How to Calculate Concrete Yards

Ordering concrete is a one-shot deal, so the math matters more than almost any other number on the job. Learning how to calculate concrete yards keeps you from the two classic mistakes: coming up short so the truck leaves before your pour is finished, or over-ordering and paying to haul away what you cannot use. Concrete is sold by the cubic yard, and every cubic yard is 27 cubic feet. Once you understand that single conversion and the basic volume formula, you can size a slab, a footing, a column, or a set of steps in a couple of minutes. Here is the method, with worked examples.

The Core Formula

Every concrete calculation comes down to finding a volume in cubic feet and dividing by 27 to get cubic yards. For any rectangular pour, the formula is:

Cubic yards = (length in feet x width in feet x thickness in feet) / 27

The catch that trips people up is thickness, which is almost always given in inches. You cannot mix inches and feet, so convert the thickness to feet first by dividing by 12. A 4-inch slab is 4 divided by 12, or 0.333 feet. A 6-inch slab is 0.5 feet. An 8-inch pour is 0.667 feet. Get that conversion right and the rest is arithmetic.

Worked Example: A Concrete Slab

Say you are pouring a 10-foot by 10-foot patio slab at 4 inches thick. Work it step by step.

1. Convert thickness: 4 inches / 12 = 0.333 feet.
2. Multiply the dimensions: 10 x 10 x 0.333 = 33.3 cubic feet.
3. Divide by 27: 33.3 / 27 = 1.23 cubic yards.

So the bare slab needs 1.23 cubic yards. But you never order the bare number, because subgrade is never perfectly flat, forms bulge, and some concrete is always lost to spillage and the chute. Add 10 percent waste: 1.23 x 1.10 = 1.35 cubic yards. That is your order figure. On a small pour like this, rounding up to 1.5 yards is cheap insurance against running short.

Worked Example: Footings

Footings are just long, narrow rectangles, so the same formula applies; you just work in the trench dimensions. Suppose you have a continuous footing 40 feet long, 16 inches wide, and 8 inches deep.

1. Convert width: 16 / 12 = 1.333 feet.
2. Convert depth: 8 / 12 = 0.667 feet.
3. Volume: 40 x 1.333 x 0.667 = 35.6 cubic feet.
4. Divide by 27: 35.6 / 27 = 1.32 cubic yards.

Add 10 percent for the uneven trench bottom that footings always have, and you land at about 1.45 cubic yards, so order 1.5. Trench footings tend to eat more concrete than the paper number because the sides slough and the bottom is irregular, so never shave this one thin.

Worked Example: Round Columns and Sonotubes

Round pours use the area of a circle instead of length times width. The area of a circle is pi times the radius squared, where the radius is half the diameter and pi is 3.1416. Then multiply by height and divide by 27.

Take a 12-inch-diameter sonotube pier, 4 feet tall. The diameter is 12 inches, so the radius is 6 inches, or 0.5 feet.

1. Area: 3.1416 x (0.5 x 0.5) = 0.785 square feet.
2. Volume: 0.785 x 4 feet tall = 3.14 cubic feet.
3. Divide by 27: 3.14 / 27 = 0.116 cubic yards per pier.

One pier barely moves the needle, but multiply by the number of piers. Twelve of these deck footings come to 12 x 0.116 = 1.4 cubic yards. Round tubes are easy to under-fill, so add 5 to 10 percent and order 1.5 yards.

Worked Example: A Set of Steps

Steps look intimidating but break down into simple boxes. Treat each step as a rectangular block, calculate each one, and add them up. For a three-step stoop where each step is 4 feet wide, and the treads stack so the bottom block is 3 feet deep by 6 inches tall, the middle 2 feet by 12 inches, and the top 1 foot by 18 inches, compute each block’s volume in cubic feet, total them, and divide by 27. Most three-step stoops land between 0.5 and 1 cubic yard. When in doubt, model the whole staircase as a solid wedge and split the difference, then add overage.

