How it’s calculated
A slab is a rectangular prism, so its volume is length × width × thickness. We then add your waste allowance.
Example: a 10 ft × 10 ft slab, 4 in thick = 10 × 10 × (4/12) = 33.3 ft³ = 1.23 yd³. Add 10% and you need about 1.36 yd³. Ready-mix suppliers usually sell in ¼ or ½ yd³ increments, so round up when ordering.
Bag yields are typical values printed by manufacturers (≈0.60 ft³ for an 80 lb bag). Check the bag you buy.
Frequently asked questions
How thick should a concrete slab be?
About 4 inches (100 mm) for patios, sidewalks and shed floors; 5–6 inches for driveways that carry cars. Heavier loads need an engineered design.
How much concrete do I need for a 10x10 slab?
At 4 in thick: 10 × 10 × (4/12) = 33.3 ft³ = 1.23 yd³. With 10% extra that is about 1.36 yd³ (order 1.5 yd³ of ready-mix), or 62 bags of 80 lb mix.
How many 80 lb bags make a cubic yard?
About 45. A cubic yard is 27 ft³, and an 80 lb bag yields roughly 0.60 ft³, so 27 ÷ 0.6 = 45 bags.
Should I add extra concrete?
Yes. 5–10% extra covers spillage, uneven ground and form bulging. Running short mid-pour causes cold joints.
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Sources
- Guide to Concrete Floor and Slab Construction (ACI 302.1R, summary) — American Concrete Institute
- QUIKRETE® Concrete Mix No. 1101 Data Sheet (bag yields) — The QUIKRETE Companies
- NIST Handbook 44, Appendix C: Units of Measurement — U.S. National Institute of Standards and Technology
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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