How the estimate works
Cubic yards = length × width × thickness (ft) ÷ 27, plus extra
Total = cubic yards × price per yard + delivery fee + sq ft × labor rate
A 20 × 20 ft slab 4 inches thick is 400 sq ft × 0.333 ft = 133.3 cubic feet, or 4.94 cubic yards, and 5.43 with 10% extra. At $155 per yard, 5.43 yards of concrete is about $842. Add your delivery fee and any finishing labor to get the total.
What changes the price
- Price per yard. Quote it by phone from two plants. Enter the figure they give you, not an average.
- Short-load fees. Many ready-mix plants add a fee when an order is below a minimum, often 3 to 5 yards. Slabs under about 120 sq ft at 4 inches fall into this range.
- Thickness. Walkways and patios are usually 4 inches. Driveways for cars are 4 to 5 inches, and for heavier vehicles 6 inches. Each extra inch on a 400 sq ft slab adds about 1.2 cubic yards.
- Extras. Gravel base, forms, reinforcement, control joints and finishing are separate lines. The gravel calculator sizes the base.
Bagged mix or ready-mix?
Bags are priced per bag and ready-mix per yard. A cubic yard is 45 bags of 80 lb mix, so a 1.36 yard slab needs 62 bags. Past about 1 cubic yard, ready-mix is usually easier. Count the bags with the concrete calculator, and see the bags per cubic yard chart.
Frequently asked questions
How much does a concrete slab cost per square foot?
Divide your total by the area. The calculator shows it. The concrete itself is about the price per yard times 0.012 per sq ft at 4 inches thick, and labor and base add to that.
How much concrete do I need for a 20x20 slab?
4.94 cubic yards at 4 inches thick, or 5.43 cubic yards with 10% extra.
Is there an extra charge for a small concrete order?
Often yes. Many plants charge a short-load fee under a minimum volume. Ask for it and enter it in the fee box.
Sources
Reference charts
More landscape material calculators
Gravel Calculator
Tons, yards and bags by stone type.
Asphalt Calculator
Tons of hot mix for driveways and lots.
Cubic Yard Calculator
Any dimensions to cubic yards.
Mulch Calculator
Yards and bags for beds and trees.
Topsoil Calculator
Soil for lawns, beds and fill.
Dirt Calculator
Fill dirt in yards, tons and truckloads.