Bagged Concrete Versus Ready-Mix

For small jobs you may skip the truck and mix bags. Know the yields: a 60-pound bag of concrete makes about 0.45 cubic feet, and an 80-pound bag makes about 0.60 cubic feet. To find how many bags you need, take your total cubic feet and divide by the bag yield.

• That 1.23-cubic-yard patio is 33.3 cubic feet. Using 80-pound bags at 0.60 cubic feet each, that is 33.3 / 0.60 = 56 bags.
• Using 60-pound bags at 0.45 cubic feet, it is 33.3 / 0.45 = 74 bags.
• As a rule of thumb, it takes about 45 of the 80-pound bags or 60 of the 60-pound bags to make one cubic yard.

Bags make sense up to roughly half a yard. Beyond that, mixing dozens of bags by hand is exhausting and the ready-mix truck becomes both cheaper and far easier.

Irregular Shapes and Slopes

Real projects rarely come as tidy rectangles, but every odd shape breaks down into pieces you already know how to calculate. An L-shaped patio is two rectangles; split it along a clean line, compute each, and add the volumes. A triangular corner slab is length times width divided by two for the area, then multiplied by thickness. A curved edge can be approximated by treating it as a rectangle to the widest point and accepting a little overage, which you want anyway.

Slopes and varying thickness are the other wrinkle. A driveway that thickens from 4 inches at the house to 6 inches at the street does not use one flat thickness; use the average, 5 inches, or 0.417 feet, in your volume math. When a subgrade dips, that low spot silently swallows concrete, so a slab you calculated at exactly 2 yards can drink 2.3 in practice. This is precisely why the overage factor exists, and why measuring your actual excavated depth in several spots beats trusting the plan.

Common Mistakes That Waste Concrete or Money

The errors I see most are avoidable with a little care. Mixing units is the classic: someone plugs 4 for a 4-inch slab straight into the formula instead of 0.333 feet, and orders twelve times too much. Forgetting overage is the opposite mistake, and running short mid-pour is far worse than having a little extra, because a cold joint where a second batch meets the first creates a weak seam.

• Always convert inches to feet before multiplying, without exception.
• Measure the actual excavated depth in several spots, not just the design number.
• Add 10 percent overage on slabs and footings, 5 to 10 percent on formed columns.
• Round the final order up to the nearest quarter yard.
• Never plan a pour that requires two separate truck loads for a single slab without a plan to keep it continuous.

Double-check your arithmetic before you call the supplier, and if the number sits right at a short-load threshold, it is often cheaper to bump the order up than to pay the small-load fee on a hair-under quantity.

A Quick Reference for the Job Site

Keep these conversions in your pocket and you can size almost any residential pour on the spot. There are 27 cubic feet in a cubic yard. A 4-inch slab is 0.333 feet thick, a 6-inch slab is 0.5 feet, and an 8-inch pour is 0.667 feet. One cubic yard covers about 81 square feet at 4 inches deep, 54 square feet at 6 inches, and 41 square feet at 8 inches. It takes roughly 45 of the 80-pound bags to equal a yard. Memorize the 81-square-feet-per-yard figure for four-inch slabs and you can estimate a patio in your head before you even reach for a calculator.

What Concrete Costs and How to Order

Ready-mix concrete typically runs $125 to $165 per cubic yard delivered, though prices vary by region and mix strength. The trap on small pours is the short-load fee: most suppliers charge extra, often $50 to $150, for orders under about 3 to 4 cubic yards, because a partly full truck still ties up the same driver and drum. Factor that in when deciding between bags and a truck.

When you place the order, give the supplier your calculated volume plus overage and the strength you need, commonly 3,000 to 4,000 psi for residential slabs and footings. Round up to the nearest quarter yard, because a truck can bring a little extra far more easily than you can conjure a second batch mid-pour. Have your forms braced, your crew ready, and a plan for any leftover concrete before the truck arrives. Nail the calculation up front and the pour itself becomes the easy part.