Concrete Slab Calculator
Total Area
Volume
Slab Diagram
3D ViewHow to Calculate Square Feet for Concrete
Concrete projects always start with the surface area measurement — the square footage of the slab. From there, you factor in the depth (thickness) of the pour to determine the volume of concrete required. Concrete is ordered and delivered in cubic yards, so converting from square feet to cubic yards is the critical second step.
Step 1: Calculate the Area
Measure the length and width of the slab in feet and multiply them together. For circular slabs (like a round patio or column footing), use the formula: Area = π × (Diameter ÷ 2)². Our calculator handles both shapes automatically.
Concrete Volume Formula
FormulaExample: 20×10 ft slab at 4 in thick = (20 × 10 × 0.333) ÷ 27 = 2.47 cu yd
Step 2: Convert to Cubic Yards
One cubic yard equals 27 cubic feet. Multiply your area (sq ft) by the depth in feet (divide inches by 12), then divide by 27. Always order an additional 10% overage — concrete must be poured without stopping, and running short mid-pour is catastrophic for curing quality.
Step 3: Bags vs. Ready-Mix
For small pours under 1 cubic yard, premixed 80 lb bags are practical. Each 80 lb bag yields approximately 0.60 cubic feet. For larger pours, ready-mix delivered by truck is more economical. The industry breakeven is typically around 1–1.5 cubic yards (27–40 cubic feet).
Recommended Concrete Slab Thickness by Project
| Project Type | Recommended Thickness | Notes |
|---|---|---|
| Walkway / Sidewalk | 3–4 inches | Foot traffic only |
| Residential Patio | 4 inches | Standard residential pour |
| Garage Floor | 4–6 inches | Vehicle weight considered |
| Driveway | 4–6 inches | Passenger cars; 6 in for trucks |
| Pool Deck | 4 inches | Non-load-bearing, decorative |
| Structural Foundation | 6–8+ inches | Engineer specification required |
Common Concrete Projects and Square Foot Estimates
Driveway Concrete
A standard two-car driveway is 20 × 20 ft = 400 sq ft. At 6 inches thick, that is (400 × 0.5) ÷ 27 = 7.41 cubic yards. Add 10% overage = 8.15 cu yd. Contact your ready-mix supplier for local pricing, typically ranging from $120–$200 per cubic yard depending on mix strength and region.
Patio Slab
A 15 × 12 ft patio (180 sq ft) at 4 inches thick requires (180 × 0.333) ÷ 27 = 2.22 cubic yards. This is near the ready-mix breakeven. If your patio is below 1.5 cubic yards, 80 lb bags may be more practical and cost-effective than ordering a truck delivery with a minimum order fee.
Column Footings
Circular column footings are measured by diameter. A 24-inch diameter footing at 12 inches deep uses: π × (1)² × 1 ft = 3.14 cubic feet = 0.116 cubic yards per footing. Multiple footings are summed — use the quantity field to calculate all footings in one step.
Fence Post Holes
Concrete fence post holes are cylindrical. For a 10-inch diameter hole at 24 inches deep, volume = π × (0.417)² × 2 = 1.09 cubic feet = 0.04 cubic yards. For 20 fence posts, that equals 0.8 cu yd — well within DIY bag territory using 80 lb Quikrete bags.
Frequently Asked Questions
Multiply the length by the width of the slab in feet. A 20 × 10 ft slab = 200 square feet. This is your surface area. Then factor in depth to find the concrete volume needed.
It depends on thickness. At 4 inches thick: (200 × 0.333) ÷ 27 = 2.47 cu yd. At 6 inches thick: (200 × 0.5) ÷ 27 = 3.70 cu yd. Always add 10% overage to your order.
One 80 lb bag yields approximately 0.60 cubic feet of concrete. Since 1 cubic yard = 27 cubic feet, you need 27 ÷ 0.60 = 45 bags per cubic yard. For 60 lb bags (0.45 cu ft each), you need 60 bags per cubic yard.
The standard residential driveway thickness is 4 inches for passenger cars. For heavy vehicles, trucks, or RVs, increase to 5–6 inches. Driveways with cold climates or expansive soils should also consider a thicker pour with proper sub-base preparation.
Installed concrete typically costs $4–$8 per square foot for a plain slab. Decorative finishes (stamped, colored, exposed aggregate) add $3–$10 per sq ft. Material-only (ready-mix delivered) runs $120–$200 per cubic yard depending on your region and mix strength (PSI).
Yes — always order 10% more than your calculated volume. Concrete cannot be paused mid-pour; running short causes cold joints that structurally weaken the slab. The small cost of extra material is insignificant compared to the risk of a failed pour.
Use the formula: Area = π × r² (where r is the radius, half the diameter). Multiply by the depth in feet, then divide by 27. Our calculator handles circular slabs automatically — just select "Circular Slab" and enter the diameter.
Square feet measures the flat surface area (2D). Cubic yards measures volume (3D) — the amount of concrete required to fill that area to a given depth. Concrete is ordered in cubic yards; square footage is just the starting point for that calculation.