How to calculate cubic yards of concrete

By Randall Strickland · Updated September 24, 2026 · 3 min read

Ready-mix is sold by the cubic yard, and running a few yards short in the middle of a pour is one of the most expensive mistakes on a concrete job. Here’s how to figure yardage for slabs, footings, and piers, and how much extra to order.

The formula for slabs

Multiply length by width by thickness, all in feet, to get cubic feet. Then divide by 27, because there are 27 cubic feet in a cubic yard (3 ft × 3 ft × 3 ft).

Cubic yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27

Thickness is usually measured in inches, so divide it by 12 to convert it to feet.

Example: a 20 × 24 slab, 4 inches thick

• Area: 20 × 24 = 480 square feet
• Thickness in feet: 4 ÷ 12 = 0.333 ft
• Volume: 480 × 0.333 = 160 cubic feet
• Cubic yards: 160 ÷ 27 = 5.93 cubic yards

Add 5% to 10% for waste, uneven subgrade, and spillage, and you’d order about 6.25 to 6.5 yards. Or skip the math with our concrete calculator.

Quick chart: cubic yards per 100 square feet

Continuous footings

Same formula, using the footing’s length, width, and depth. For 100 linear feet of footing, 24 inches wide and 12 inches deep:

100 × 2 × 1 = 200 cu ft ÷ 27 = 7.41 cubic yards

Footings are often dug wider or deeper than drawn, so measure the actual trench before you order.

Round piers and columns

For a cylinder, use π × radius² × height:

Cubic feet = 3.1416 × (Diameter ÷ 2)² × Depth

A 12-inch pier (radius 0.5 ft) that’s 4 feet deep: 3.1416 × 0.25 × 4 = 3.14 cubic feet, or 0.116 cubic yards. Twenty of them is about 2.33 yards.

Small pours: how many bags?

For small jobs you might use bagged mix instead of a truck. An 80-lb bag of standard concrete mix makes about 0.60 cubic feet; a 60-lb bag makes about 0.45 cubic feet. Check the bag, because yields vary by product. At those yields, one cubic yard (27 cubic feet) takes about 45 80-lb bags or 60 60-lb bags.

How much extra to order

• 5% to 10% is a common allowance for flat slabs on good, compacted subgrade.
• More for uneven or over-excavated subgrade, footings dug by hand, or when you’re pumping, since the line holds concrete.
• Measure thickness in several spots. A 4-inch slab that’s actually 4½ inches across the whole pour needs about 12% more concrete.
• Ask your supplier about their minimum load and short-load fees, and how they bill for extra time on site. Many charge more for small loads.

Track what you ordered against what you bid

Your batch tickets tell you exactly how many yards each pour actually took. Compare them to your estimate on every job and you’ll learn your real waste factor, which makes your next bid sharper. In JobMargin’s Concrete & Flatwork setup, Ready-Mix Concrete is its own cost category and bid lines can be priced by the cubic yard, so each job shows concrete spent against budget as the tickets come in.

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