For most concrete slabs, put rebar 12 to 18 inches on center in both directions. A 4 inch patio slab needs about 15 inches with #3 bar, and a 5 or 6 inch driveway can use #4 bar at the 18 inch maximum. The calculator below checks the spacing against the ACI 318 minimum and counts the bars and pounds you need.
| 4 in slab | #3 at 15 in · #4 at 18 in |
|---|---|
| 5 in slab | #3 at 12 in · #4 at 18 in |
| 6 in slab | #3 at 10 in · #4 at 18 in |
| 8 in slab | #4 at 13 in · #5 at 18 in |
Rebar spacing chart by slab thickness
This chart gives the widest spacing that still meets the ACI 318-19 minimum for shrinkage and temperature steel, with the 18 inch cap. Round down to a whole inch when you lay out the grid.
| Slab thickness | Steel needed (in²/ft) | #3 bar | #4 bar | #5 bar |
|---|---|---|---|---|
| 4 in | 0.086 | 15 in | 18 in | 18 in |
| 5 in | 0.108 | 12 in | 18 in | 18 in |
| 6 in | 0.130 | 10 in | 18 in | 18 in |
| 8 in | 0.173 | 7 in | 13 in | 18 in |
Values are for Grade 60 bar (0.0018 × thickness × 12). For Grade 40 or 50 bar the ratio is 0.0020, so spacing gets about 10 percent tighter.
Typical rebar spacing by project
These are common field choices for ground-supported slabs. They are practice, not code, so use them as a starting point.
| Project | Slab thickness | Common rebar layout |
|---|---|---|
| Walkway or patio | 4 in | #3 at 18 to 24 in each way, or fiber / mesh |
| Shed or small garage slab | 4 to 5 in | #3 or #4 at 16 to 18 in each way |
| Driveway (cars) | 4 to 5 in | #3 at 12 to 18 in, or #4 at 18 in |
| Driveway (trucks, RV) | 6 in | #4 at 12 to 18 in each way |
| Garage floor | 5 to 6 in | #4 at 18 in each way |
| Basement floor | 4 in | #3 at 18 in, or mesh |
| Pool deck | 4 to 5 in | #3 at 12 to 18 in each way |
How the minimum steel is worked out
ACI 318-19 section 24.4.3.2 sets the minimum shrinkage and temperature steel at 0.0018 × gross concrete area for Grade 60 bar and 0.0020 for Grade 40 or 50. Section 24.4.3.3 caps the spacing at the smaller of 5 × slab thickness or 18 inches. A ground-supported slab is not always designed to this code, but it is the most careful way to check a layout.
- Steel needed per foot of width: As = ratio × 12 × thickness.
- Bar area: #3 = 0.11 in², #4 = 0.20 in², #5 = 0.31 in².
- Spacing: s = bar area × 12 ÷ As, rounded down, and never over 18 in.
Worked example: a 20 × 20 ft, 5 in driveway
- As = 0.0018 × 12 × 5 = 0.108 in²/ft.
- #4 bar: s = 0.20 × 12 ÷ 0.108 = 22 in, so the 18 in cap governs. Use #4 at 18 in each way.
- Keep bars 3 in from the edges, so each bar is 19.5 ft long and each direction has 14 bars.
- Total = 28 bars, 546 ft, about 365 lb (28 bars of 20 ft).
Need the weight of other sizes? See the rebar weight chart. For the concrete itself, use the cubic yard calculator.
Where the rebar goes inside the slab
- Height: in the middle or upper third of the slab, not on the ground. Use plastic or concrete chairs every 3 to 4 feet.
- Cover: ACI 318 asks for 3 in when concrete is cast against soil. Keep 2 to 3 in clear at the edges.
- Laps: overlap bars about 40 bar diameters, which is 15 in for #3 and 20 in for #4. Tie them with wire.
- Joints: stop bars 2 in short of control joints, or let them run through if your engineer asks.
- Base: compact 4 in of gravel first. A good base matters as much as the steel.
Rebar, wire mesh or fiber?
| Option | Good for | Weak point |
|---|---|---|
| Rebar grid | Driveways, garages, heavy loads | Needs chairs and tying labor |
| Welded wire mesh | Light patios and walks | Often ends up at the bottom, where it does little |
| Fiber in the mix | Cutting plastic shrinkage cracks | Does not carry load or hold wide cracks |
Common mistakes
- Leaving rebar on the ground and hoping it gets pulled up. Use chairs.
- Spacing wider than 18 in on a driveway or garage.
- Stopping bars right at the form. Keep the clearance.
- Adding steel to fix a bad base or poor drainage. Steel does not cure that.
- Using rebar to stop cracks. Control joints decide where it cracks; rebar only keeps cracks tight.
FAQ
How far apart should rebar be in a concrete slab?
For most residential slabs, 12 to 18 inches on center each way. A 4 inch patio slab with #3 bar needs about 15 inches under the ACI 318 minimum. A 6 inch driveway with #4 bar can go to the 18 inch maximum. Heavy loads or poor soil need closer spacing from an engineer.
What size rebar should I use for a driveway?
#3 (3/8 in) is common for 4 inch driveways for cars. #4 (1/2 in) at 18 inches on center is the usual choice for 5 to 6 inch driveways, garages and anything that sees trucks.
Is 24 inch rebar spacing enough for a slab?
24 inches is common on light patios and walks, but it is wider than the 18 inch maximum in ACI 318 for structural slabs. For a driveway or garage, use 18 inches or less.
Do I need rebar in a 4 inch concrete slab?
Not by code for most residential slab-on-grade work. Rebar does not stop cracks, but it holds the cracks tight. It is worth adding for driveways, garages, slabs over fill and any slab where cracking would cause damage.
How far should rebar be from the edge of the slab?
Keep bars about 2 to 3 inches inside the edge and sides. Where concrete is cast against soil, ACI 318 requires 3 inches of cover, so keep bars off the ground with chairs.
Rebar or wire mesh: which is better for a slab?
Rebar is stiffer, easier to keep at the right height and holds cracks tighter. Wire mesh is faster to roll out but is often left at the bottom of the slab, where it does very little. Most contractors prefer a rebar grid for driveways and garages.
How much does a 20 by 20 ft rebar grid weigh?
With #4 bar at 18 inches on center each way, a 20 by 20 ft slab needs 28 bars of 20 ft, about 546 ft of steel, and weighs about 365 lb.
References: ACI CODE-318-19, sections 24.4.3.2 and 24.4.3.3 (minimum shrinkage and temperature reinforcement and spacing); ACI 360R, Guide to Design of Slabs-on-Ground; ASTM A615/A615M bar sizes and areas. This guide is general information, not a design. Structural, suspended or heavily loaded slabs need a licensed engineer, and local codes can ask for more.
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