Rebar Calculator
Lay out a slab rebar grid — pieces running each way and the total linear feet to order, including an overlap allowance.
How it works
Give the calculator your slab's length and width in feet and the on-center spacing of the bars in inches. It works out how many bars run each direction: bars running the long way are counted across the width, bars running the short way are counted across the length, and each direction gets one extra bar to close out the edge.
Total linear feet is where the money is. Each bar's length is the slab dimension it spans, plus the overlap you add for splices where two sticks of rebar meet. Add both directions together and you have the footage to order — divide by 20 if you're buying standard 20-foot sticks.
If you leave overlap at zero you get the bare grid length; bump it up to cover lap splices at the ends of long runs. The result assumes a simple rectangular mat, which covers most residential slabs, footings, and driveways.
Frequently asked questions
How is the number of bars figured out?
For each direction the tool divides the perpendicular slab dimension by the spacing, drops to a whole number, then adds one so there's a bar at both edges. A 12-foot width at 12-inch spacing gives 12 gaps plus a closing bar, so 13 bars running the length.
What spacing should I use?
Twelve to eighteen inches on center is common for residential slabs and driveways; tighter spacing means more steel and more strength. Your engineer or local code will specify the real number for anything structural — this tool just does the counting once you pick it.
Why add an overlap allowance?
Rebar comes in fixed lengths, so long runs need two sticks lapped together, and code usually wants that overlap to be dozens of bar diameters. Adding a few inches per bar keeps your order from coming up short at the splices.