Concrete Yardage Calculator
Enter your slab’s length, width, and thickness to get cubic yards needed, bag counts, and an optional cost estimate.
4″ for sidewalks/patios, 5-6″ for driveways, 6-8″ for heavy vehicles
Covers subgrade unevenness, spillage, and slab depth variation
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Concrete Yardage Calculator
Short answer
Multiply length × width × thickness (in feet) to get cubic feet, then divide by 27 to get cubic yards. A 10×10 ft slab at 4 inches thick needs about 1.23 cubic yards — round up and add 5-10% for waste when you order.
Formula
Cubic yards = Cubic feet ÷ 27
Concrete is sold and ordered by the cubic yard for ready-mix trucks, or by the bag for smaller pours.
Typical slab thickness by use
| Application | Typical thickness |
|---|---|
| Walkway / patio | 4 in |
| Standard driveway | 5-6 in |
| Garage floor | 4-6 in |
| Heavy vehicle / RV pad | 6-8 in |
| Shed / small structure base | 4 in |
Worked example
Slab: 20 ft × 12 ft driveway section at 5 inches thick.
- Cubic feet = 20 × 12 × (5 ÷ 12) = 100 ft³
- Cubic yards = 100 ÷ 27 = 3.70 yd³
- With 10% waste = 3.70 × 1.10 = 4.07 yd³ — order 4.25-4.5 yd³ from the ready-mix truck
Assumptions
- Slab is a uniform rectangle at a single consistent thickness.
- 80 lb bag yields ≈ 0.6 ft³ of mixed concrete; 60 lb bag yields ≈ 0.45 ft³.
- Waste percentage is added on top of the exact calculated volume.
- Ready-mix trucks are typically ordered in quarter-yard increments — round up.
Common mistakes
- Forgetting to convert thickness from inches to feet before multiplying — this is the single most common yardage error.
- Ordering the exact calculated amount with no waste buffer, then running short mid-pour.
- Using bag concrete for large pours where a ready-mix truck would be far more practical and cost-effective.
- Not accounting for a sloped or uneven subgrade, which increases actual thickness in low spots.
FAQ
How much concrete do I need for a 10×10 slab?
At a standard 4 inch thickness, a 10×10 ft slab is 100 sq ft x 0.333 ft = 33.3 cubic feet, or about 1.24 cubic yards. Add 5-10% for waste, which brings it to roughly 1.3-1.4 cubic yards.
How many bags of concrete equal a yard?
An 80 lb bag yields about 0.6 cubic feet, so it takes about 45 bags to make one cubic yard (27 cubic feet). A 60 lb bag yields about 0.45 cubic feet, so it takes about 60 bags per yard.
Should I order extra concrete for waste?
Yes. Most contractors add 5-10% over the calculated volume to cover slab thickness variation, spillage, and uneven subgrade. For irregular forms or hand-mixed pours, use the higher end of that range.
How thick should a concrete slab be?
Typical thicknesses are 4 inches for sidewalks and patios, 5-6 inches for driveways, and 6-8 inches for areas that carry heavy vehicles like trucks or RVs.
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