Concrete Slab Calculator
A standard 4-inch residential slab needs roughly 0.012 cubic yards per square foot—a 10×10 takes about 1.23 yards, a 20×20 takes just under 5. Enter your dimensions to get exact cubic yards, bag count, and cost.
Pro Tips
- →Most residential slabs need 4-6 inches thickness
- →Add 10% extra concrete for waste
- →Include wire mesh or rebar for reinforcement
- →Compact gravel base to 4-6 inches
- →Use control joints to manage cracking
Includes 10% waste factor
That's typically a professional pour. See costs ↓
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Cost Estimate
Estimated material costs for your project
Recommendation: Ready-Mix Concrete
For projects over 1 cubic yard, ready-mix is typically more economical and easier to work with.
245 bags × 80lb
5.43 cubic yards + delivery
400 sq ft × $3.00–$8.00/sq ft
Prices vary by location and time. Contact local suppliers for accurate quotes.
For general step-by-step instructions, read our complete Concrete Slab Cost and How To Calculate Concrete.
This calculator tells you how much concrete a rectangular slab needs: the volume in cubic yards and cubic feet, the equivalent count of pre-mixed bags, and where the ready-mix crossover sits. It works for patios, garage floors, driveways, walkways, and shed bases. Enter length, width, and thickness to get exact numbers. (For pricing, use the concrete cost calculator once you know your volume.)
How to Calculate Slab Volume
Concrete volume is length × width × thickness, converted to cubic yards because that is how ready-mix is sold.
- Convert every dimension to feet. Thickness is usually in inches, so divide by 12 (4 in ÷ 12 = 0.333 ft).
- Multiply length × width × thickness to get cubic feet.
- Divide cubic feet by 27 to get cubic yards (27 cubic feet per cubic yard).
Worked example, 20×20 at 4 inches: 20 × 20 × 0.333 = 133.3 cubic feet ÷ 27 = 4.94 cubic yards. Add 10% waste and you order 5.43 cubic yards. The same arithmetic drives the calculator above, so you can sanity-check any result by hand. For a deeper walk-through, see how to calculate concrete.
Cubic Yards per Square Foot by Thickness
For a flat slab, volume scales directly with thickness. A handy shortcut: a 4-inch slab needs about 0.0123 cubic yards per square foot. Multiply your area by the factor below, then add waste.
| Thickness | Cu Yd per sq ft | 100 sq ft | 200 sq ft | 400 sq ft |
|---|---|---|---|---|
| 4 in | 0.0123 | 1.23 cu yd | 2.47 cu yd | 4.94 cu yd |
| 5 in | 0.0154 | 1.54 cu yd | 3.09 cu yd | 6.17 cu yd |
| 6 in | 0.0185 | 1.85 cu yd | 3.70 cu yd | 7.41 cu yd |
| 8 in | 0.0247 | 2.47 cu yd | 4.94 cu yd | 9.88 cu yd |
These figures are the bare slab volume. Always add 10% before ordering to cover spillage, uneven subgrade, and form overfill.
Volume for Common Slab Sizes
The table below gives ready-to-use cubic-yard figures for popular slab dimensions at the standard 4-inch residential thickness, both bare and with 10% waste added.
| Slab Size | Square Feet | Bare Volume | + 10% Waste |
|---|---|---|---|
| 8×8 ft | 64 | 0.79 cu yd | 0.87 cu yd |
| 10×10 ft | 100 | 1.23 cu yd | 1.36 cu yd |
| 12×12 ft | 144 | 1.78 cu yd | 1.96 cu yd |
| 12×20 ft | 240 | 2.96 cu yd | 3.26 cu yd |
| 16×20 ft | 320 | 3.95 cu yd | 4.35 cu yd |
| 20×20 ft | 400 | 4.94 cu yd | 5.43 cu yd |
| 24×24 ft | 576 | 7.11 cu yd | 7.82 cu yd |
| 30×30 ft | 900 | 11.11 cu yd | 12.22 cu yd |
Bag Count for Small Slabs
For small pours you can mix bagged concrete on-site instead of ordering a truck. Each bag yields a fixed volume, so divide your total cubic feet by the per-bag yield. Typical dry-mix yields are about 0.60 cu ft for an 80-lb bag, 0.45 cu ft for a 60-lb bag, and 0.30 cu ft for a 40-lb bag (always confirm against the printed yield on your brand).
