
How many bags of concrete for a fence post?
A typical 4×4 post in a 10-inch-wide, 3-foot-deep hole needs about 1.4 cubic feet — around two to three 50–60 lb bags of fast-setting mix, depending on the brand.
Why order extra concrete?
Uneven ground, forms that bow slightly and spills all use more concrete than the math suggests. Running short in the middle of a pour causes weak joints.
Last reviewed November 14, 2026.
