Concrete Cubic Yard Calculator for Contractors

The concrete cubic yard calculator formula

A concrete cubic yard calculator runs on one formula. Length in feet times width in feet times thickness in feet, divided by 27. That single calculation turns a slab, driveway, footing, or sidewalk into the cubic yards of ready-mix you order. The number 27 is the count of cubic feet in one cubic yard. Get the thickness conversion right, add a waste factor, and round up. You will rarely run short on a pour.

Concrete is sold by the cubic yard at the plant, and the load is not returnable once it leaves the yard. The National Ready Mixed Concrete Association notes that the overwhelming majority of ready mixed concrete reaches the site in truck-mounted rotating drum mixers. It also puts the typical truck mixer at a 9 to 11 cubic yard unit. Most contractors still size the pour with a tape and mental math on the tailgate. That is where short orders and surprise short-load fees start. SimplyWise Cost Estimator turns a site photo into an itemized estimate in about 6 seconds instead.

Where contractors trip

The formula is fixed. Cubic yards equals length in feet times width in feet times thickness in feet, all divided by 27. The only place contractors trip is the thickness term. Slab thickness is specified in inches, and the formula needs feet. So a 4-inch slab is not 4 in the formula. It is 0.333 ft. The order of operations is measure, convert thickness to feet, multiply for cubic feet, then divide by 27.

Inches to feet thickness conversion table

Slab and footing thickness is called out in inches on nearly every plan. The conversion to feet has to happen before the multiplication. The table below converts the common pour thicknesses. Divide any inch value by 12 to get the decimal foot value.

Why you divide by 27

A cubic yard is a cube that measures one yard, or 3 ft, on every side. Its volume is 3 ft times 3 ft times 3 ft, which equals 27 cubic feet. Once a pour is expressed in cubic feet, dividing by 27 converts straight to cubic yards. That is the unit a ready-mix plant quotes and bills in. No rounding hides in the 27. It is an exact unit conversion.

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Add a waste factor of 7 to 10 percent

The raw formula gives the volume of a perfectly formed pour. In the field that number always runs short. Add 7 to 10 percent on top of the calculated cubic yards. On rough or hand-dug subgrade, push the waste factor toward 10 percent or higher. The ground rarely cuts as clean as the plan assumes.

Where the concrete actually goes

• Subgrade variation. A slab specified at 4 in pours closer to 4.5 in over a low spot. Every fraction of extra depth adds volume across the whole pad.
• Over-excavation. Hand-dug footings and trenches run wider and deeper than the form dimension, so the concrete fills more than the calculated box.
• Spillage and waste. Concrete left in the chute, dropped during placement, or stiffening before it moves is gone.
• Form deflection. Forms bow outward under the weight of wet concrete, widening the pour past the nominal dimension.

A calculated 10.0 cubic yard order becomes a 10.7 to 11.0 yard order once waste is applied. Then it rounds up to 11.0 yards for the truck. Running short on a pour that has already started is the expensive failure. A cold joint or a second short-load delivery costs far more than the extra quarter yard.

Worked examples: slab, driveway, footing, and sidewalk

The same formula handles every common pour. Below are four worked examples. Each shows the full chain: measure, convert thickness to feet, multiply, divide by 27, add waste. The free SimplyWise concrete estimate template carries the same line items, so the math repeats job to job.

Example 1: 4-inch residential slab, 20 ft by 30 ft

The pad is 20 ft long, 30 ft wide, and 4 in thick. Convert thickness: 4 divided by 12 equals 0.333 ft. Multiply: 20 times 30 times 0.333 equals 200 cubic feet. Divide by 27: 200 divided by 27 equals 7.41 cubic yards. Add 10 percent waste: 7.41 times 1.10 equals 8.15 cubic yards. Order 8.25 cubic yards, rounded up to the nearest quarter yard.

Example 2: 6-inch driveway, 16 ft by 40 ft

The driveway is 16 ft wide, 40 ft long, and 6 in thick. Convert thickness: 6 divided by 12 equals 0.5 ft. Multiply: 16 times 40 times 0.5 equals 320 cubic feet. Divide by 27: 320 divided by 27 equals 11.85 cubic yards. Add 8 percent waste: 11.85 times 1.08 equals 12.80 cubic yards. Order 13.0 cubic yards to clear the pad with a small reserve.

