How Do You Calculate Concrete for a Slab?

Ready mixed concrete "is sold by volume, in cubic yards (cubic meters)," as the National Ready Mixed Concrete Association puts it in CIP 31, Ordering Ready Mixed Concrete. A cubic yard is 27 cubic feet (3 × 3 × 3), which is why the industry's own yield formula divides by 27 (CIP 8, Discrepancies in Yield). Work every dimension in feet, find the cubic feet of each shape, add them, and divide once.

Take dimensions from the structural drawings, not the architectural plan, and use the detail's thickness, which may be more at edges, under bearing walls or at equipment pads. If you bid the job as a concrete sub, the same takeoff feeds your unit prices; see how concrete subs bid and bill.

A fictional detached garage: a 24 by 24 foot slab, 4 inches thick; a continuous footing 16 inches wide and 8 inches deep in a 96-foot centerline run; and twelve 12-inch round piers, 4 feet deep, for a porch.

Check it a second way: the slab alone is 7.11 cubic yards (192 ÷ 27), the footing 3.16 and the piers 1.40, which add to 11.67. At this size the order is more than one truck can carry if trucks run 8 to 12 cubic yards (CIP 31), so ask the producer how they will split it and pour the footing and piers first.

NRMCA's guidance is that the "quantity of concrete ordered should be 4% to 10% more than an estimate calculated from the plan dimensions to account for contingencies" (CIP 31). Its yield bulletin lists why forms take more than the plan says: thickness or dimensions bigger than assumed "even by a small amount," forms that deflect under the concrete's pressure, and "placement on irregular subgrade or granular fill and settlement" (CIP 8).

• Thickness first. CIP 8's example: a 1/8-inch difference on a 4-inch slab means a 3% shortage, or 1 cubic yard on a 32-yard order. On the garage slab, 4 1/8 inches is 7.33 cubic yards instead of 7.11.
• Subgrade next. Fine-grade and compact the base, and check it with a string line before the pour.
• Forms. Brace them; a bowed form adds volume you never measured.
• Small leftovers count. Concrete returned each day or used in mud sills and small footings adds up over a job (CIP 8).

Use the low end of the range for formed work you can measure, such as walls, and the high end for slabs on uneven ground and for drilled piers in loose soil. Then order once, with a clean-up load as a last resort: CIP 31 advises avoiding "excessive concrete or small clean-up loads."

The volume is one line of the order. CIP 31 lists what else the producer needs: the size of coarse aggregate, the slump, air content where the concrete is exposed to freezing, the strength or performance requirement (it gives 3,500 to 5,000 psi as typical for most applications), and the delivery address, placement method, rate and time. A loaded truck weighs about 80,000 pounds and "may not be able to maneuver on loose dirt or over residential curbs," so plan the access or a pump.

Write the quantity and the waste factor on the estimate, so a change in slab thickness or footing depth can be priced from the same math. Price the concrete as a cost, then the markup, the way markup vs margin explains.

Concrete is length times width times thickness in feet, divided by 27, shape by shape, then 4% to 10% more for the subgrade, the forms and the pour. Measure thickness carefully, because it moves the number most.

If you are a sub bidding the work, carry the takeoff into your unit prices and billing; the subcontractor's guide covers the rest.

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1. CIP 31, Ordering Ready Mixed Concrete — National Ready Mixed Concrete Association. Read October 22, 2026.
2. CIP 8, Discrepancies in Yield — National Ready Mixed Concrete Association. Read October 22, 2026.