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Concrete Calculator
Enter your slab's length, width, and thickness. We compute the exact volume in cubic yards and feet, plus the number of pre-mix bags you'll need at the size you choose.
Slabs are commonly 4–6 inches.
10% covers spillage and edge overpour.
Bag size
80 lb bags (incl. waste)
62
61.1 exact, rounded up
At 1.23 yd³, ready-mix from a truck is usually cheaper and faster than mixing this many bags by hand.
Examples
10 ft × 10 ft × 4 in slab
≈ 1.23 yd³ · 56 × 80 lb bags
20 ft × 12 ft × 6 in patio
≈ 4.44 yd³ · ready-mix recommended
3 ft × 3 ft × 4 in pad
≈ 0.11 yd³ · 5 × 80 lb bags
How it works
Volume in cubic feet is length × width × thickness, with thickness converted from inches to feet. We divide by 27 to get cubic yards, then divide by each bag's yield to get bag count.
Volume · L × W × (T ÷ 12) ÷ 27 = yd³
Bag yield: 40 lb ≈ 0.30 ft³ · 60 lb ≈ 0.45 ft³ · 80 lb ≈ 0.60 ft³
Frequently asked questions
About 90 bags of 40 lb mix, 60 bags of 60 lb mix, or 45 bags of 80 lb mix. We compute the exact bag count from your project's volume.
We add a 10% waste allowance to the bag count by default — it covers spillage, uneven subgrade, and over-pour at edges. For very small jobs, consider rounding up further.
Common minimums: 4 inches for sidewalks and patios, 5–6 inches for driveways, 6+ inches for heavy loads. Always check your local codes and any structural plans.
Past about 1 cubic yard (roughly 60 × 80 lb bags), ready-mix from a truck is usually cheaper and far easier than mixing bags by hand.
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