How to Calculate Concrete Needed for a Slab or Patio
Ordering too little concrete mid-pour is a contractor's nightmare. Here's the formula to get your quantity right before you start.
Concrete needed in cubic yards = (Length × Width × Thickness in feet) ÷ 27. A 10×10 ft slab at the standard 4-inch thickness requires approximately 1.23 cubic yards, or about 75 bags of 60lb concrete mix.
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The Concrete Calculation Formula
Cubic Yards = (Length × Width × Thickness in feet) ÷ 27
Convert thickness from inches to feet by dividing by 12 first.
Worked Example
A 10×10 ft patio slab at 4 inches thick:
Thickness in feet = 4 ÷ 12 = 0.333 ft
Cubic feet = 10 × 10 × 0.333 = 33.3 cu ft
Cubic yards = 33.3 ÷ 27 = 1.23 cubic yards
Standard Thickness by Project
| Project | Standard Thickness |
|---|---|
| Sidewalk / Patio | 4 inches |
| Driveway (car) | 5–6 inches |
| Garage Floor | 4–6 inches |
| Footings | Varies — check local code |
Bag Conversions
One cubic yard of concrete requires approximately:
- 60 bags of 60lb concrete mix
- 45 bags of 80lb concrete mix
Bagged vs Ready-Mix
For projects under about 1 cubic yard, bagged concrete mix is usually more practical. For larger pours, ready-mix delivery is more cost-effective and ensures consistent quality throughout the entire pour.
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