How Do I Calculate Yards of Concrete?

If you’ve ever stood at a job site staring at an empty slab form and thought, “How many yards of concrete do I actually need?” — you’re in the right place. Getting this number wrong is one of the most expensive mistakes on any concrete project, whether you’re pouring a backyard patio, a driveway, footings for a fence, or a full garage floor.

I’ve spent years working with concrete estimating tools, helping homeowners and contractors get accurate material estimates before they ever pick up the phone to call a ready-mix supplier. The good news? The math is simpler than you think. Let me walk you through it the right way.

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Why Concrete Is Measured in Cubic Yards

Before we get into the formula, it helps to understand why cubic yards are the standard unit in the U.S. concrete industry. Concrete is a three-dimensional material — it fills a space that has length, width, and depth. Volume is the right way to measure it, and in the U.S., that volume is expressed in cubic yards.

One cubic yard is a cube that measures 3 feet on each side: 3 ft × 3 ft × 3 ft = 27 cubic feet. That number — 27 — is the key to every concrete calculation you’ll ever do. Ready-mix trucks are dispatched by the yard. Bag quantities are described in relation to a yard. Once you understand that relationship, everything else clicks into place.

Quick fact: One cubic yard of concrete placed at a 4-inch depth covers approximately 81 square feet — that’s roughly a 9-foot by 9-foot area. Keep that number in your back pocket as a quick mental anchor whenever you’re sizing up a project.

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The Core Formula: Step-by-Step

For any rectangular or square concrete pour, the formula is always the same:

Cubic Yards = (Length × Width × Depth) ÷ 27

All measurements must be in feet before you multiply.

That’s it. Three numbers multiplied together, then divided by 27. The only thing that trips most people up is the depth, because thickness is usually given in inches — not feet. You must convert inches to feet before plugging into the formula.

To convert inches to feet: Divide the number of inches by 12.

Slab Thickness (inches)	Converted to Feet
3 inches	0.25 ft
4 inches	0.333 ft
5 inches	0.417 ft
6 inches	0.5 ft
8 inches	0.667 ft

The 5 Steps, Plain and Simple

Step 1 — Measure your length and width in feet. Use a tape measure. If one side is 12 feet 6 inches, write that as 12.5 feet (since 6 inches = 0.5 ft).

Step 2 — Convert your depth from inches to feet. If you’re pouring a 4-inch slab: 4 ÷ 12 = 0.333 feet. Write this number down before you forget it.

Step 3 — Multiply all three dimensions together. Length × Width × Depth = Total cubic feet.

Step 4 — Divide by 27. This converts cubic feet into cubic yards, which is the unit your supplier uses to price and deliver concrete.

Step 5 — Add 5–10% for waste and overage. Always round up slightly. Concrete is unforgiving when you come up short mid-pour.

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Real-World Examples

Example 1: A 10×10 Concrete Patio (4 inches thick)

This is a classic weekend project. Here’s the math broken down:

• Length: 10 ft
• Width: 10 ft
• Depth: 4 inches = 0.333 ft
• 10 × 10 × 0.333 = 33.3 cubic feet
• 33.3 ÷ 27 = 1.23 cubic yards

Add a 10% waste buffer: 1.23 × 1.10 = about 1.35 cubic yards. You’d order 1.5 yards to be safe. That’s a comfortable margin without over-spending.

Example 2: A Standard Driveway (20×30 ft, 5 inches thick)

• Length: 30 ft
• Width: 20 ft
• Depth: 5 inches = 0.417 ft
• 30 × 20 × 0.417 = 250.2 cubic feet
• 250.2 ÷ 27 = 9.27 cubic yards

With 10% overage: approximately 10.2 cubic yards. You’d call the plant and order 10.5 yards to keep a cushion. At this volume, a ready-mix truck is absolutely the right call over bagged concrete.

Example 3: A Concrete Footing (24 in wide × 12 in deep × 40 ft long)

Footings get a little trickier because the dimensions are smaller and easier to misread. Convert everything to feet first:

• Length: 40 ft
• Width: 24 inches = 2 ft
• Depth: 12 inches = 1 ft
• 40 × 2 × 1 = 80 cubic feet
• 80 ÷ 27 = 2.96 cubic yards

You’d order 3.25 cubic yards to account for waste and slight variations in trench width. Even a trench that’s an inch wider than planned adds up fast over 40 feet.

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Quick Reference Chart for Common Projects

Here’s a table covering the most common pour sizes. All figures assume a 4-inch slab thickness unless noted, and do not include the 10% overage buffer — add that yourself before ordering.

Project Size	Thickness	Cubic Yards Needed	80-lb Bags (approx.)
10×10 slab	4″	1.23 yd³	56 bags
12×12 slab	4″	1.78 yd³	80 bags
16×16 slab	4″	3.16 yd³	143 bags
20×20 slab	4″	4.94 yd³	222 bags
24×24 slab	4″	7.11 yd³	320 bags
20×30 driveway	5″	9.27 yd³	Ready-mix recommended
24×40 driveway	6″	21.3 yd³	Ready-mix recommended
10×10 garage floor	6″	1.85 yd³	84 bags

Pro shortcut for 4-inch slabs: Divide your total square footage by 81. That gives you the cubic yards needed almost instantly. Example: 300 sq ft ÷ 81 = 3.7 cubic yards. Experienced contractors use this trick on job sites every single day.

