Concrete Slab Size Chart: Cubic Yards and Bags

Slab size chart at 4 inches thick

All quantities below assume a uniform 4-inch slab with no thickened edges, footings, steps or separate pads. The last column adds 10% to exact volume before rounding to whole bags. It is an example allowance, not an instruction to use 10% on every pour.

Calculate a slab without relying on a chart

Cubic yards = length in feet × width in feet × thickness in inches ÷ 324. The 324 combines 12 inches per foot and 27 cubic feet per cubic yard. Keep thickness in inches for this version of the formula.

For a 10 × 10 ft slab, 10 × 10 × 4 ÷ 324 = 1.234567… yd³. The same slab is 33.333… ft³. Dividing by a 0.60 ft³ bag yield gives 55.555… bags, so the no-allowance purchase is 56 bags.

For the 10% example, 33.333… × 1.10 = 36.666… ft³. Dividing by 0.60 gives 61.111… bags, rounded to 62. Adding 10% to an already rounded bag count can give a different result; apply the allowance to volume first.

What changes at 5 or 6 inches?

Increasing thickness changes volume in direct proportion. Moving from 4 to 6 inches adds 50% more concrete for the same footprint. The difference is a quantity comparison only: this article does not select a structurally suitable slab thickness.

Add details without counting them twice

A thickened edge needs its additional volume. If the basic slab calculation already includes the top 4 inches across the footprint, an edge takeoff must add only the extra depth below that layer, or use a fully separated takeoff that avoids overlapping volumes.

Treat disconnected pads or different thicknesses as separate sections. Combine compatible concrete quantities before supplier order rounding. Reinforcement, forms, vapor barriers, subbase and labor are separate items; the concrete volume does not price or specify them.

Decide between bags and a ready-mix order

Bag counts make the handling requirement visible. The chart’s 20 × 20 ft slab needs 223 nominal 80 lb bags even before allowance. That represents 17,840 lb of packaged dry product to move and mix; it is not the weight of the finished wet concrete.

Compare the complete supply plan: quoted ready-mix volume, minimum load charges, delivery, access, mixing equipment and the placement schedule. A low bag price alone does not settle that decision. Use the ready-mix versus bags calculator to compare alternatives for one shared required volume.

Checks before ordering

Measure the formed area, confirm the specified thickness and check for changes in level. A depth error across a large footprint matters more than the third decimal place in a calculator result.

• Confirm product yield instead of converting dry bag weight directly to volume.
• Account for all edge and footing details on the plan.
• Keep the allowance and supplier rounding visible as separate steps.
• Confirm required concrete properties and the placement plan with the responsible contractor.

Common questions

Sources and product references

• QUIKRETE — Concrete Mix No. 1101 data sheet

  Product-specific nominal yields: 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, and 0.60 ft³ for 80 lb bags. Verify the label on the mix you buy.

• NIST — SI units: volume

  Reference for volume units. Coverage tables and worked examples on this page are Calcstead calculations, not product performance guarantees.