How to Calculate Concrete for a Slab
Calculating concrete for a slab is straightforward but getting it wrong is expensive. Here's the formula every concrete contractor should know:
The Formula
Cubic Yards = (Length × Width × Thickness) ÷ 27
All measurements must be in feet. If your thickness is in inches, divide by 12 first. One cubic yard equals 27 cubic feet.
Example: 20×20 Slab at 4 Inches
- Length: 20 ft
- Width: 20 ft
- Thickness: 4 in = 0.333 ft
- Volume: 20 × 20 × 0.333 = 133.3 cu ft
- Cubic yards: 133.3 ÷ 27 = 4.94 yards
- With 10% waste: 5.43 yards
Why 10% Waste Factor?
In practice, you'll always use more concrete than the theoretical calculation:
- Uneven subgrade — the ground is never perfectly flat
- Form deflection — forms bow slightly under pressure
- Spillage during pour
- Over-excavation
Professional contractors always order 10% extra. Running short during a pour creates a cold joint — the #1 structural weakness in concrete slabs.
Concrete Slab Thickness Guide
| Application | Minimum | Recommended |
|---|---|---|
| Sidewalk | 4" | 4" |
| Patio | 4" | 4-6" |
| Residential driveway | 4" | 5-6" |
| Garage floor | 4" | 6" |
| Commercial parking | 6" | 6-8" |
How Much Does Concrete Cost?
As of 2026, ready-mix concrete costs between $125-$175 per cubic yard depending on region, mix design, and delivery distance. Short loads (under 5 yards) typically incur an additional fee of $50-$100.
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