Calculate the number of rebar bars needed for a slab or wall. Enter span, spacing, and cover to get bar count for both directions.
Reinforcing steel (rebar) gives concrete its tensile strength, preventing cracks from becoming structural failures. The spacing of rebar determines how well the reinforcement distributes loads across the concrete section. Too wide a spacing leaves unreinforced zones; too tight wastes material and makes concrete placement difficult.
This calculator computes the number of rebar bars needed in each direction for a rectangular slab, wall, or footing. Enter the span dimensions, the on-center spacing, and the concrete cover distance. The tool calculates bars in both directions (for a grid) and gives you total bar counts and total linear feet of rebar.
Whether you're laying out a #4 bar grid at 12 inches on center for a driveway slab or #5 bars at 18-inch spacing for a foundation wall, this calculator ensures you order the right number of bars and don't run short during placement.
This measurement supports better project estimation, enabling contractors and engineers to deliver accurate bids and avoid costly overruns during the construction process.
Rebar is sold by the piece (typically 20-foot lengths), and running short during placement means a delay while you source more bars. Over-ordering wastes money on heavy material that's expensive to return. This calculator gives you exact counts for both directions of a reinforcement grid. Data-driven calculations reduce financial risk by ensuring that material orders, labor estimates, and project budgets reflect actual requirements rather than rough approximations.
Bars in length direction = floor((Width − 2 × Cover) / Spacing) + 1 Bars in width direction = floor((Length − 2 × Cover) / Spacing) + 1 Total bars = Bars(L) + Bars(W) Total LF = Bars(L) × Length + Bars(W) × Width
Result: 29 bars total
A 20×12 ft slab with #4 rebar at 12" on center and 2" cover: bars running the 20 ft direction = (12 − 4) / 1 + 1 = 9 bars. Bars running the 12 ft direction = (20 − 4) / 1 + 1 = 17 bars. Total = 26 bars needing 20 ft × 9 + 12 ft × 17 = 384 linear feet.
A rebar grid consists of bars running in two perpendicular directions, tied together at intersections. The grid provides multi-directional tensile strength to resist cracking from loads, settlement, and thermal changes. Grid spacing is specified by the structural engineer based on load requirements.
Residential slabs: #4 at 18" OC both ways. Driveways: #4 at 12" OC both ways. Foundation walls: #4 or #5 at 12" OC vertically, #4 at 24" OC horizontally. Always verify with your structural plans or local building code.
Order 20-foot bars and cut to length on site using a rebar cutter or reciprocating saw with a metal blade. Account for lap splice length when calculating total material. A typical lap splice adds 24–40 bar diameters of extra length per splice.
On-center (OC) spacing measures from the center of one bar to the center of the next bar. 12-inch OC means bars are 12 inches apart, center to center. This is different from clear spacing, which measures the gap between bar edges.
#3 (3/8") or #4 (1/2") rebar is standard for residential slabs. #4 is most common for driveways and garage floors. #5 or larger is used for structural slabs, commercial applications, and heavily loaded areas.
Cover is the distance from the nearest concrete surface to the nearest rebar. It protects the steel from moisture and corrosion. ACI 318 requires 1.5" cover for slabs on grade, 2" for formed surfaces exposed to weather, and 3" for concrete cast against soil.
For circular slabs, lay bars in a grid pattern across the full diameter in both directions. The number of bars in each direction is: (diameter − 2 × cover) / spacing + 1. Bars will vary in length — outer bars are shorter.
Yes, for slabs subjected to loads, rebar should run in both directions (a grid). One-way reinforcement is only used in specific structural elements like beams and one-way slabs where the load path is clearly directional.
Rebar is typically sold in 20-foot lengths (sometimes 30 or 40 feet for large projects). Where bars must be longer than available stock, lap splices are used to connect bars end-to-end.