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Bags vs. Cubic Yards: Which Should You Order?

This is one of the most common questions I get, and the answer depends almost entirely on your project size.

Use bagged concrete (Quikrete, Sakrete, etc.) when your project requires fewer than about 1.5 cubic yards total. Small patios, fence post holes, small repair jobs, and single footings all fall into this category. An 80-lb bag yields about 0.022 cubic yards, and you can grab them from any hardware store without scheduling a delivery window.

Order ready-mix concrete by the yard when your project needs more than 1.5–2 cubic yards. Mixing bag after bag by hand for a full driveway or garage floor is exhausting, inconsistent, and far slower than a truck. Ready-mix plants typically have a minimum order of 1 yard, and most trucks carry a maximum load of 10 cubic yards per trip.

Project Volume	Best Option	Why
Under 0.5 yd³	Bagged mix	Cost-effective, no delivery needed
0.5 – 1.5 yd³	Bagged mix or short load	Weigh cost vs. effort
Over 1.5 yd³	Ready-mix truck	Faster, stronger, more consistent

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Pro Tips to Avoid Running Short (or Wasting Money)

Always order 5–10% extra. No subgrade is perfectly flat. Concrete spills. Forms flex slightly under pressure. A small buffer is not wasteful — it’s essential. Running out mid-pour forces you to stop, create a cold joint, and either scramble for a new delivery or start mixing bags on the fly. That cold joint weakens the slab structurally and can become a cracking point years down the road.

Round up to the nearest quarter yard. Most ready-mix suppliers charge in quarter-yard increments. If your calculation gives you 3.1 yards, just order 3.25 or 3.5. The small upcharge is a cheap insurance policy.

Account for your subgrade condition. If your base has dips, soft spots, or uneven areas, your actual concrete consumption will be higher than what the formula predicts. For rough or problem subgrades, bump your overage up to 15%.

Verify your form dimensions right before you call. Measure twice — seriously. A very common mistake is measuring the outside of the forms instead of the interior where the concrete actually goes. Check length, width, and depth at multiple points, especially depth, since it can vary if the ground isn’t perfectly level beneath the forms.

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Calculating Irregular and Non-Rectangular Shapes

Not every project is a neat rectangle. Here’s how to handle the most common irregular situations:

L-shaped slabs: Break the L into two rectangles. Calculate the cubic yardage for each section separately using the standard formula, then add the two totals together. This approach works for any shape you can divide into simpler sections, no matter how complex it looks at first.

Circular slabs (round patios, pool decks): For circles, the formula changes slightly. Cubic Yards = 3.14159 × (Radius in ft)² × Depth in ft ÷ 27. Remember that the radius equals the diameter divided by 2. For example, a circular patio 12 feet in diameter at 4 inches thick: Radius = 6 ft. So 3.14159 × 36 × 0.333 ÷ 27 = 1.40 cubic yards.

Concrete columns and round footings (Sonotubes): Use the same circle formula, but depth becomes the height of the tube or column. A 10-inch diameter tube (radius = 0.417 ft) that is 4 feet tall gives: 3.14159 × 0.174 × 4 ÷ 27 = 0.081 cubic yards per tube. If you have 20 tubes, multiply: 0.081 × 20 = 1.62 cubic yards total. Easy.

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What to Say When You Order Ready-Mix Concrete

Once you’ve done your calculation, calling the plant is straightforward — but have a few things ready before you dial:

Your cubic yardage. Give them your calculated amount already including the 10% overage. Don’t make them do that math for you.

Mix design and PSI strength. Residential slabs and patios typically use 3,000 PSI. Driveways and garage floors often call for 3,500–4,000 PSI. Structural work may require higher depending on your local code. If you’re unsure, the plant’s sales rep can guide you — they do this every day.

Slump. This measures how fluid or stiff the mix is, on a scale of 1 to 8 inches. A 4–5 inch slump is standard for most residential pours. Higher slump means easier to place but weaker if overdone with excess water.

Delivery date and site access. Concrete trucks are heavy — around 70,000 lbs fully loaded. Confirm gate widths, overhead clearance, and how close the truck can park to your pour site. The farther the chute has to reach, the more manual work you’ll be doing with wheelbarrows.

Tip: Many ready-mix plants have their own online concrete calculator. After you do your own math, cross-check with theirs. If the numbers are close, you’re in great shape. If there’s a big discrepancy, re-measure before you place the order.

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Frequently Asked Questions

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Wrapping It Up

Calculating yards of concrete doesn’t have to feel intimidating. With the right formula and a tape measure, you can nail your estimate whether you’re pouring a small weekend patio or a full two-car driveway. The key things to remember: always work in feet, always divide by 27, and always order a little extra. Those three habits will save you money, time, and a whole lot of stress on pour day.

If your project has multiple sections or an unusual shape, just break it into simple rectangles or circles, calculate each one separately, and add the totals together. And when in doubt, call your local ready-mix plant — most of their sales reps are happy to double-check your math before you place the order.

Good luck with your pour. Measure twice, order smart, and finish strong.