How to Calculate Concrete in Cubic Yards

Concrete is sold by the cubic yard. Slabs are measured in feet, and their thickness in inches. That mismatch is behind nearly every ordering mistake on a small pour, and unlike paint, getting it wrong is not a quick trip back to the store — a partially poured slab with a cold joint through the middle is a permanent defect.

The conversion is one number: 27. There are 27 cubic feet in a cubic yard, because a yard is 3 feet and 3 × 3 × 3 = 27.

The formula

cubic yards = (length_ft × width_ft × thickness_ft) ÷ 27

The only trap is thickness, which is virtually always given in inches. Convert it first by dividing by 12:

• 4 inches = 0.333 ft
• 5 inches = 0.417 ft
• 6 inches = 0.5 ft
• 8 inches = 0.667 ft

A 20 ft × 12 ft patio at 4 inches:

20 × 12 × 0.333 = 79.9 cubic feet
79.9 ÷ 27 = 2.96 cubic yards

So just under 3 cubic yards before any allowance for waste. The concrete calculator handles slabs, footings and cylindrical postholes, including the inch-to-foot conversion that causes most manual errors.

Add a waste factor — always

Never order the calculated figure exactly. Standard practice is 5 to 10% extra, and there are concrete reasons for it:

• Subgrade is never perfectly flat. A slight dip across a 240 sq ft slab adds real volume. Half an inch of average over-excavation on that patio is nearly a third of a cubic yard on its own.
• Forms bulge. Even well-braced forms flex slightly under the weight of wet concrete.
• Spillage and residue. Some stays in the chute, the wheelbarrow and the mixer.
• Edges get over-dug, particularly where a slab meets a thickened edge or footing.

On our 2.96-yard patio, add 10% and order 3.25 yards — most ready-mix suppliers sell in quarter-yard increments.

Why running short is worse than over-ordering

Ordering extra costs perhaps $20 to $40 a yard for material you do not use. Running short costs vastly more.

Concrete begins hydrating as soon as it is mixed and you have roughly 60 to 90 minutes of working time depending on temperature. If a pour stops while a second delivery is arranged, the first section starts to set. When the new concrete arrives you get a cold joint — a plane of weakness through the slab where the two pours never properly bonded. It cracks along that line, often within a season.

Most suppliers also charge a short-load fee on anything under about 3 to 4 cubic yards, frequently $50 to $150, and a second small delivery means paying it twice on top of another trip charge.

The asymmetry is stark: over-ordering wastes tens of dollars, under-ordering can compromise the structure. Round up.

Bagged concrete and when it stops making sense

For small jobs, bagged mix is easier than arranging a truck. The yields are roughly:

Bag size	Approximate yield	Bags per cubic yard
40 lb	0.30 cu ft	~90
60 lb	0.45 cu ft	~60
80 lb	0.60 cu ft	~45

Ninety bags of 40 lb mix is 3,600 pounds to move, open and mix by hand. The practical crossover is around 1 cubic yard: below that, bags are usually simpler; above it, order ready-mix.

For that 3-yard patio you would be mixing roughly 135 eighty-pound bags. Nobody should do that by hand.

Thickness and what it is for

Thickness is not a free variable — it follows the load:

• 4 inches: patios, walkways, shed floors, general foot traffic.
• 5–6 inches: driveways for passenger vehicles.
• 6+ inches: anything carrying trucks or heavy equipment.

Going from 4 to 6 inches raises volume by 50%, so it is a real cost decision, not a detail. Under-thickness on a driveway is a common and expensive failure — the slab cracks under wheel loads and cannot be repaired without replacement.

Local building codes govern minimum thickness, reinforcement and frost depth for footings, and they vary enormously by climate. Check before ordering, not after.

Footings and postholes

Footings are the same formula in different proportions. A 20 ft footing, 16 inches wide and 8 inches deep:

20 × 1.333 × 0.667 = 17.8 cubic feet ÷ 27 = 0.66 cubic yards

Round holes use the cylinder volume:

cubic feet = π × radius_ft² × depth_ft

A 12-inch-diameter posthole 3 feet deep is π × 0.5² × 3 = 2.36 cubic feet, or about 0.087 cubic yards — roughly four 80 lb bags per hole. Multiply by the number of posts before deciding to mix by hand; a 20-post fence is 80 bags.

A worked checklist

1. Measure length and width in feet.
2. Convert thickness from inches to feet (divide by 12).
3. Multiply the three together for cubic feet.
4. Divide by 27 for cubic yards.
5. Add 10%.
6. Round up to the supplier's increment.

The same measure-and-divide-by-a-coverage-figure pattern shows up across home projects — it is exactly how mulch is ordered by volume, how flooring is ordered by area with a cut-waste allowance, and how you work out how much paint a room needs. Learn it once and it transfers.

For technical standards on mix design, curing and reinforcement, the American Concrete Institute publishes the codes US inspectors work from, and the Portland Cement Association has accessible guidance on slab thickness and joint spacing for residential work.