Rebar Calculator
Enter your slab's length, width, and rebar spacing to calculate how many rebar pieces you need in a two-way grid and the total linear footage to order.
How the Rebar formula works
Bars in each direction, based on spacing:
Bars running one way = ceil(Width × 12 / Spacing) + 1 Bars running the other way = ceil(Length × 12 / Spacing) + 1 Total linear feet = (Bars one way × Length) + (Bars other way × Width)
Step-by-step calculation
- Divide the slab width by the rebar spacing (converted to the same units) and add 1 to find bars running lengthwise.
- Do the same with slab length to find bars running widthwise.
- Multiply each set of bars by the perpendicular dimension and add the two totals for total linear feet.
Worked example
A 20 ft × 15 ft slab with rebar on 18-inch spacing: Bars one way = ceil(15×12/18)+1 = 11. Bars other way = ceil(20×12/18)+1 = 14. Total linear feet = (11×20) + (14×15) = 220 + 210 = 430 ft.
Frequently asked questions
What rebar spacing is typical for a residential slab?
12 to 18 inches on-center is common for residential slabs, though exact requirements depend on the slab's purpose, thickness, and local building code — always check specific project requirements.
Does this account for overlap between rebar sections?
No — standard rebar comes in fixed lengths (often 20 or 40 feet), so longer runs require overlapping sections, typically by 12-18 inches at each splice. Add extra length for overlaps when placing your final order.