To figure out concrete yardage, you mostly need one core idea: concrete is ordered in cubic yards , and there are 27 cubic feet in 1 cubic yard.

Basic formula (for most slabs)

For a standard patio, driveway, or slab, use:

Length (ft) × Width (ft) × Thickness (ft) ÷ 27 = cubic yards of concrete

Steps:

  1. Measure:
    • Length in feet
    • Width in feet
    • Thickness in feet (convert inches to feet by dividing by 12)
  2. Multiply length × width × thickness → this gives cubic feet.
  1. Divide that result by 27 → this gives cubic yards.

Converting thickness

Common slab thicknesses:

  • 4 inches = 4 ÷ 12 = 0.33 ft (often rounded to 0.333).
  • 5 inches = 5 ÷ 12 ≈ 0.42 ft (0.416 is often used).
  • 6 inches = 6 ÷ 12 = 0.5 ft.

Quick example: sidewalk

You want a 25 ft long, 5 ft wide sidewalk, 4 inches thick.

  1. Convert thickness: 4 in ÷ 12 = 0.333 ft.
  1. Find cubic feet: 25 × 5 × 0.333 ≈ 41.6 cubic feet.
  1. Convert to yards: 41.6 ÷ 27 ≈ 1.54 cubic yards.

You’d order about 1.6–2.0 cubic yards to be safe, depending on how comfortable you are with a little extra.

Another example: driveway

Say an 80 ft long, 24 ft wide driveway, 5 inches thick.

  1. Thickness: 5 in ÷ 12 ≈ 0.416 ft.
  1. Cubic feet: 80 × 24 × 0.416 ≈ 799.7 cubic feet.
  1. Cubic yards: 799.7 ÷ 27 ≈ 29.6 cubic yards.

So you’d order about 30–31 yards.

Handy shortcuts by slab thickness

Some pros use shortcuts if the thickness is standard. For example:

  • For a 4‑inch slab:
    Square footage ÷ 80 ≈ cubic yards.
  • For a 6‑inch slab:
    Square footage ÷ 50 ≈ cubic yards.

Example: 20 ft × 40 ft slab = 800 sq ft.

  • 4 in thick → 800 ÷ 80 = 10 yards.
  • 6 in thick → 800 ÷ 50 = 16 yards.

These are approximations but very practical in the field.

Allowing for waste and variations

Real jobs are never perfectly uniform, so it’s smart to order a bit extra.

  • Add about 5–10% more than the exact calculation for:
    • Slightly uneven ground
    • Forms that bow out a bit
    • Spills and overfill
  • Some guides specifically recommend adding around 10% to your yardage.

If you need a lot of hose when pumping concrete, one rule of thumb is to order roughly an extra 1/4 yard for every 50 feet of hose.

Different shapes: not just rectangles

If your slab isn’t a simple rectangle, break it into simple shapes, figure each one, and add them together.

Common approach:

  • Irregular slabs:
    • Split into rectangles and triangles, calculate each, sum the cubic feet, then divide by 27.
  • Circular pads:
    • Use area of a circle in feet: πr2\pi r^2πr2 for square feet, multiply by thickness in feet, then ÷ 27 for yards.

Once you treat each part as a simple shape, the math is the same.

Online tools and calculators

If you don’t like doing math by hand:

  • Many concrete suppliers have calculators where you plug in length, width, and thickness, and they output yards.
  • There are also mobile “concrete calc” apps mentioned often in contractor discussions, which help if your slab is a mix of shapes.

These tools still use the same basic formula under the hood.

Mini recap

  • Convert thickness from inches to feet (divide by 12).
  • Use: length × width × thickness (ft) ÷ 27 = cubic yards.
  • Add 5–10% extra so you don’t run short.
  • For 4" and 6" slabs, you can shortcut using square-foot ÷ 80 or ÷ 50.
  • Break odd shapes into simple ones and add them up.

If you tell me your specific dimensions (length, width, and thickness), I can walk you step by step through your exact concrete yardage. Information gathered from public forums or data available on the internet and portrayed here.