| Slab (4 in) | Cu Ft (+10%) | 40-lb bags | 60-lb bags | 80-lb bags |
|---|---|---|---|---|
| 3×3 ft | 3.3 | 11 | 8 | 6 |
| 4×4 ft | 5.9 | 20 | 14 | 10 |
| 5×5 ft | 9.2 | 31 | 21 | 16 |
| 8×8 ft | 23.5 | 79 | 53 | 40 |
| 10×10 ft | 36.7 | 123 | 82 | 62 |
Bag math gets impractical fast. A 10×10 slab already means hauling and mixing more than 60 bags of 80-lb mix by hand. For step-by-step bag counting, see how many bags of concrete you need.
Bagged vs. Ready-Mix: The Crossover
The practical break point is about 1 cubic yard, which equals roughly 45 bags of 80-lb mix (or 60 bags of 60-lb). Below that, bagged concrete is convenient. Above it, ordering ready-mix is faster, cheaper per yard, and gives a more uniform pour.
| Volume | Equivalent 80-lb bags | Recommendation |
|---|---|---|
| Under 0.5 cu yd | Under ~23 | Bagged mix |
| 0.5–1 cu yd | ~23–45 | Bagged or ready-mix |
| 1–10 cu yd | ~45–450 | Ready-mix delivery |
| Over 10 cu yd | Over ~450 | Multiple truck loads |
A standard NRMCA ready-mix truck holds about 10 cubic yards fully loaded, so a slab larger than one truck is typically split into sections at planned construction joints that let the crew finish each pour within its set window. Compare the economics in our bagged vs. ready-mix guide.
Choosing Slab Thickness
Thickness sets your volume, so pick it before you calculate. For interior residential slabs, the IRC sets a minimum of 3.5 inches (per IRC R506.2.3), and 4 inches is the standard for patios, walkways, and most garage floors. Step up to 5–6 inches for driveways, heavier vehicles, or freeze-thaw climates, and 6–8 inches for RV pads and heavy equipment, which usually need engineered design.
| Use | Thickness | Notes |
|---|---|---|
| Patio / walkway | 4 in | Standard for foot traffic |
| Garage floor | 4–6 in | 4,000–5,000 PSI mix |
| Driveway | 5–6 in | Supports passenger-vehicle weight |
| RV pad / heavy equipment | 6–8 in | Typically engineered |
The jump from 4 to 6 inches adds 50% to your concrete volume, so confirm the load before over-ordering. See 4-inch vs. 6-inch slabs for the full comparison.
Measuring Irregular Shapes
The calculator assumes a rectangle. For an L-shape or U-shape, break the slab into rectangles, calculate each volume separately, and add them together. For a triangle, use (base × height ÷ 2) for the area, then multiply by thickness; round pads use π × radius². Where thickness varies (a sloped or thickened section), measure the average so you do not under-order.
Ordering Tips
- Round up, never down. Running short mid-pour means a cold joint or a second delivery fee. The built-in 10% waste allowance covers most overruns.
- Know your access. Ready-mix trucks need clear, firm access within chute reach (about 18 ft); otherwise budget for a pump or wheelbarrow relay.
- Order one number. Give the plant your total cubic yards including waste, the PSI (4,000 is typical residential), and any admixtures.
- Have the volume, then price it. Once the calculator gives you cubic yards, run the concrete cost calculator for a full material-and-labor estimate.