How a rebar grid is measured
Rebar runs in two perpendicular directions over the slab: bars across the width at your spacing, and bars along the length between them. The calculator counts both sets — rows across plus columns along — converts each to its full bar length, and sums the linear footage. 18-inch spacing is the common residential choice; 12-inch spacing doubles the steel for heavy loads.
Placement beats quantity
Rebar belongs in the middle third of the slab — on chairs so the pour pushes it down to center, not lying on the ground. Overlap splices 40 times the bar diameter (about 20 inches for #4). Order 10% over for laps, offcuts and the bars that get cut around penetrations.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- ASTM International — C94 for ready-mixed concrete, C150 for cement and C143 for slump testing.
- American Concrete Institute — ACI 318 code requirements and ACI 302 slab construction guidance.
- Portland Cement Association — mix design, yield and curing data published by the US cement industry.
Figures, formulas and reference tables last verified: 19 September 2026.