Concrete Calculator
To find how much concrete you need, multiply length × width × thickness to get the volume, then convert to cubic yards (divide cubic feet by 27). Add 5–10% for waste. A 10×10 ft slab at 4 inches needs about 1.23 cubic yards, costing roughly $150–$220 in ready-mix.
- 1 cubic yard
- 27 ft³
- 80 lb bag yield
- 0.60 ft³
- Typical slab
- 4" thick
Your measurements
Accounts for spillage and uneven subgrade.
Override the default ready-mix price range.
Enter your measurements
Fill in the fields to see your estimate.
The formula
Volume (yd³) = (Length ft × Width ft × Thickness ft) ÷ 27 × waste factorHow to calculate it manually
- 1
Measure the area
Record length and width in feet.
- 2
Set the thickness
Convert inches to feet by dividing by 12 (4 in = 0.333 ft).
- 3
Calculate volume
Multiply length × width × thickness for cubic feet.
- 4
Convert to cubic yards
Divide cubic feet by 27.
- 5
Add waste
Multiply by 1.10 for a standard 10% allowance.
Example calculation
A 10 ft × 10 ft slab at 4 in thick: 10 × 10 × 0.333 = 33.3 ft³ ÷ 27 = 1.23 yd³, plus 10% waste ≈ 1.36 yd³.
Concrete cost by slab size
Cubic yards and ready-mix material cost for a 4-inch slab. Add labor for a finished price.
| Slab size | Cubic yards (4") | Ready-mix cost | 80 lb bags |
|---|---|---|---|
| 10 × 10 ft | 1.23 yd³ | $150 – $205 | ~56 |
| 12 × 12 ft | 1.78 yd³ | $215 – $295 | ~80 |
| 20 × 20 ft | 4.94 yd³ | $595 – $815 | — |
| 24 × 24 ft | 7.11 yd³ | $855 – $1,175 | — |
Concrete bag yield
How much each bag size yields and how many make a cubic yard.
| Bag size | Yield | Bags per yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | ~90 |
| 60 lb | 0.45 ft³ | ~60 |
| 80 lb | 0.60 ft³ | ~45 |
Common concrete estimating mistakes
Forgetting the waste factor
Uneven subgrade and spillage mean you always need a bit more. Add 5–10% so you don't run short mid-pour.
Using the wrong thickness
4 inches suits patios and walkways, but driveways need 5–6 inches. Too thin and the slab cracks under load.
Bag-mixing a large pour
Above ~1 cubic yard, mixing bags by hand is slow and risks cold joints. Order ready-mix instead.
Ignoring the base and rebar
A compacted gravel base and reinforcement aren't in the concrete volume but are essential — budget for them.
Frequently asked questions
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Ready to start your concrete project?
Run the numbers above, then explore related tools and guides to plan every step.