Concrete Volume Calculator: How to Calculate Cubic Yards for Any Pour

Ordering concrete comes down to one number: volume. Get it right and the pour goes smoothly. Get it wrong and you're either paying for a truckload you didn't need — or staring at a half-finished slab while the crew waits.

Here's how to calculate concrete volume for any project, with the exact formulas.

The core formula

For any rectangular pour (slab, footing, wall):

Volume = Length × Width × Thickness

Measure everything in feet, and the result is cubic feet. Then convert:

Cubic yards = Cubic feet ÷ 27

Why yards? Because ready-mix concrete is sold by the cubic yard. One cubic yard = 27 cubic feet.

Quick example: 10×10 ft patio, 4 inches thick

1. Convert thickness to feet: 4" ÷ 12 = 0.333 ft
2. Volume: 10 × 10 × 0.333 = 33.3 cubic feet
3. Convert: 33.3 ÷ 27 = 1.23 cubic yards

You'd order 1.25–1.5 cubic yards (always round up and add waste — more on that below).

Formulas for common shapes

Slabs and footings (rectangular)

Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft)

| Thickness | Decimal feet | |---|---| | 3" | 0.25 | | 4" | 0.333 | | 5" | 0.417 | | 6" | 0.50 | | 8" | 0.667 |

Round footings and sonotubes (cylinder)

Volume (cu ft) = π × radius² × height

For a 12" diameter × 36" deep footing:

• Radius = 0.5 ft, height = 3 ft
• Volume = 3.1416 × 0.25 × 3 = 2.36 cubic feet

Columns (cylinder, same formula)

A 16" diameter × 8 ft tall column:

• Radius = 0.667 ft
• Volume = 3.1416 × 0.444 × 8 = 11.2 cubic feet (0.41 cubic yards)

Steps

Calculate each step as a rectangle and add them up. For 3 steps, each 4 ft wide:

• Step 1 (bottom): 4 × 1 × 0.58 = 2.33 cu ft
• Step 2: 4 × 0.67 × 0.58 = 1.56 cu ft
• Step 3 (top): 4 × 0.33 × 0.58 = 0.78 cu ft
• Total: 4.67 cubic feet

Always add a waste factor

Real pours never match the math exactly — forms flex, subgrade is uneven, spillage happens. Standard practice:

• Slabs on prepared base: add 5–10%
• Footings in rough trenches: add 10%
• Uneven or hand-dug areas: add 10–15%

That 1.23 cubic yard patio? Order 1.5 cubic yards. The extra $20–$30 of concrete is far cheaper than a short load fee or a cold joint from running out mid-pour.

Volume cheat sheet (4" thick slabs)

| Project | Cubic feet | Cubic yards | With 10% waste | |---|---|---|---| | 8×8 ft shed base | 21.3 | 0.79 | 0.87 | | 10×10 ft patio | 33.3 | 1.23 | 1.36 | | 12×12 ft patio | 48.0 | 1.78 | 1.96 | | 20×20 ft garage | 133.3 | 4.94 | 5.43 | | 10×20 ft driveway | 66.7 | 2.47 | 2.72 |

Bags vs. truck: where's the cutoff?

• Under ~0.5 cubic yards (about 20–25 bags): mixing bags is practical
• 0.5–1 cubic yard: gray zone — bags are exhausting but a truck has minimum-load fees
• Over 1 cubic yard: call the ready-mix plant

One 80-lb bag of concrete yields about 0.6 cubic feet. So our 10×10 patio (33.3 cu ft + waste ≈ 37 cu ft) would need roughly 62 bags — that's a truck job, not a wheelbarrow job.

4 mistakes that waste concrete (and money)

1. Measuring in inches and forgetting to convert. The #1 math error. Always convert to feet first, or divide inch-measurements by 12.
2. Ignoring the gravel base depth. If your forms are 4" tall but the base is uneven, you'll pour more than the math says. Measure the actual form depth in several spots.
3. No waste factor. Ordering exactly 1.23 yards guarantees a stressful pour. Round up.
4. Forgetting footings. A slab with thickened edges or separate footings needs those volumes added in — they're easy to overlook.

The 30-second method

For a quick estimate on any rectangular slab:

(Length × Width × Thickness in inches) ÷ 324 = cubic yards

Check: (10 × 10 × 4) ÷ 324 = 1.23 cubic yards. Same answer, one step.

Skip the calculator app on your phone — our free concrete calculator handles slabs, footings, columns, and steps, and converts to cubic yards AND bag counts instantly.