Example 3: continuous footing, 1.5 ft by 60 ft by 12 in

Footings pour in feet on width and depth, so the conversion is simpler. The footing is 1.5 ft wide, 60 ft long, and 12 in deep, which is 1.0 ft. Multiply: 1.5 times 60 times 1.0 equals 90 cubic feet. Divide by 27: 90 divided by 27 equals 3.33 cubic yards. Add 10 percent waste, because hand-dug trenches over-excavate. That gives 3.67 cubic yards, so order 3.75.

Example 4: sidewalk, 4 ft by 50 ft by 4 in

The walk is 4 ft wide, 50 ft long, and 4 in thick, which is 0.333 ft. Multiply: 4 times 50 times 0.333 equals 66.6 cubic feet. Divide by 27: 66.6 divided by 27 equals 2.47 cubic yards. Add 10 percent waste: 2.47 times 1.10 equals 2.72 cubic yards. Order 2.75 cubic yards, rounded up to the nearest quarter yard. On a pour this small, ask the plant for its posted minimum, since a short-load fee can apply either way.

Reference table: slab size to cubic yards

For fast field checks, the table below pre-runs the concrete cubic yard calculator. It covers common square and rectangular slabs at 4 in and 6 in thick. These figures are the raw calculated volume before waste. Add 7 to 10 percent on top, then round up for the order.

Each figure is the calculated volume before waste. For a working order, multiply by 1.07 to 1.10 and round up. The 20 ft by 30 ft slab at 4 in shows 7.41 yards in the table. That becomes the 8.25-yard order from Example 1 once 10 percent waste and quarter-yard rounding are applied.

Short-load fees: why rounding up beats ordering short

Ready-mix plants charge a short-load fee on deliveries below their posted minimum. The threshold and the fee are set plant by plant and by delivery area, so ask the dispatcher for the minimum before you order. It is the most common surprise line on a small-pour invoice. The math on rounding is asymmetric. A quarter yard of extra concrete costs a small fraction of what a second delivery costs. Ordering short risks a second-truck fee plus a cold joint where the first pour set up before the second arrived.

The short-load math that favors rounding up

Ordering exactly 7.41 yards on a calculated 7.41-yard slab leaves zero margin for subgrade variation. A single low spot then forces a hand-mix patch or a partial second delivery. Ordering 8.25 yards adds about 0.84 of a yard, which is far cheaper than a return trip. The field rule is simple. Calculate, add waste, round up to the nearest quarter yard, and never order to the exact theoretical number.

Ordering ready-mix from the plant

Once the calculator has sized the pour, the order carries more than a yard count. The dispatcher needs the volume, the mix design, and the timing so trucks arrive ready to place. The deeper field workflow lives in the SimplyWise guide to estimating concrete work. It covers labor, finishing, and pricing on top of the volume math.

What the plant needs on the order

• Cubic yards. The calculated volume plus the waste factor, rounded up to the nearest quarter yard.
• Mix design or strength. The specified compressive strength in psi, commonly 3,000 to 4,000 psi on residential flatwork.
• Slump. The workability target, called out by the plan or by the placement method.
• Additives. Air entrainment for freeze-thaw climates, accelerator or retarder for temperature, fiber mesh if specified.
• Delivery window. The pour start time and the spacing between trucks, so concrete does not stiffen in the drum.
• Site access. Chute, pump, or buggy, so the plant sends a truck that can reach the placement point.

A clean order pairs the cubic yard figure with the mix spec. That keeps the pour on schedule and avoids a load that arrives wrong for the job.

Speed up the concrete cubic yard calculator on site

Most contractors size a pour the same way. Walk the pad, measure with a tape, run the divide-by-27 math in your head, then call the plant from the truck. That works. It also leaves the thickness conversion and the waste factor to memory, which is where short orders start. A phone workflow keeps the math honest.

Photo to estimate with SimplyWise

SimplyWise Cost Estimator prices the job from a phone photo on site. The contractor photographs the slab, driveway, footing, or sidewalk and types the measured length, width, and thickness into the project description. The app returns an itemized estimate in about 6 seconds, with material quantities. Every line item carries unit, quantity, and price, so you can adjust a quantity on the spot. On Pro iPhones, a LiDAR room scan captures measurements. You can hand over a professional, branded quote the client signs to approve before the truck is even booked.

What is included

• Photo to itemized estimate in about 6 seconds.
• Estimates that come back with material quantities.
• Editable material, labor, and fee line items.
• Unit, quantity, and price on every line item.
• LiDAR room scans on Pro iPhones to capture measurements.
• Professional, branded quotes the client signs to approve.