Rebar Spacing Chart — ACI 318 Minimum & Maximum Spacing Requirements
Rebar spacing in reinforced concrete controls crack width, ensures proper concrete consolidation, and governs the distribution of reinforcement across the section. ACI 318-19 provides both minimum and maximum spacing limits for beams, slabs, columns, and walls.
Minimum Clear Spacing Between Bars (ACI 318-19 Cl 25.2.1)
The minimum clear spacing between parallel nonprestressed reinforcing bars must be the greatest of the following three conditions (ACI 318-19 Section 25.2.1):
| Governing Condition | Minimum Clear Spacing |
|---|---|
| (a) Absolute minimum | 1.0 in (25 mm) |
| (b) Aggregate clearance | 4/3 × nominal max aggregate size |
| (c) Bar diameter | db (one bar diameter) |
Where db = nominal bar diameter. The controlling value is whichever of (a), (b), or (c) is largest.
For bars in separate horizontal layers, the clear spacing between layers must be at least 1.0 in (25 mm) (ACI 318-19 Cl 25.2.2).
Center-to-center spacing note: Because clear spacing must be ≥ db, the minimum center-to-center spacing between same-size bars is 2×db. In practice, with 1.0 in and aggregate controls, c-c spacing is typically 2 in or more for #4–#6 bars.
Why this matters: Too little spacing prevents concrete from flowing between bars (segregation), leaving voids and weak spots in the structure.
Maximum Bar Spacing in Slabs (ACI 318-19 Cl 7.7.2, 8.7.2)
One-Way Slabs and Footings
| Bar Type | Maximum Spacing |
|---|---|
| Primary flexural steel | min(3h, 18 in) |
| Temperature & shrinkage steel | min(5h, 18 in) |
Where h = slab thickness.
Two-Way Slabs
| Bar Type | Maximum Spacing |
|---|---|
| Main reinforcement (both ways) | min(2h, 18 in) |
| Column strip | min(2h, 18 in) |
Maximum Bar Spacing in Beams (ACI 318-19 Cl 9.7.2)
For crack control in beams, the maximum center-to-center spacing of tension reinforcement:
s_max = 15(40,000/fs) - 2.5cc ≤ 12(40,000/fs)
Where:
- fs = steel stress at service load (psi) = 2/3 × fy for most cases
- cc = clear cover to the flexural tension reinforcement (in)
For Grade 60 (fy = 60,000 psi) with typical cover:
- fs = 40,000 psi (2/3 × 60,000)
- s_max = 15(40,000/40,000) - 2.5(1.5) = 11.25 in ≤ 12(1.0) = 12 in
Rebar Spacing Chart — Common Slab Configurations
| Slab Thickness | Bar Size | Spacing | As Provided (in²/ft) |
|---|---|---|---|
| 4 in (100mm) | #4 | 12 in o.c. | 0.20 |
| 4 in (100mm) | #4 | 9 in o.c. | 0.27 |
| 5 in (125mm) | #4 | 12 in o.c. | 0.20 |
| 5 in (125mm) | #5 | 12 in o.c. | 0.31 |
| 6 in (150mm) | #4 | 9 in o.c. | 0.27 |
| 6 in (150mm) | #5 | 9 in o.c. | 0.41 |
| 8 in (200mm) | #5 | 12 in o.c. | 0.31 |
| 8 in (200mm) | #6 | 12 in o.c. | 0.44 |
| 8 in (200mm) | #5 | 9 in o.c. | 0.41 |
| 10 in (250mm) | #6 | 12 in o.c. | 0.44 |
| 10 in (250mm) | #7 | 12 in o.c. | 0.60 |
| 12 in (300mm) | #7 | 9 in o.c. | 0.80 |
| 12 in (300mm) | #8 | 12 in o.c. | 0.79 |
Temperature and Shrinkage Reinforcement (ACI 318-19 Cl 24.4.3)
Minimum As for temperature and shrinkage control:
| Steel Type | Minimum ρ (As/Ag) |
|---|---|
| Grade 40 or 50 deformed bars | 0.0020 |
| Grade 60 deformed bars | 0.0018 |
| Grade 60 welded wire reinforcement | 0.0018 |
| Reinforcement with fy > 60,000 psi | 0.0018 × 60,000/fy (min 0.0014) |
Example: 6 in slab, Grade 60 bars As,min = 0.0018 × 6 × 12 = 0.130 in²/ft → Use #3 @ 10 in o.c. (0.132 in²/ft)
Wall Reinforcement Spacing (ACI 318-19 Cl 11.7.2)
For structural walls, vertical and horizontal reinforcement must not exceed:
| Direction | Maximum Spacing |
|---|---|
| Vertical bars | min(3h, 18 in) |
| Horizontal bars | min(3h, 18 in) |
Where h = wall thickness. Walls with factored in-plane shear force exceeding 2√f'c × Acv require two curtains of reinforcement.
