Concrete Slab Calculator: Cubic Yards & Bags
Instantly compute cubic yards, cubic feet, and exact 60 lb / 80 lb bag counts for any slab. Enter length, width, and thickness below.
slab Dimensions
Typical finished slab: $6–12 / sq.ft (labor + materials)
Total Material Needed
or 78 bags of 60 lb
Cost Comparison
Cheapest option highlighted in green
Contractor estimate based on $8/sq.ft. Adjust above to match local quotes.
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Concrete Slab Volume Formula
Every concrete slab calculation starts from a single formula: Volume = Length × Width × Thickness, with all three values in the same unit (feet). The calculator above handles the inch-to-feet conversion and waste factor automatically, but knowing the math helps you sanity-check the result.
Volume (cu ft) = Length(ft) × Width(ft) × Thickness(in) ÷ 12
Volume (cu yd) = Cubic feet ÷ 27
80 lb bags = Cubic feet ÷ 0.60, rounded up
60 lb bags = Cubic feet ÷ 0.45, rounded up
Worked example — 12×12 patio at 4 inches: 12 × 12 × (4÷12) = 48 cu ft = 1.78 cu yd. Exact: 48 ÷ 0.60 = 80 bags of 80 lb; with 5% waste: 50.4 ÷ 0.60 = 84 bags to buy.
Common Slab Sizes (4" Thick)
Quick reference for the residential slab sizes people search for most. All figures assume the 4-inch thickness standard for patios and walkways. Multiply by 1.5 for 6-inch driveway thickness.
| Size | Area (sq ft) | Volume (cu ft) | Volume (cu yd) | 80 lb Bags (exact) | Buy (5% waste) |
|---|---|---|---|---|---|
| 4 × 4 | 16 | 5.3 | 0.20 | 9 | 10 |
| 8 × 10 | 80 | 26.7 | 0.99 | 45 | 47 |
| 10 × 10 | 100 | 33.3 | 1.23 | 56 | 59 |
| 10 × 12 | 120 | 40.0 | 1.48 | 67 | 70 |
| 12 × 12 | 144 | 48.0 | 1.78 | 80 | 84 |
| 12 × 20 | 240 | 80.0 | 2.96 | 134 | 140 |
| 20 × 20 | 400 | 133.3 | 4.94 | 223 | 234 |
| 24 × 24 | 576 | 192.0 | 7.11 | 320 | 336 |
* Exact = no waste (80 lb bag yield 0.60 cu ft). The calculator above defaults to +5% waste, so it matches the “Buy” column.
Choosing the Right Slab Thickness
- 3 inches: Walkways, decorative pads, garden stepping stones. Minimum for any finished concrete surface — anything thinner cracks.
- 4 inches (most common): Patios, shed bases, AC pads, hot tub pads up to 2-person size, small hardscape. Default for residential pours.
- 5 inches: Overbuilt patio for heavy furniture, large hot tubs, or freeze-thaw regions where extra mass helps.
- 6 inches: Driveways for passenger cars, garage floors, small workshop slabs. Add wire mesh or #3 rebar on 18-inch centers.
- 8 inches: Driveways for RVs, boat trailers, heavy trucks. Commercial shop floors. Full rebar grid required.
Frequently Asked Questions
How do I calculate the volume of a concrete slab?
Multiply length × width × thickness (all in feet). For a 10×10 slab at 4 inches: 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. To convert inches to feet for the thickness, divide by 12.
How thick should my concrete slab be?
4 inches for patios, walkways, and AC pads. 6 inches for driveways carrying passenger vehicles. 8 inches for driveways that see RVs, boat trailers, or heavy work trucks. Every extra inch adds about 25% to the concrete volume and cost.
How do I calculate concrete for an L-shaped or irregular slab?
Split the shape into rectangles, calculate each rectangle's volume separately (length × width × thickness), and add them together. Any layout with the same total area and thickness uses the same number of bags — only the forming labor differs.
How many cubic feet in a cubic yard of concrete?
Exactly 27 cubic feet. Since one 80 lb bag yields 0.6 cu ft, it takes 45 bags of 80 lb concrete to make 1 cubic yard.
Should I order ready-mix or buy bags for a concrete slab?
Under about 1 cubic yard (45 bags of 80 lb — an 8×10 slab at 4 inches is 0.99 cu yd), bags are almost always cheaper. Above 1 cubic yard, ready-mix usually wins once you factor in mixing labor and time.
Do I need rebar in a concrete slab?
Optional for patios under 4 inches thick. Strongly recommended for any slab carrying vehicles, heavy equipment, or attached structures. Wire mesh (6×6 W1.4/W1.4) or #3 rebar on an 18-inch grid is the standard for residential driveways and garages.
How much does a concrete slab cost per square foot?
Materials alone: $1.50 to $2.50 per sq ft (bags) or $1.50 to $3 per sq ft (ready-mix). Turnkey including labor: $4.40 to $11.40 per sq ft installed in 2026. See our slab cost calculator for a full side-by-side comparison.