One of the most common mistakes on a concrete project is ordering too little — or wasting money by ordering too much. Getting your concrete calculation right before you call the supplier saves time, money, and stress.
The Basic Formula
Concrete volume is measured in cubic yards (in the US) or cubic meters. The formula for a rectangular slab is:
Volume = Length × Width × Thickness
All measurements must be in the same unit. Convert inches to feet by dividing by 12. Then divide the total cubic feet by 27 to get cubic yards.
Step-by-Step: Calculating a Concrete Slab
- Measure the length and width of your slab in feet
- Convert your thickness from inches to feet (e.g., 4 inches ÷ 12 = 0.333 ft)
- Multiply: Length × Width × Thickness = Cubic Feet
- Divide by 27 to convert to cubic yards
- Add 10% extra for waste and spillage
Example Calculation
You're pouring a driveway that is 20 ft long, 10 ft wide, and 4 inches thick:
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Volume: 20 × 10 × 0.333 = 66.6 cubic feet
- In cubic yards: 66.6 ÷ 27 = 2.47 cubic yards
- With 10% waste: 2.47 × 1.10 = 2.72 cubic yards
You'd order 3 cubic yards to be safe.
Common Concrete Thicknesses
| Project Type | Recommended Thickness |
|---|---|
| Sidewalk / Walkway | 4 inches |
| Residential Driveway | 4–6 inches |
| Garage Floor | 4–6 inches |
| Patio | 4 inches |
| Foundation Footing | 8–12 inches |
| Commercial Driveway | 6–8 inches |
Calculating for Footings and Columns
For round columns, use the formula: Volume = π × radius² × height
For square or rectangular footings, use the same length × width × depth formula as slabs.
How Many Bags of Concrete Do You Need?
| Bag Size | Yield (cubic feet) |
|---|---|
| 40 lb bag | 0.30 cubic feet |
| 60 lb bag | 0.45 cubic feet |
| 80 lb bag | 0.60 cubic feet |
For small jobs under 1 cubic yard, bagged concrete is easier. For anything larger, order ready-mix concrete from a supplier — it's far more cost-effective.
When to Use Ready-Mix vs Bagged Concrete
- Bagged concrete — Best for small repairs, fence posts, and projects under 0.5 cubic yards
- Ready-mix concrete — Best for driveways, slabs, foundations, anything over 1 cubic yard
Skip the Math — Use Our Free Calculator
Enter your dimensions and get instant concrete volume in cubic yards, cubic feet, and number of bags needed.
Use Concrete Calculator →Final Tips Before You Pour
- Check the weather — don't pour concrete if rain or freezing temperatures are expected
- Prepare your subbase properly — compact the soil and add gravel if needed
- Have everything ready before the truck arrives — ready-mix has a limited working time
- Use expansion joints every 8–10 feet to prevent cracking