Concrete Slab Calculator — Cubic Yards & Bags
Calculate concrete volume for any flat slab — patio, garage, driveway, or shed pad. Returns cubic yards, 60 and 80-pound bag counts, and material cost.
Enter Dimensions
4 in for patios/walkways · 5–6 in for garages
10% recommended for most slabs
Results
Bags Needed
62bags
80 lb bags including 10% waste
Ready-Mix Volume
1.358cu yd
Order this amount from ready-mix supplier
Slab Volume
33.33cu ft
1.235 cu yd before waste
Editorial Standards
Each calculator is reviewed for formula accuracy, unit consistency, and alignment with current U.S. building practices before publication. We verify outputs against published engineering references and real-world project data. Learn more about our methodology.
Concrete slab volume is straightforward: length × width × thickness. The difficulty is in the unit conversions — thickness is measured in inches, but volume is sold in cubic yards (ready-mix) or converted to bags. This calculator handles all unit math and includes a configurable waste factor.
For rebar reinforcement quantities on slabs, see the Concrete Slab Rebar Calculator.
How to Calculate Concrete for a Slab
Volume (cu ft) = Length × Width × (Thickness ÷ 12). Cubic yards = cu ft ÷ 27. Bags (80 lb) = cu ft ÷ 0.60.
Common Slab Quick Reference
- 10 × 10 × 4" patio: 1.23 cu yd · 56 bags 80 lb
- 12 × 14 × 4" patio: 2.07 cu yd · ready-mix recommended
- 20 × 20 × 4" patio: 4.94 cu yd
- 20 × 22 × 6" garage: 8.15 cu yd
- 20 × 25 × 5" driveway: 7.72 cu yd
Frequently Asked Questions
How thick should a concrete slab be?
4 inches for residential patios, walkways, and shed pads. 5–6 inches for garage floors and driveways. 6–8 inches for shop slabs, RV pads, or heavy equipment. IRC R506.1 specifies a minimum 3.5-inch thickness for residential slabs on grade.
How much concrete do I need for a 10×10 slab at 4 inches thick?
10 × 10 × (4 ÷ 12) = 33.3 cu ft = 1.23 cu yd. With 10% waste, order 1.35 cu yd from ready-mix, or 56 bags of 80 lb mix. For projects under 1 yd, bags are practical; above 1 yd, ready-mix is usually cheaper per cubic yard.
When should I use ready-mix instead of bags?
Above 1 cubic yard (roughly an 8×8 ft slab at 4 inches). Ready-mix averages $160–$195/yd nationally (NRMCA 2024 data), plus a $50–$150 short-load fee for orders under 8–10 yd. At $6–$8/bag for 80 lb mix, bagging a 1.5 cu yd slab costs about $270–$360 and takes hours of mixing — ready-mix wins on both cost and quality above 1 cu yd.
Do I need a vapor barrier under a concrete slab?
Yes for any slab inside a building or under a finished floor. IRC R506.2.3 requires a vapor barrier between the gravel base and concrete — 6-mil polyethylene is the code minimum under both IRC 2021 and IRC 2024. Skipping it causes moisture migration that damages floor finishes and creates efflorescence.
How much rebar does a 4-inch slab need?
#4 rebar at 12–18 inches on-center grid is standard for residential slabs. For a 10×10 ft slab at 12-inch spacing: approximately 200 LF of #4 rebar (10 sticks of 20 ft). For light-duty non-structural slabs, 6×6 wire mesh is a code-compliant alternative.