MaterialScopeHow much concrete for a 12 × 12 slab?
Compare thicknesses, extra material and bag quantities for a 12-foot-square slab.
Thickness changes the order
These figures assume straight-sided, level forms and a uniform thickness across 144 square feet. The table compares quantities only; it does not select a structurally suitable thickness.
| Thickness | Net concrete | With 10% extra | 80-lb bags* |
|---|---|---|---|
| 4 in | 1.78 yd³ | 1.96 yd³ | 88 bags |
| 5 in | 2.22 yd³ | 2.44 yd³ | 110 bags |
| 6 in | 2.67 yd³ | 2.93 yd³ | 132 bags |
*Bag counts assume 0.60 ft³ per bag and include the same 10% allowance. Ready-mix and bags are alternative purchase methods.
Work through the dimensions
12 × 12 × (4 ÷ 12) = 48.00 cubic feet. Divide by 27 to get cubic yards. Multiply the unrounded volume by 1.10 to add the allowance. Round only at the end, and confirm the supplier’s order increments.
Plan the job around the delivery
For a 12-foot-square slab, a small change in depth can noticeably change the order. One extra inch across the whole slab adds 12 cubic feet, or about 0.44 cubic yards before allowance. Compare the cost of that change using your supplier’s actual price.
See what half an inch changes
Across 144 square feet, an extra half inch adds 6 ft³, or 0.22 yd³ before allowance. With the same 10% extra, that difference is 6.6 ft³: eleven more bags at a 0.60 ft³ yield. A nominal 4-inch slab with a measured average depth of 4½ inches needs 1.80 yd³ net, 1.98 yd³ with allowance, or 99 of those bags.
Record depth measurements before ordering
Check several representative points across the prepared base. If distinct sections have different designed depths, calculate each section separately and add them, avoiding overlaps. Keep your measured volume and allowance on separate lines. The allowance should cover uncertainty in the order, not substitute for measuring a known deeper section.
Check the base before the pour
An uneven excavation can consume more material than the nominal dimensions suggest. Measure several depths and confirm that the base and forms match the design. Do not use this table to choose load-bearing capacity, reinforcement or local code requirements.
Use the bag calculator to change product yield, or the cost calculator to enter a supplier quote. Compare the 4-inch and 6-inch quantities before discussing the design with your project professional.
Product reference: QUIKRETE concrete mix technical data. Confirm the current label for your chosen product.
Change the measurements and allowance to match your plan.
Open Concrete Slab CalculatorMaterialScope editorial · Updated September 29, 2026 · Calculation methodology