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ACI 318 spacing · Updated 2026

Rebar Spacing Calculator: ACI 318 Compliant Spacing, Bar Count & Steel Area

Calculate rebar spacing for concrete slabs, walls, footings, and column ties. Use dimensions to get a recommended spacing and bar count, or enter required steel area to reverse-calculate exact spacing by bar size.

Mode A - Dimensions to spacing

Enter slab size, wall height, footing depth, or column tie data to get ACI-compliant spacing, count, cost, and layout.

Mode B - Steel area to spacing

Enter required As in in2/ft and compare #3 through #8 spacing options side by side.

Rebar Spacing Quick Reference - 2026

Standard residential spacing that stays inside the common ACI 318 maximum spacing rule.

Application	Bar size	Spacing (o.c.)
4 in slab	#4	12 in max (ACI 318 §7.7.2.3)
5 in slab	#4	15 in max
6 in slab	#4 or #5	18 in max
Wall (8 in)	#4 or #5	18 in max (ACI 318 §11.7.3)
Footing	#5	12-18 in typical
Column ties	#3	8-12 in (ACI 318 §10.7.6)

Memory rule: max spacing = min(3 x thickness, 18 in). A 4 in slab gives 12 in; a 6 in slab gives 18 in.

Member type

Concrete reinforcement calculator inputs

Length

Width

ThicknessBar sizeConcrete strengthExposureLoad typeWaste factor

Live layout preview

20 ft x 20 ft | #4 @ 12 in

Recommended

#4 @ 12 in o.c.

ACI max spacing: 12 in.

Bar count

47 bars

21 one way + 21 the other, including 10% waste.

Steel area

0.200 in2/ft

Required As: 0.086 in2/ft.

Status

ACI compliant

Interior exposure selected.

Five-dimensional output

Panel A - recommended spacing: #4 @ 12 in o.c.; ACI maximum is 12 in, so spacing is within limit.

Panel B - bar count: 21 bars running the length and 21 bars running the width. Total with waste: 47 bars / 925 linear ft.

Panel C - steel area: required 0.0864 in2/ft; provided 0.2000 in2/ft; ratio 2.3x.

Panel D - spacing cost comparison:

Spacing	Bars	Lin ft	Cost	ACI status
#4 @ 6 in	91	1,805	$1,326	Compliant
#4 @ 8 in	69	1,364	$1,002	Compliant
#4 @ 10 in	56	1,100	$808	Compliant
#4 @ 12 in *	47	925	$680	At limit
#4 @ 16 in	36	704	$517	Exceeds limit
#4 @ 18 in	33	660	$485	Exceeds limit
#4 @ 24 in	25	485	$356	Exceeds limit

Panel E - optimization: Current spacing is already the widest standard compliant option for this setup. A stronger option is #5 @ 12 in when higher load reserve is needed.

Rebar Spacing Chart - Bar Count Per 20-Foot Span

Number of bars needed for a 20-foot span at various spacings. Includes one bar at each end and no waste factor.

Bar Size	@ 6 in	@ 8 in	@ 12 in	@ 16 in	@ 18 in	@ 24 in
#3	41	31	21	16	14	11
#4	41	31	21	16	14	11
#5	41	31	21	16	14	11
#6	41	31	21	16	14	11
#8	41	31	21	16	14	11

Bar count is independent of bar size; spacing controls count. Bar size controls weight, cost, and steel area.

Steel Area Chart - As Provided in in2/ft

Use this matrix as a fast steel area calculator for concrete reinforcement.

Bar Size	@ 6 in	@ 8 in	@ 12 in	@ 16 in	@ 18 in	@ 24 in
#3 (0.11 in2)	0.220	0.165	0.110	0.083	0.073	0.055
#4 (0.20 in2)	0.400	0.300	0.200	0.150	0.133	0.100
#5 (0.31 in2)	0.620	0.465	0.310	0.233	0.207	0.155
#6 (0.44 in2)	0.880	0.660	0.440	0.330	0.293	0.220
#8 (0.79 in2)	1.580	1.185	0.790	0.593	0.527	0.395

ACI 318-19 Rebar Spacing Requirements - Complete Reference

Reference values for ACI 318 rebar spacing, minimum steel checks, and constructability limits. Always verify project drawings and local code.

Slabs, Walls, and Footings

Slabs (§7.7.2.3): s_max = min(3h, 18 in).

Walls (§11.7.3.1): vertical and horizontal spacing commonly use min(3h, 18 in), where h is wall thickness.

Footings (§13.3.3.3): s_max = min(3h, 18 in), where h is footing depth.

