Rebar Spacing Calculator
Calculate rebar spacing, bars per foot, and steel ratio for concrete slabs. Checks ACI code compliance for maximum spacing.
About this calculator
Once a structural engineer determines how much steel a slab needs per foot of width, this calculator converts that requirement into a buildable bar layout. It looks up the actual cross-sectional area of your chosen rebar size (from #3 at 0.11 in² up to #11 at 1.56 in²) and divides it by your required steel area per foot, then multiplies by 12 to get the center-to-center spacing in inches — bigger bars naturally mean wider spacing for the same total steel area, while smaller bars need to go in tighter. From that spacing it back-calculates bars per foot and the actual steel area you'd end up providing, which is useful for checking whether rounding your spacing to a practical increment (like 6" or 8" o.c.) still meets or exceeds the design requirement.
Effective depth is found by subtracting concrete cover and half the bar diameter from the total slab thickness, since rebar sits at its centerline, not its outer edge, and that value feeds directly into the reported steel reinforcement ratio. The calculator's compliance check applies ACI 318's maximum spacing rule for slabs — the lesser of three times slab thickness or 18 inches — flagging when a spacing would be too generous even if it technically satisfies the steel area requirement, since ACI's spacing cap exists to control crack width and ensure load transfer, independent of the raw area calculation. This tool assumes a single layer of reinforcement in one direction; a real slab design typically needs both top and bottom mats, and two-way slabs need this check run separately in each direction.
Inputs
ACI 318: one-way slab minimum L/20 (simple) to L/28 (one end continuous); 4–8" typical
ACI 318 Table 20.6.1.3: slabs not exposed 1.5"; footings in soil 3"; exposed to weather 2"
Results
Bar Spacing
7.74 in
≈ 2 credit cards
ACI Code Compliant
1
How to Use This Calculator
- Enter slab thickness (inches) and select rebar size (#3 through #10).
- Set required steel area (in²/ft) from your structural design.
- Review maximum allowable spacing (inches) and actual provided steel area.
- Spacing must not exceed 3× the slab thickness or 18 inches per ACI 318.
How the result changes with Required Steel Area
| Required Steel Area | Bar Spacing | ACI Code Compliant |
|---|---|---|
| 0.16 | 15.48 in | 1 |
| 0.23 | 10.34 in | 1 |
| 0.47 | 5.16 in | 1 |
| 0.78 | 3.1 in | 1 |
What each input means
- Slab Thickness
- Total thickness of the concrete slab. ACI 318 Table 7.3.1.1 specifies minimum slab thicknesses based on span. Typical one-way slabs: 4–8 inches; two-way slabs: 5–12 inches.
- Rebar Size
- Standard rebar size number. #4 and #5 are most common for slabs; #8+ for beams and columns.
- Concrete Cover
- Minimum clear cover over rebar per ACI 318 Table 20.6.1.3. Cast-in-place: slabs 1.5" (not exposed), beams 1.5"–2", columns 1.5", footings in contact with soil 3", exposed to weather 2" (#6 and larger).
- Required Steel Area
- Required area of steel per linear foot of slab width, from structural design calculations.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSlab Thickness = 6, Rebar Size = 4, Concrete Cover = 1.5, Required Steel Area = 0.31 = 4 input(s) provided
- Calculate Bar SpacingBar Spacing7.74 = 7.74
- Calculate ACI Code CompliantACI Code Compliant1 = 1
- Calculate Bars per FootBars per Foot1.55 = 1.55
- Calculate Steel RatioSteel Ratio0.61 = 0.61
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does a smaller rebar size require tighter spacing than a larger one for the same steel area?
Spacing here comes directly from dividing the chosen bar's cross-sectional area by your required steel area per foot, then multiplying by 12. A #4 bar has only 0.20 in² of area versus 1.56 in² for a #11, so to deliver the same total steel area per foot, the #4 bars have to be placed much closer together while the #11 bars can be spread much farther apart — the calculator's rebarAreas lookup table is what drives this trade-off.
What is the ACI 318 maximum spacing rule, and why does it apply even when the steel area is already met?
This calculator caps allowable spacing at the lesser of three times the slab thickness or 18 inches, per ACI 318. That cap exists independently of the steel-area math because widely spaced bars — even if they technically supply enough total steel per foot — leave gaps wide enough for cracks to open unevenly and for load to transfer poorly between bars, so the code enforces a maximum gap regardless of how the raw area calculation comes out.
How is effective depth calculated, and why does the bar diameter matter?
Effective depth is the slab thickness minus the concrete cover minus half the bar diameter, because rebar's structural effectiveness is measured from its centerline, not its outer surface. A larger bar size therefore reduces effective depth slightly more than a smaller one for the same cover and slab thickness, which is why the calculator looks up your specific bar's diameter rather than using a single fixed value.
Does this calculator handle two-way slabs or top-and-bottom reinforcement mats?
No — it models a single layer of reinforcement running in one direction only. A real slab typically needs both a top and bottom mat, and any two-way slab (spanning in two directions) needs this same spacing check run independently for each direction's steel requirement, since the required steel area per foot is usually different in each direction.
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