The Basic Concrete Yardage Formula

Every concrete calculation starts with the same basic formula. Doesn't matter if it's a patio, driveway, sidewalk, or garage floor — the math is the same.

Length (ft) × Width (ft) × Thickness (ft) ÷ 27 = Cubic Yards

The key thing most people forget: thickness has to be in feet, not inches. A 4-inch slab is 0.333 feet. A 6-inch slab is 0.5 feet. If you punch in "4" instead of "0.333," your answer will be off by a factor of 12.

📐 Converting Inches to Feet

Divide inches by 12. So 4 inches = 4/12 = 0.333 feet. Or 6 inches = 6/12 = 0.5 feet. Simple, but forgetting this is the #1 mistake in concrete ordering.

Slabs: Step-by-Step Example

Slabs are the most common concrete project for homeowners. Patios, shed pads, basketball courts, garage floors — they're all flat slabs.

Example: A 20 × 20 ft Patio, 4 Inches Thick

  1. Convert thickness: 4 ÷ 12 = 0.333 ft
  2. Calculate volume: 20 × 20 × 0.333 = 133.2 cubic feet
  3. Convert to yards: 133.2 ÷ 27 = 4.93 cubic yards
  4. Add 10% waste: 4.93 × 1.10 = 5.43 cubic yards
  5. Round up: Order 5.5 cubic yards
Key Takeaway

A 20×20 ft slab at 4 inches thick needs about 5.5 cubic yards of concrete (including 10% extra for waste).

Example: A 10 × 10 ft Pad, 4 Inches Thick

  1. 10 × 10 × 0.333 = 33.3 cubic feet
  2. 33.3 ÷ 27 = 1.23 cubic yards
  3. With 10% waste: 1.36 cubic yards
  4. Round up: Order 1.5 cubic yards

For a project this small, you could also use bagged concrete — about 56 bags of 80 lb mix. But a ready-mix truck is faster if you can meet their minimum order.

Skip the Math — Use the Calculator

Enter length, width, and thickness. The calculator handles unit conversion, waste factor, and bag estimates.

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Driveways: How to Figure Concrete Yardage

Driveways use the same formula as slabs, but they're usually thicker and longer. Standard driveway thickness is 4 inches for cars and 5-6 inches for heavier vehicles (trucks, RVs, trailers).

Example: A Single-Car Driveway (12 × 40 ft, 5 Inches Thick)

  1. Convert thickness: 5 ÷ 12 = 0.417 ft
  2. Volume: 12 × 40 × 0.417 = 200 cubic feet
  3. Convert: 200 ÷ 27 = 7.41 cubic yards
  4. With 10% waste: 8.15 cubic yards
  5. Order: 8.5 cubic yards

Example: A Double-Wide Driveway (20 × 40 ft, 5 Inches Thick)

  1. 20 × 40 × 0.417 = 333.6 cubic feet
  2. 333.6 ÷ 27 = 12.36 cubic yards
  3. With 10%: 13.59 cubic yards
  4. Order: 14 cubic yards
💡 Driveway Depth Guide

4 inches — standard for passenger cars on stable, compacted base. 5 inches — recommended for most residential driveways. 6 inches — heavy vehicles, frequent truck traffic, or weak soil conditions.

Footings and Foundations

Footings are different from slabs because they're usually narrow and deep instead of wide and thin. The formula is the same — you're still calculating length × width × depth ÷ 27 — but the shape changes.

Continuous Strip Footing

A strip footing runs along the perimeter of a building or wall. You measure its total linear length, width, and depth.

Example: A footing that runs 80 feet total, 16 inches wide, and 8 inches deep.

  1. Convert width: 16 ÷ 12 = 1.333 ft
  2. Convert depth: 8 ÷ 12 = 0.667 ft
  3. Volume: 80 × 1.333 × 0.667 = 71.1 cubic feet
  4. Convert: 71.1 ÷ 27 = 2.63 cubic yards
  5. With 10% waste: 2.90 cubic yards
  6. Order: 3 cubic yards

Pier / Pad Footings

Pier footings are square or rectangular pads poured at specific points (deck posts, column bases). Calculate each pad individually, then add them up.

Example: 8 pier footings, each 24 × 24 inches × 12 inches deep.

