Rebar Spacing Calculator
This calculator tells you how many rebar bars you need for a given span and bar spacing. The formula is straightforward: number of bars = floor(span / spacing) + 1, where both span and spacing are in the same units (inches). The first bar goes at one end, the last at the other end, and the remainder fill the space between. This tool also calculates total linear feet of rebar needed. For slabs, run the calculator once for each grid direction. Per ACI 318, maximum temperature and shrinkage reinforcement spacing is 18 inches for slabs on grade, though structural requirements may call for closer spacing.
Rebar quantity formula
Bars = floor(Span (in) / Spacing (in)) + 1
Total linear feet = Bars x Span (ft)
Stock bars = ceil(Total linear feet / Bar stock length)
The first bar is placed at one edge of the slab and the last bar at the other edge. Additional bars fill between them at the specified spacing. If a bar spans the full slab length, multiply bar count by slab length to get total linear footage, then divide by stock length to find how many full bars to purchase.
ACI rebar spacing guidelines
- Temperature and shrinkage reinforcement maximum spacing: 18 inches (ACI 318 Section 7.7.6).
- Minimum concrete cover for rebar: 1.5 inches for slabs not exposed to weather (ACI 318 Section 20.6.1).
- Minimum bar spacing: the greater of bar diameter, 4/3 of maximum aggregate size, or 1 inch (ACI 318 Section 25.2).
- For seismic zones, special detailing requirements apply per ACI 318 Chapter 18.
Rebar spacing calculator: frequently asked questions
How do I calculate the number of rebar bars needed?
Use the formula: Bars = floor(Length / Spacing) + 1. For example, a 10-foot span with bars every 12 inches needs floor(120/12) + 1 = 11 bars.
What is standard rebar spacing for a concrete slab?
Per ACI 318, typical spacing for temperature and shrinkage reinforcement in residential slabs is 18 inches on center maximum. Structural slabs often use 12 inches on center or less depending on design loads.
How much overlap (lap splice) do I add for rebar?
The standard lap splice length per ACI 318 is typically 40 to 60 times the bar diameter. For #4 rebar (0.5-inch diameter), that is 20 to 30 inches. Add lap splice length when ordering extra rebar.
What size rebar is used in residential construction?
#3 (3/8 inch) and #4 (1/2 inch) are the most common sizes for residential slabs, footings, and walls. Larger structural members use #5, #6, or bigger.
Do I need rebar in both directions?
Yes. For slabs, rebar is placed in a grid pattern with bars in both the longitudinal and transverse directions. Run this calculator once for each direction and add the totals.
Official sources
- American Concrete Institute: ACI 318 Building Code Requirements for Structural Concrete.
- International Code Council: International Building Code (IBC).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.