Pouring a concrete patio, driveway, or sidewalk? One of the most confusing parts of ordering concrete is the unit mismatch: you measure your slab's surface in square feet or square yards, but ready-mix suppliers sell concrete in cubic yards, while home improvement stores sell it in 60-lb or 80-lb bags.
Because concrete begins curing within minutes of being poured, running short by even a half-yard can ruin an entire project. This guide shows you how to convert surface area and slab thickness into precise cubic yards and bag counts.
Square Yards (Area) vs. Cubic Yards (Volume)
A square yard measures 2D flat surface area (3 ft × 3 ft = 9 sq ft). But a concrete slab has a third dimension: thickness (depth).
A cubic yard is a 3D block measuring 3 ft × 3 ft × 3 ft, which equals 27 cubic feet. When concrete is poured 4 inches thick (which is ⅓ of a foot), each cubic yard covers:
- 27 cubic feet ÷ 0.333 ft depth = 81 square feet
- 81 sq ft ÷ 9 = 9 square yards
Anatomy of a Poured Concrete Slab
- 4 Inches: Patios, walkways, sidewalks, and shed floors.
- 5 Inches: Standard residential driveways and minivan/SUV parking.
- 6 Inches: Heavy equipment pads, RV parking, and commercial approaches.
The Concrete Formulas
To calculate cubic yards directly from your dimensions:
Live Concrete & Bag Calculator
Interactive Slab, Cubic Yard & Bag Estimator
How Many Bags of Concrete in a Cubic Yard?
If you choose to mix your own concrete with bagged Quikrete or Sakrete:
- 80-pound bags: 1 bag yields 0.60 cubic feet. You need 45 bags to make 1 cubic yard.
- 60-pound bags: 1 bag yields 0.45 cubic feet. You need 60 bags to make 1 cubic yard.
- 50-pound bags: 1 bag yields 0.375 cubic feet. You need 72 bags to make 1 cubic yard.
Ready-Mix Truck vs. Bags: When to Order
Mixing 20 bags in a wheelbarrow is doable in an afternoon. Mixing 90 bags by hand is exhausting and causes "cold joints" where previous batches set before the next is poured.
Standard Slab Dimension & Concrete Yardage Table
| Slab Size (ft) | Area (sq ft / sq yd) | 4" Depth (yd³) | 6" Depth (yd³) | 80-lb Bags (4" depth) |
|---|---|---|---|---|
| 8 × 10 ft | 80 ft² (8.9 yd²) | 0.99 yd³ | 1.48 yd³ | 45 bags |
| 10 × 10 ft | 100 ft² (11.1 yd²) | 1.23 yd³ | 1.85 yd³ | 56 bags |
| 12 × 12 ft | 144 ft² (16.0 yd²) | 1.78 yd³ | 2.67 yd³ | 80 bags |
| 10 × 20 ft | 200 ft² (22.2 yd²) | 2.47 yd³ | 3.70 yd³ | 112 bags |
| 12 × 20 ft | 240 ft² (26.7 yd²) | 2.96 yd³ | 4.44 yd³ | 134 bags |
| 20 × 20 ft | 400 ft² (44.4 yd²) | 4.94 yd³ | 7.41 yd³ | 223 bags |
Common Mistakes When Pouring Concrete Slabs
- Ordering exact volume without a waste buffer. Dirt subgrade is rarely laser-flat. If your form sits 0.5 inches deeper than planned, you will run out of concrete before finishing the slab. Always order 10% extra.
- Using 2x4 lumber assuming it is 4 inches. Standard 2x4 form boards actually measure 3.5 inches. If your slab is framed with 2x4s, calculate depth as 3.5 inches.
- Skipping the compacted gravel subbase. Never pour directly onto soft topsoil. A 3-to-4-inch layer of compacted crushed gravel prevents frost heaving and cracking. Use our gravel calculator to estimate crushed stone.
More Concrete & Heavy Material Calculators
- Specialized Concrete Calculator — slab, footing, and column calculations
- Area with Depth Calculator — converts any 2D surface to 3D volume
- Gravel Calculator — subbase stone and aggregate estimator
- Square Yards to Cubic Yards — volumetric conversion tool