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.
Specialized Square Feet Calculators
Each tool is purpose-built with its own dedicated calculator, visual diagrams, and step-by-step guidance.
Triangle
Find the area of any triangle using base × height or Heron's formula with three side lengths.
Irregular Shape
Break any complex shape into rectangles and triangles. Add sections and sum them automatically.
Inches
Enter measurements in inches and get instant square feet results. Includes a quick-reference conversion table.
From Meters
Convert metric measurements to square feet. Perfect for international real estate and imported materials.
Linear Feet
Convert linear footage to square feet for lumber, carpet, fencing, decking, and rolled materials.
Sq Meters
Convert sq ft to sq m with live dual display. Essential for international real estate and architecture.
Sq Yards
Divide sq ft by 9 to get sq yd. Perfect for carpet ordering, sod purchases, and fabric estimates.
Acres
Convert property area from sq ft to acres. Ideal for land sales, farming, zoning, and lot size comparisons.
Cubic Feet
Add depth to any area to calculate volume. Essential for concrete, mulch, soil, gravel, and pool planning.
House
Add rooms one by one and get your home's total square footage, with a live bar chart breakdown per room.
Room
Measure rectangular or L-shaped rooms for flooring, paint, and renovation budgeting with instant cost estimates.
Wall
Get net wall area for paint, wallpaper, or tile by subtracting doors and windows from the gross surface.
Tile
Enter room size and tile dimensions to calculate tile count, boxes to buy, and total material cost including waste.
Concrete
Get slab area, cubic yards of concrete, bags needed, and pour cost for any project.
Land
Measure any property lot or parcel in sq ft and acres. Supports rectangular, triangular, and irregular shapes.
Cost
Estimate total project cost from area and price per square foot. Compare scenarios with tax and markup.
Flooring
Find exact flooring material for hardwood, laminate, vinyl, or carpet with waste factor and box count.
Roof
Get true roof surface area using pitch multiplier. Includes roofing squares, shingle bundles, and cost.
Paint Area
Get paintable wall area with door and window deductions. Calculates gallons, coats, and total paint cost.
Estimation
Add multiple areas, apply buffer percentages, and get quick estimates for budgeting any project.
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.