Rebar spacing chart
Bar size and spacing by application, the ACI maximums that actually govern, and a quick bar count for your slab.
Bar count calculator
Spacing by application
| Application | Bar | Spacing (each way) | Notes |
|---|---|---|---|
| Patio / shed slab, 4 in | #3 | 16–18 in | Mesh is often substituted |
| Driveway, 5–6 in | #4 | 12–18 in | Two mats for heavy vehicles |
| Garage slab, 4–6 in | #4 | 12 in | Chairs at 4 ft grid |
| Basement slab | #4 | 12–18 in | Vapor barrier below |
| Strip footing | 2–3 × #4 | Continuous | 3 in cover, lapped corners |
| Concrete wall | #4 | 12 in | Two curtains for structural walls |
The two limits are different rules, and they get confused
ACI 318 sets two separate maximum spacings, and almost every chart online quotes one while describing the other.
- Primary flexural reinforcement — the steel carrying bending — is limited to the lesser of three times the slab thickness and 18 inches.
- Shrinkage and temperature reinforcement — the steel whose only job is to hold cracks closed — is limited to the lesser of five times the thickness and 18 inches.
On a residential slab on grade it is the second rule that usually applies, and because five times any normal slab thickness exceeds 18 inches, the 18-inch cap is what actually binds:
| Slab thickness | 3h flexural limit | 5h crack-control limit | Which binds |
|---|---|---|---|
| 3 in | 9 in | 15 in | Flexural |
| 4 in | 12 in | 18 in | Flexural |
| 5 in | 15 in | 18 in | Flexural |
| 6 in | 18 in | 18 in | Both |
| 8 in | 18 in (capped) | 18 in (capped) | 18-inch cap |
The steel area binds before either spacing does
Spacing is only half the requirement. ACI also sets a minimum area of crack-control steel: 0.0018 times the gross concrete area for Grade 60 deformed bars. On a 4-inch slab that is 0.086 square inches per foot of width, and several common arrangements do not reach it:
| Reinforcement | Area provided (in²/ft) | Against 0.0018 on a 4 in slab |
|---|---|---|
| 6 × 6 - W2.9 × W2.9 mesh | 0.058 | Fails — short even for a 3 in slab |
| #3 at 18 in | 0.073 | Fails |
| #3 at 12 in | 0.110 | Passes |
| #4 at 18 in | 0.133 | Passes |
This is why the welded wire mesh that gets rolled into most residential slabs is close to decorative. At 0.058 square inches per foot it does not meet the temperature-steel minimum for any slab thickness, and because it is light it usually ends up trampled to the bottom of the pour, which is where it helps least.
Worth keeping in perspective: a residential patio is not a structural slab, and the IRC does not require reinforcement in a slab on grade at all. So this is a crack-control decision rather than a code minimum. But if crack control is the reason you are adding steel, #3 at 18 inches is thinner than what ACI would call minimum temperature steel — the chart above describes field practice, not the code floor.
Placement rules
- Cover: 3 in where concrete is cast against earth, 1½–2 in for interior slabs.
- Position: the mat belongs at mid-depth. Rebar on the ground does nothing for crack control.
- Chairs: one every 4 ft each way, plus at edges.
- Laps: 40 bar diameters (20 in for #4), staggered rather than stacked.
When the plan and this chart disagree
An engineered drawing always wins. The chart here describes ordinary residential work, and the moment a plan specifies bar size, spacing, or a mat layout, those numbers came from a load calculation that knows things this page does not: soil pressure, span, superimposed load, and the consequences of a crack in that particular slab.
The chart is useful in exactly two situations. Before a plan exists, when you are sizing a shed pad or a patio and want a defensible default. And after the fact, when you want to know whether what was installed is in the normal range for residential work. It is not a substitute for a design, and a contractor who treats it as one is guessing with your money.
Where the two commonly diverge: garage slabs carrying a vehicle, slabs with a lift or a heavy bench, slabs over expansive clay, and anything with a structural wall sitting on it. All four push spacing down and bar size up, sometimes past what the 18-inch cap allows.
Walls and footings answer a different question
The 5h and 3h limits are slab rules. Vertical and horizontal steel in a wall, and longitudinal steel in a footing, are governed by their own provisions, and the numbers look different enough that a slab chart applied to them will mislead.
- Strip footings typically carry two or three continuous bars with 3 inches of cover. Spacing between them is set by the footing width and the design, not by a spacing table.
- Concrete walls usually get a grid on both faces at 12 inches, with closer spacing near openings and at the top of a retaining wall.
- Footing dowels and column ties follow separate rules again, and they are the ones most often missing on a residential drawing.
The common thread is that slabs control cracking across a surface, while walls and footings carry load along a line. Different problem, different steel.
Frequently asked questions
What is the maximum rebar spacing for a slab?
The lesser of three times the thickness or 18 inches — 12 inches for a 4-inch slab.
What size rebar for a 4-inch slab?
#3 or #4 at 12–18 inches on center. Use #4 at 12 inches for driveways.
How far apart in a footing?
Footings usually use two or three continuous bars with 3 inches of cover; spacing is set by the design.
Is the maximum spacing 3 times the thickness or 5 times?
Both rules exist. Flexural reinforcement is limited to the lesser of 3h and 18 inches; shrinkage and temperature reinforcement to the lesser of 5h and 18 inches. On a slab on grade the 18-inch cap usually binds.
Does welded wire mesh meet the minimum steel area?
Usually not. 6 x 6 - W2.9 x W2.9 provides 0.058 sq in per foot, below the 0.0018 ratio even for a 3-inch slab. #3 at 12 inches provides 0.110 and clears it.
Does a residential patio legally need rebar?
The IRC does not require reinforcement in a slab on grade. Adding steel is a crack-control decision, and if you are paying for it you should get enough to actually control cracking.
What is the minimum steel for crack control?
0.0018 times the gross concrete area for Grade 60 deformed bars — 0.086 square inches per foot on a 4-inch slab.
Note: Spacing here follows standard residential practice and ACI maximums. Engineered plans control where they exist.
Citing this page? FigureMill, “Rebar spacing chart” (last reviewed October 30, 2026). Maximum bar spacing for flexural reinforcement under ACI 318-19 §7.7.2.3 and for shrinkage and temperature reinforcement under §24.4.3.3, side by side, with the minimum reinforcement ratio that often controls before spacing does. https://figuremill.com/materials/rebar-spacing-chart
Related: Rebar calculator · Concrete calculator · All material calculators