Slab Rebar Spacing Calculator
Estimate slab rebar spacing for your project with our free calculator. Get accurate material quantities, costs, and specifications.
Formula
Spacing (in) = (Bar Area / Required As per ft) x 12
Divide the cross-sectional area of one bar by the required steel area per linear foot of slab width, then multiply by 12 to convert to inches. The result is the on-center spacing. This must not exceed 18 inches or 3 times the slab thickness (for flexural steel). Temperature and shrinkage steel minimum is As = 0.0018 x b x h for Grade 60 bars.
Worked Examples
Example 1: 6-inch Slab with #4 Bars
Problem: Find the spacing for #4 bars to provide 0.36 sq in/ft of steel in a 6-inch slab (20 x 20 ft).
Solution: Spacing = (0.20 / 0.36) x 12 = 6.67 in\nMax spacing = min(18, 3x6) = 18 in\nPractical spacing = 6 in o.c.\nActual As = (0.20 / 6) x 12 = 0.40 sq in/ft (OK)
Result: #4 at 6 in o.c., As = 0.40 sq in/ft
Example 2: 4-inch Slab Temperature Steel
Problem: Determine temperature steel for a 4-inch slab using #3 bars.
Solution: Required As = 0.0018 x 12 x 4 = 0.0864 sq in/ft\nSpacing = (0.11 / 0.0864) x 12 = 15.3 in\nMax spacing = min(18, 5x4) = 18 in\nUse #3 at 15 in o.c.
Result: #3 at 15 in o.c. for temperature steel
Frequently Asked Questions
How do you determine rebar spacing in a concrete slab?
Rebar spacing is determined by dividing the cross-sectional area of one bar by the required steel area per linear foot, then multiplying by 12 inches. The result tells you the on-center spacing needed to provide the required reinforcement. This spacing must not exceed 18 inches or 3 times the slab thickness, whichever is less, per ACI 318. Common spacings are 6, 8, 12, and 16 inches on center. The required steel area comes from structural design calculations based on the loading, span, and concrete strength.
What is the minimum reinforcement for a concrete slab?
ACI 318 requires minimum temperature and shrinkage reinforcement in slabs. For Grade 60 deformed bars, the minimum steel ratio is 0.0018 times the gross concrete area (As = 0.0018 x b x h). For a 6-inch thick slab using a 12-inch wide strip, this equals 0.0018 x 12 x 6 = 0.1296 sq in per foot. This can typically be provided by #4 bars at 18-inch spacing (As = 0.133 sq in/ft). This minimum applies in both directions and ensures the slab can resist cracking from shrinkage and temperature changes.
What is the maximum rebar spacing allowed in slabs?
ACI 318 limits the maximum spacing of flexural reinforcement in slabs to the lesser of 18 inches or 3 times the slab thickness. For temperature and shrinkage reinforcement, the maximum spacing is the lesser of 18 inches or 5 times the slab thickness. For example, in a 4-inch slab, the maximum flexural spacing is min(18, 12) = 12 inches, and the maximum temperature steel spacing is min(18, 20) = 18 inches. These limits ensure that reinforcement is distributed closely enough to control cracking.
Should rebar be placed in one layer or two in a slab?
For slabs up to about 6 inches thick, a single layer of rebar placed in the bottom third is typical. The rebar grid runs in both directions within that single layer. For thicker slabs (8 inches and above), two layers of reinforcement may be required: one near the bottom for positive moment and one near the top for negative moment, especially over supports. In two-way slabs, the bottom layer has bars running in the long direction placed first, then the short direction on top, with proper chair support to maintain the correct concrete cover.
What is the correct rebar spacing for concrete slabs?
Standard residential slabs use #3 or #4 rebar on 18-inch centers both ways, placed at mid-depth. Driveways and heavy-load areas use #4 rebar on 12-inch centers. Rebar should have 2-3 inches of concrete cover on the bottom. Wire mesh (6x6 W1.4xW1.4) is an alternative for light-duty slabs.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.