Construction · Concrete
Concrete Slab CalculatorVolume, Bags & Installed Cost
Looking for a different region?
Example — 20 × 20 ft driveway slab
4.94
cubic yards
297
bags (80 lb)
4 in
thickness
$2,750–$4,200
est. installed
A standard 20 × 20 ft garage floor at 4 inches needs just under 5 cubic yards — around $2,750–$4,200 installed with basic broom finish and no reinforcement. Adding rebar and a stamped finish can push that to $8,000+.
Use this concrete slab calculator to estimate how much concrete you need for slabs, driveways, patios, and floors. Enter length, width, and thickness to calculate cubic yards, bags of concrete, and installed cost estimates instantly.
How to calculate concrete for a slab
All concrete in the US is sold by the cubic yard. To calculate how much you need, multiply the length, width, and thickness of your slab, then divide by 27 (the number of cubic feet in one cubic yard).
Thickness is the most overlooked variable. A slab poured at 6 inches instead of 4 inches uses 50% more concrete for the same footprint — so always confirm your thickness requirement before calculating.
For quick volume-only estimates, use the concrete volume calculator to get cubic yards before estimating cost.
Concrete slab calculation formula
Formula
Volume (cu yd) = Length (ft) × Width (ft) × (Thickness (in) ÷ 12) ÷ 27
Divide thickness in inches by 12 to convert to feet. Multiply all three dimensions to get cubic feet, then divide by 27 to get cubic yards. Always add 10% for waste before ordering.
10×10 ft, 4 in thick
1.23 cu yd
≈ 74 bags
20×20 ft, 4 in thick
4.94 cu yd
≈ 297 bags
20×20 ft, 6 in thick
7.41 cu yd
ready-mix
Material calculator for concrete slabs
Once you have your cubic yards, choose between bagged concrete and ready-mix delivery. The right choice depends on project size.
Bagged concrete (DIY)
60 bags per cu yd
80 lb bags — ≈ 0.45 cu ft each
Best for small slabs under 1 cubic yard — fence posts, steps, and patches. Bags cost $5–$8 each, working out to $300–$480 per cubic yard. For larger slabs, mixing bags by hand is impractical.
Ready-mix (truck delivery)
$120–$200 per cu yd
Plus delivery fee, typically $100–$200
The right choice for anything over 1–2 cubic yards. Uniform mix, faster pour, lower cost at scale. For a standard 20×20 ft garage floor at 4 inches (≈5 cu yd), ready-mix saves significant time and money.
Need an exact bag count? Use the concrete bag calculator →
Concrete slab thickness guide
Getting thickness wrong is the most expensive concrete mistake. Use this as your quick reference before entering dimensions above.
3–4 inches
Patios & walkways
Light foot traffic only
4–6 inches
Residential driveways
Standard passenger vehicles
4–6 inches
Garage floors
Add rebar for vehicle weight
6–8+ inches
Heavy loads
Trucks, RVs, commercial use
A 6-inch slab uses 50% more concrete than a 4-inch slab of the same footprint. Always confirm thickness requirements with a contractor or structural engineer for load-bearing or structural applications.
How much does a concrete slab cost?
Installed concrete slab costs run $4–$15 per square foot, depending on thickness, finish, reinforcement, and local labour rates. Use the table below as a quick reference, then use the calculator above for a full project estimate.
| Slab size | 4" thick | 6" thick |
|---|---|---|
| 10×10 ft (100 sq ft) | $600–$1,500 | $900–$2,000 |
| 12×12 ft (144 sq ft) | $865–$2,160 | $1,300–$2,880 |
| 20×20 ft (400 sq ft) | $2,400–$6,000 | $3,600–$8,000 |
| 24×24 ft (576 sq ft) | $3,456–$8,640 | $5,180–$11,520 |
| 30×30 ft (900 sq ft) | $5,400–$13,500 | $8,100–$18,000 |
What affects concrete slab cost?
Size and thickness
The biggest cost driver. A 4-inch slab uses 1.23 cu yd per 100 sq ft. Doubling thickness nearly doubles material cost.
Reinforcement
Wire mesh adds $0.15–$0.30/sq ft. Rebar adds $0.50–$1.50/sq ft but is required for driveways, foundations, and heavy loads.
Finish type
Broom finish is cheapest. Exposed aggregate adds $2–$4/sq ft. Stamped or coloured concrete adds $8–$20+/sq ft.
Site preparation
Excavation, gravel base, and formwork can add $1–$4/sq ft depending on access and soil conditions.
Location
Labour costs vary significantly. Urban areas and the Northeast/West Coast typically cost 20–40% more than rural Midwest.
Accessibility
Difficult access for concrete trucks or tight residential lots can add $500–$2,000 to the overall cost.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length (ft) × width (ft) × thickness (in ÷ 12), then divide by 27 to get cubic yards. A 10×10 ft slab at 4 inches thick needs 1.23 cubic yards. Always add 10% for waste before ordering.
How many bags of concrete do I need for a slab?
An 80 lb bag covers about 0.45 cubic feet. You need roughly 60 bags per cubic yard. A 10×10 ft slab at 4 inches thick (1.23 cu yd) needs about 74 bags including 10% waste. For anything over 1–2 cubic yards, ready-mix is more practical.
What thickness should a concrete slab be?
Patios and walkways: 3–4 inches. Residential driveways: 4–6 inches. Garage floors: 4–6 inches. Heavy vehicles, commercial use, or high loads: 6–8 inches minimum. Always confirm with a contractor for structural applications.
How do you calculate cubic yards of concrete?
Formula: (Length ft × Width ft × Thickness in ÷ 12) ÷ 27. For example, a 20×20 ft slab at 4 inches thick = (20 × 20 × 0.333) ÷ 27 = 4.94 cubic yards. Add 10% for waste: 5.43 cubic yards to order.
How much does a concrete slab cost per square foot?
A basic concrete slab costs $4–$8 per square foot installed. With premium finishes, reinforcement, and site prep, costs can reach $12–$20+ per square foot.
Can I pour a concrete slab myself?
Small slabs under 50 sq ft are DIY-friendly with bagged concrete. Larger pours require a concrete truck, proper forming, and finishing skills. Mistakes in levelling or curing can cause cracking, so hiring a professional is usually worthwhile for driveways and foundations.
This calculator provides estimates for planning purposes only. Actual concrete quantities depend on ground conditions, form accuracy, pour technique, and material yield. Cost figures are approximate US national averages for 2026 and may not reflect local pricing. Always confirm quantities and costs with your supplier and contractor before ordering.