How to Calculate Concrete for a Slab
A step-by-step guide with formulas, examples, and conversion tables to calculate exactly how much concrete you need.
The Concrete Slab Formula
Cubic Feet = Length (ft) × Width (ft) × Thickness (ft)
Cubic Yards = Cubic Feet ÷ 27
Thickness (ft) = Thickness (inches) ÷ 12
With waste: Cubic Yards × 1.10 = Total to Order
Measure the Slab Area
Measure the length and width of your slab in feet. For rectangular slabs, multiply length × width to get the area in square feet.
Convert Thickness to Feet
Concrete thickness is measured in inches but must be converted to feet for the volume formula. Divide your thickness in inches by 12.
Calculate Cubic Feet
Multiply length × width × thickness (in feet) to get the volume in cubic feet.
Convert to Cubic Yards
Concrete is ordered by the cubic yard. Divide cubic feet by 27 (since 1 cubic yard = 27 cubic feet).
Add Waste Factor (10%)
Always add 10% extra for spillage, uneven ground, and form irregularities. Multiply your cubic yards by 1.10.
Convert to Bags (if using bagged concrete)
If mixing your own: each 80lb bag yields ~0.60 cu ft, each 60lb bag yields ~0.45 cu ft. Divide total cubic feet by bag yield.
Worked Examples
Common slab sizes with full calculations
10×10 Slab at 4" Thick
Area: 10 × 10 = 100 sq ft
Thickness: 4" ÷ 12 = 0.333 ft
Volume: 100 × 0.333 = 33.3 cu ft
Cubic yards: 33.3 ÷ 27 = 1.23 yd³
With 10% waste: 1.23 × 1.10 = 1.36 yd³
80lb bags: 33.3 ÷ 0.60 = 56 bags
60lb bags: 33.3 ÷ 0.45 = 75 bags
12×12 Slab at 4" Thick
Area: 12 × 12 = 144 sq ft
Thickness: 4" ÷ 12 = 0.333 ft
Volume: 144 × 0.333 = 48.0 cu ft
Cubic yards: 48.0 ÷ 27 = 1.78 yd³
With 10% waste: 1.78 × 1.10 = 1.95 yd³
80lb bags: 48.0 ÷ 0.60 = 80 bags
60lb bags: 48.0 ÷ 0.45 = 107 bags
Thickness Conversion Table
| Inches | Feet | Decimal | Best For |
|---|---|---|---|
| 3" | 0' 3" | 0.250 | Light-duty walkway |
| 4" | 0' 4" | 0.333 | Patio, shed, walkway |
| 5" | 0' 5" | 0.417 | Standard driveway |
| 6" | 0' 6" | 0.500 | Garage, heavy-duty driveway |
| 8" | 0' 8" | 0.667 | Foundation footing |
| 10" | 0' 10" | 0.833 | Thickened edge / footing |
| 12" | 1' 0" | 1.000 | Full foot of concrete |
Concrete Bag Yield
How much concrete each bag produces
| Bag Size | Yield (cu ft) | Bags per Cubic Yard | Typical Price |
|---|---|---|---|
| 40 lb | 0.30 | 90 | $3-5 |
| 60 lb | 0.45 | 60 | $4-6 |
| 80 lb | 0.60 | 45 | $5-7 |
| 90 lb | 0.68 | 40 | $6-8 |
Pro Tips
- Always round up when ordering concrete — running short mid-pour is a disaster.
- Ready-mix trucks typically have a 1-yard minimum order, with a small-load fee under 3 yards.
- For odd shapes, break the area into rectangles, calculate each separately, then add together.
- Double-check your measurements. A 1-foot error on a 20×20 slab means 1.23 yards difference at 4".
- Order 10% extra for waste. It's cheaper to have a little left over than to run short.
Skip the Math — Use Our Calculator
Enter your dimensions and get instant results in cubic yards, bags, and cost estimates.