  1. Convert: 24/12 = 2 ft wide, 24/12 = 2 ft long, 12/12 = 1 ft deep
  2. Volume per pad: 2 × 2 × 1 = 4 cubic feet
  3. Total: 8 × 4 = 32 cubic feet
  4. Convert: 32 ÷ 27 = 1.19 cubic yards
  5. With 10%: 1.30 cubic yards → Order 1.5 yards

Cylinders and Columns (Round Pours)

Sonotubes, round columns, and pier holes need a different formula because they're circular, not rectangular.

π × radius² × height ÷ 27 = Cubic Yards

Or in plainer terms: 3.14159 × (diameter ÷ 2)² × height ÷ 27

Example: 6 Sonotubes, 12 Inches Diameter × 48 Inches Deep

  1. Convert diameter: 12 ÷ 12 = 1 ft → radius = 0.5 ft
  2. Convert height: 48 ÷ 12 = 4 ft
  3. Volume per tube: 3.14159 × 0.5² × 4 = 3.14 cubic feet
  4. Total: 6 × 3.14 = 18.85 cubic feet
  5. Convert: 18.85 ÷ 27 = 0.70 cubic yards
  6. With 10%: 0.77 cubic yards → Order 1 cubic yard

For round pours, our cylinder cubic yards calculator handles this automatically — just enter diameter and height.

Slab Size Quick-Reference Table

These are the two most common slab thicknesses — 4 inches and 6 inches. Waste factor is not included in these numbers, so add 10% to whatever you look up.

Slab Size At 4″ Thick At 6″ Thick
8 × 8 ft0.79 yd³1.19 yd³
10 × 10 ft1.23 yd³1.85 yd³
10 × 20 ft2.47 yd³3.70 yd³
12 × 12 ft1.78 yd³2.67 yd³
12 × 20 ft2.96 yd³4.44 yd³
15 × 15 ft2.78 yd³4.17 yd³
20 × 20 ft4.94 yd³7.41 yd³
20 × 30 ft7.41 yd³11.11 yd³
20 × 40 ft9.88 yd³14.81 yd³
24 × 24 ft7.11 yd³10.67 yd³
30 × 30 ft11.11 yd³16.67 yd³
30 × 40 ft14.81 yd³22.22 yd³

Remember: add 10% to these numbers for waste. A 20×20 slab at 4 inches needs about 4.94 yards, so order 5.5 yards.

The 10% Waste Rule

This is non-negotiable. Always add 10% to your concrete yardage calculation. Here's why:

  • Uneven subgrade — the ground under your forms is never perfectly flat. Low spots eat up extra concrete.
  • Spillage — concrete flows where it wants. Some always ends up outside the forms.
  • Form bulging — wood forms flex slightly under the weight of wet concrete, expanding the actual volume.
  • Over-excavation — your actual hole or trench is often a little bigger than planned.
  • Finishing loss — screeding and floating push material around, and some gets displaced.
⚠️ Running Short Is Expensive

If you run out mid-pour, you'll need a second truck delivery — which means another delivery charge, another short-load fee, and a cold joint where the two pours meet. A cold joint is a weak point that can crack over time. It's always cheaper to order a little extra than to come up short.

What About Irregular Shapes?

Not every project is a clean rectangle. Here's how to handle the common weird shapes:

L-shaped slab: Split it into two rectangles. Calculate each one separately, then add them together.

T-shaped slab: Same approach — break it into two or three rectangles.

Curved edge: Approximate the area as a rectangle that's slightly larger than the actual shape. The waste factor will cover the difference.

Multiple thicknesses: If one section is 4 inches and another is 6 inches (like a thickened edge), calculate each thickness zone separately and add the results.

Concrete Ordering Tips

Once you've got your cubic yardage number, there are a few practical things to sort out before calling the supplier:

  • Confirm the mix design. Standard residential is 3,000-4,000 PSI. Driveways and garage slabs should be at least 4,000 PSI.
  • Ask about short-load fees. Most plants charge extra if you order less than their minimum (usually 3-5 yards, sometimes up to 10). This fee is typically $30-$75 per yard short.
  • Check truck access. A loaded concrete truck weighs about 60,000 lbs and is 30+ feet long. Make sure it can actually reach your pour site.
  • Round up to the nearest quarter yard. That's how most suppliers increment their loads — you can't order 4.13 yards. You'll get 4.25.
  • Don't subtract for rebar. Rebar and wire mesh take up minimal volume. It's not worth adjusting your order for reinforcement.

Get Your Exact Yardage

Enter your project dimensions and get cubic yards, bags, tons, and cost estimates — all in one calculation.

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