Column Spiral and Tie Spacing (ACI 318-19 Cl 25.7)
Lateral Ties
Maximum vertical spacing of ties:
- 16 × longitudinal bar diameter
- 48 × tie bar diameter
- Least column dimension
Spirals
Clear spacing between spiral turns: 1 in to 3 in (25 to 75 mm)
Frequently Asked Questions
What is the minimum spacing between rebar bars? ACI 318-19 Section 25.2.1 requires the clear spacing between parallel bars to be the greatest of: (a) 1.0 inch, (b) 4/3 times the nominal maximum aggregate size, or (c) the bar diameter db. In practice, with 3/4" aggregate (most common), condition (b) gives 1.0" and condition (c) often controls for #5 and larger bars. For #8 bars, minimum clear spacing = 1.0" (the bar diameter).
What is the maximum rebar spacing in a one-way slab? For primary flexural reinforcement in a one-way slab, ACI 318-19 Section 7.7.2.3 limits spacing to the lesser of 3h or 18 inches, where h is the slab thickness. For a 5" slab: max spacing = min(15", 18") = 15". Temperature and shrinkage bars in the transverse direction follow the less restrictive min(5h, 18") = 18" limit for the same slab.
How is beam bar spacing calculated for crack control? ACI 318-19 Section 9.7.2.3 uses the formula: s_max = 15(40,000/fs) − 2.5cc ≤ 12(40,000/fs). For Grade 60 bars with 1.5" clear cover, this gives approximately 11.25" ≤ 12.0" — so 11.25" controls. This limit is based on service-level steel stress, not factored load, and is specifically for crack width control in beams, not slabs.
What is the minimum As for temperature and shrinkage steel? Grade 60 deformed bars require As,min = 0.0018 × b × h per ACI 318-19 Section 24.4.3.2. For a 6" thick slab: As,min = 0.0018 × 12" × 6" = 0.130 in²/ft. Use #3 @ 10" o.c. (0.132 in²/ft) or #4 @ 18" o.c. (0.133 in²/ft). Maximum spacing for T&S steel = min(5h, 18") = 18" for a 6" slab.
What are the tie spacing rules for columns? ACI 318-19 Section 25.7.2 limits maximum vertical tie spacing to the least of: 16 × longitudinal bar diameter, 48 × tie bar diameter, or the least column dimension. For a 12" × 16" column with #8 longitudinal bars and #3 ties: 16 × 1.0" = 16", 48 × 0.375" = 18", least dimension = 12" — ties at 12" o.c. maximum. Closer ties are required in seismic zones.
Run This Calculation
→ Concrete Footing Calculator — spread footing bearing, punching shear, and flexural reinforcement checks per ACI 318.
→ Two-Way Slab Calculator — ACI 318 direct design method for two-way slabs with moment distribution and steel area.
→ Punching Shear Calculator — ACI 318 punching shear check at column-slab connections.
Related Reference Tables
- Rebar Size Chart — Bar Diameters, Areas & Weights
- Rebar Development Length — ACI 318-19 ld, ldc, Hook
- Concrete Spread Footing Design — ACI 318
- Anchor Bolt Embedment Depth — ACI 318 Chapter 17
- Rebar size chart
- Base plate and anchors calculator
- Unit converter
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