Every DIY concrete project starts with the same question: how many cubic yards do I order? Ready-mix suppliers sell by the cubic yard, but we measure slabs in feet and inches. Here is a tiny, dependable formula you can drop into any project.

The math

  1. Measure length and width in feet.
  2. Convert the thickness (depth) from inches to feet: inches / 12.
  3. Multiply length x width x depth to get cubic feet.
  4. Divide by 27, because one cubic yard is 3 ft x 3 ft x 3 ft = 27 cubic feet. 
cubic yards = (L ft x W ft x (D in / 12)) / 27

A small JavaScript helper 

function concreteYards(lengthFt, widthFt, depthIn, waste = 0.1) { const depthFt = depthIn / 12; const cubicFeet = lengthFt * widthFt * depthFt; const yards = cubicFeet / 27; return +(yards * (1 + waste)).toFixed(2); } console.log(concreteYards(20, 10, 4)); // 2.72

The optional waste factor adds 10% for spillage, uneven subgrade and over-excavation, which is what most contractors recommend.

Worked example

A 20 ft x 10 ft patio slab poured 4 inches thick:

  • Depth: 4 / 12 = 0.333 ft
  • Volume: 20 x 10 x 0.333 = 66.7 cubic feet
  • Yards: 66.7 / 27 = 2.47 cubic yards
  • With 10% extra: about 2.72 cubic yards, so order 3 yards.

Edge cases worth handling

  • Mixed units like 10 ft 6 in: convert to 10.5 ft before multiplying.
  • Round up when ordering, since suppliers usually sell in quarter- or half-yard increments.
  • For bagged concrete, one 80 lb bag yields roughly 0.6 cubic feet.

If you want to skip the code and just get a number (plus bag counts), try the CalcQuarry concrete calculator. It uses the same formula and also covers gravel, mulch and paver base.