Rebar Spacing Calculator — Bars Per Slab
Determine rebar bar count for any slab dimension. Supports 6, 8, 10, 12, 16, and 18-inch on-center spacing for #3 through #8 bar sizes per ACI 318.
Enter Dimensions
Add 10% for standard cuts and overlaps
Results
Bars Needed
38bars
Total Lineal Feet
608lin ft
Total rebar length including waste
Total Weight
406lb
#4 rebar weight per CRSI
Slab Area
240sq ft
Editorial Standards
Each calculator is reviewed for formula accuracy, unit consistency, and alignment with current U.S. building practices before publication. We verify outputs against published engineering references and real-world project data. Learn more about our methodology.
Rebar spacing is the single biggest predictor of how much steel you'll buy for a slab — wider spacing means fewer bars, but it has to satisfy ACI 318 maximum spacing rules and deliver enough steel area to control shrinkage cracking and meet structural demand.
This calculator translates spacing into piece counts and total linear feet for both directions of a slab. Use it to compare 12-inch versus 18-inch grids before ordering.
How to Plan Rebar Spacing
Bars one direction = ⌈Dimension ÷ Spacing⌉ + 1. Total linear feet = (X-direction bars × X-length) + (Y-direction bars × Y-width).
Common Slab Spacing Reference
- 6″ OC #4: heavy commercial slabs, ramps, areas with surcharge loading.
- 10–12″ OC #4: residential slabs on grade, garages, light commercial floors.
- 16–18″ OC #4: patios, light-duty pads, sheds (where code permits).
- 10–12″ OC #5: footings, foundation walls, structural slabs.
- Maximum (ACI 318): 3× slab thickness or 18 inches, whichever is less.
Frequently Asked Questions
What is the standard rebar spacing for a concrete slab?
12-inch on-center grid (#4 bars) is typical for residential 4-inch slabs. 18-inch grid is used for lightly loaded slabs (slabs on grade, sheds). Heavier or structural slabs use 6-, 8-, or 10-inch spacing per engineered design.
Can I use 24 inch rebar spacing?
ACI 318 sets a maximum spacing of 18 inches for primary slab reinforcement, or 3× the slab thickness, whichever is less. 24-inch spacing is non-compliant for most slab applications.
Does rebar size affect spacing?
Yes — larger bars (#5, #6) can be spaced farther apart while delivering the same steel area per square foot. A 12″ grid of #4 (≈ 0.20 in²/ft) is approximately equivalent to an 18″ grid of #5 (≈ 0.21 in²/ft).
How do I lay out rebar spacing on a slab?
Snap chalk lines on the prepped subgrade at your specified spacing in both directions. Place bars with concrete cover (typically 2 inches from soil side, 1.5 inches from slab top) using rebar chairs or precast blocks.
How much rebar do I need per square foot at 12 inch spacing?
12-inch grid (both directions) needs ~2 linear feet of rebar per square foot of slab. A 400 sq ft slab needs ~800 linear feet of rebar — roughly 40 pieces of 20 ft #4.
Looking for the general calculator?
Calculate rebar quantity, spacing, and total weight for concrete slabs, footings, and walls. Covers #3 through #8 bar sizes using CRSI published weight tables.
Open the Rebar Calculator: Bar Count & Weight →