By Zencalcy Editorial TeamLast reviewed About us
Concrete is ordered by the cubic yard from a ready-mix truck or mixed from bags for small jobs. The break-even is around one cubic yard: below that, bags are practical; above it, ready-mix is cheaper and far less work. This calculator gives the volume and both bag counts.
Cubic yards and bags for a slab, footing or column.
Slab / footing: V = L × W × T Column: V = π × (d/2)² × h
Volume is converted to cubic yards (1 yd³ = 27 ft³). An 80 lb bag yields about 0.6 cu ft and a 60 lb bag 0.45 cu ft. Order ready-mix in quarter-yard increments and never order less than the calculated volume.
| Symbol | Meaning |
|---|---|
| L, W | Length and width of the pour. |
| T | Slab or footing thickness. |
| V | Volume in cubic yards. |
12 × 20 × 0.333 = 80 cu ft, or 2.96 cu yd. With 10% extra, order 3.25 yards — or about 147 bags of 80 lb if mixing by hand.
Continue with the Cement Calculator, Concrete Block Fill Calculator, Concrete Column Calculator, or the Concrete Stairs Calculator — or browse more cement & concrete and all construction calculators.
At 4 inches thick, 100 sq ft × 0.333 ft = 33.3 cu ft, or 1.23 cu yd — about 56 bags of 60 lb.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026