Thickness	3h	18 in cap	s_max	Rec. bar
3.5 in	10.5 in	18 in	10.5 in	#4
4.0 in	12.0 in	18 in	12.0 in	#4
5.0 in	15.0 in	18 in	15.0 in	#4/#5
6.0 in	18.0 in	18 in	18.0 in	#5
8.0 in	24.0 in	18 in	18.0 in	#5/#6

Columns, Beams, and Clear Spacing

Columns - tie spacing (§10.7.6.1.2): s_max = min(16 x d_long, 48 x d_tie, b_min).

Example: #6 long bars with #3 ties in a 12 in column gives min(12 in, 18 in, 12 in) = 12 in.

Beams - stirrup spacing (§9.7.6.2.2): end zones commonly use d/2 and mid zones use d, based on effective depth.

Minimum clear spacing (§25.8.1): greatest of 1.5 x db, 1.5 in, or 4/3 x aggregate size.

Recommended Rebar Spacing by Project Type

Based on ACI 318-19 spacing checks and common industry practice. Use engineered drawings where provided.

Project	Thickness	Bar Size	Spacing	As Provided	ACI Status	Notes
Residential patio	4 in	#4	12 in o.c.	0.200 in2/ft	Compliant	Standard practice
Residential driveway	4 in	#4	12 in o.c.	0.200 in2/ft	Compliant	Minimum for vehicles
Heavy driveway	5 in	#4	12 in o.c.	0.200 in2/ft	Compliant	RVs, heavy trucks
Garage floor	4 in	#4	12-16 in	0.150-0.200	Compliant	Passenger vehicles
Sidewalk	4 in	#3 or #4	12-18 in	0.073-0.200	Compliant	Pedestrian only
Shed base	3.5 in	#3	12 in o.c.	0.110 in2/ft	Compliant	Light loads
Pool deck	4 in	#4	12 in o.c.	0.200 in2/ft	Compliant	Wet exposure
Basement floor	4 in	#4	12-18 in	0.133-0.200	Compliant	-
Foundation wall	8 in	#5	12 in o.c.	0.310 in2/ft	Compliant	Both faces
Retaining wall	10 in	#5	12 in o.c.	0.310 in2/ft	Compliant	Vertical + horizontal
Column (12 x 12 in)	-	#6 long	-	-	Compliant	4 bars minimum
Column ties	-	#3	8-12 in	-	Compliant	Per ACI §10.7.6

How to Choose Rebar Spacing: A Decision Framework

Check ACI maximum

Use s_max = min(3 x thickness, 18 in). This is the upper limit.

Check minimum As

For Grade 60 slab shrinkage steel, As_min = 0.0018 x 12 x thickness.

Consider load

Light patios can be near minimum; vehicles usually need more reserve.

Choose standard spacing

Prefer 6, 8, 10, 12, 16, or 18 inches. 12 inches is easiest to lay out.

Verify constructability

Check clear spacing, cover, aggregate size, chairs, and lap details.

For quantity-first planning, use the rebar calculator. For light-duty reinforcement comparisons, compare welded fabric with the wire mesh calculator. For concrete volume, start with the concrete slab calculator or the general concrete calculator. Transport checks often need concrete weight.

Rebar Spacing Calculator FAQ

How far apart should rebar be in a concrete slab?+

For a standard 4-inch residential slab, ACI 318 §7.7.2.3 limits maximum spacing to min(3 x 4 in, 18 in) = 12 inches. In practice, #4 rebar at 12 inches on center both ways is the common residential detail for patios, driveways, and garage floors.

What is the minimum rebar spacing per ACI 318?+

ACI 318 §25.8.1 sets minimum clear spacing between parallel bars as the greatest of 1.5 x bar diameter, 1.5 inches, or 4/3 x maximum aggregate size. For #4 bars, the practical center-to-center minimum is about 2 inches, though residential layouts usually start at 6 inches for constructability.

What spacing should I use for rebar in a concrete driveway?+

For passenger vehicles, #4 rebar at 12 inches on center both directions in a 4-inch slab is the common starting point. Heavy vehicles usually justify a 5-6 inch slab, #4 or #5 bars, and engineering review.

How do I calculate rebar spacing from required steel area?+

Use spacing in inches = bar area in square inches / required As in square inches per foot x 12. Example: As = 0.20 in2/ft with #4 bars (0.20 in2) gives 12 inches.

Can I use 18-inch rebar spacing for a concrete slab?+

For a 6-inch or thicker slab, ACI 318 allows up to 18 inches using the min(3h, 18 in) limit. For a 4-inch slab, 18 inches exceeds the 12-inch maximum spacing reference.

What is the difference between rebar spacing and rebar on center?+

On center means measured from the center of one bar to the center of the next. Clear spacing means the gap between bar edges. For #4 at 12 inches on center, clear spacing is 11.5 inches.

How does rebar spacing affect concrete strength?+

Rebar spacing affects crack control, tensile capacity, ductility, and load distribution. Concrete compressive strength still comes from the mix. Closer spacing adds steel area, but residential slabs can reach diminishing returns quickly.