Rebar Spacing Calculator

About Rebar Spacing Calculator

The Rebar Spacing Calculator finds the center-to-center spacing of reinforcing bars across a slab, wall or beam width — or the number of bars needed to stay within a maximum spacing. It works in inches or millimeters, has one-click US bar sizes (#3–#8), and checks the result against the ACI 318 minimum and maximum spacing limits.

Spacing controls crack width, load distribution and whether concrete can flow between bars. Too wide and the section cracks or is under-reinforced; too tight and the concrete honeycombs around the steel.

How to Use

1Pick a unit: Choose inches or millimeters; values already typed are converted.
2Select a mode: Calculate Spacing from a bar count, or Calculate Number of Bars from a maximum spacing.
3Enter the element: Type the width of the slab, wall or beam and the clear cover on each side.
4Choose the bar: Tap #3–#8 (or a metric size) or type a diameter, and optionally the maximum aggregate size.
5Read the result: See the center-to-center and clear spacing, with any ACI 318 warning.

Calculation Formulas

Calculate Spacing:

Effective Width = Total Width − (2 × Clear Cover) − Bar Diameter

Center-to-Center Spacing = Effective Width ÷ (Number of Bars − 1)

Clear Spacing = Spacing − Bar Diameter

Calculate Number of Bars:

Effective Width = Total Width − (2 × Clear Cover) − Bar Diameter

Number of Bars = ⌈Effective Width ÷ Maximum Spacing⌉ + 1

The count is rounded up: the spacing you enter is treated as a maximum, so the actual spacing always comes out equal to or tighter than it — never wider.

Worked Example (Imperial)

A 48 in wide slab strip, 1½ in clear cover each side, #5 bars at no more than 12 in on center:

Effective width (outer bar centers) = 48 − (2 × 1.5) − 0.625 = 44.375 in
Bars = ⌈44.375 ÷ 12⌉ + 1 = 4 + 1 = 5 bars
Actual spacing = 44.375 ÷ 4 = 11.09 in on center — within the 12 in maximum
Clear spacing = 11.09 − 0.625 = 10.47 in — above the 1 in ACI minimum, below the 18 in slab maximum

ACI 318 Spacing Limits

The calculator checks the two limits that most often govern (ACI 318-19):

Minimum clear spacing between parallel bars in a layer (25.2.1): the greatest of 1 in, the bar diameter, and 4/3 × the maximum aggregate size. With ¾ in aggregate that is 1 in; with 1½ in aggregate it rises to 2 in.
Maximum spacing in slabs: flexural reinforcement in one-way slabs may not exceed the lesser of 3h and 18 in (7.7.2.3); shrinkage and temperature reinforcement the lesser of 5h and 18 in (24.4.3.3), where h is the slab thickness. The calculator flags anything over 18 in; check 3h or 5h for thin slabs.

Beams, columns, walls and footings have further member-specific limits (crack-control spacing, tie spacing, development and splice lengths). Treat these checks as a first pass — the engineer of record and your local code have the final say.

US Rebar Sizes

US bar numbers give the nominal diameter in eighths of an inch — a #5 bar is 5/8 in. Standard ASTM A615 values:

Bar	Diameter (in)	Diameter (mm)	Area (in²)	Weight (lb/ft)
#3	0.375	9.5	0.11	0.376
#4	0.500	12.7	0.20	0.668
#5	0.625	15.9	0.31	1.043
#6	0.750	19.1	0.44	1.502
#7	0.875	22.2	0.60	2.044
#8	1.000	25.4	0.79	2.670

What Clear Cover to Enter

Specified concrete cover for cast-in-place, non-prestressed concrete (ACI 318-19, Table 20.5.1.3.1):

Cast against and permanently exposed to earth (footings)	3 in
Exposed to weather or earth — #6 through #18 bars	2 in
Exposed to weather or earth — #5 bars and smaller	1½ in
Not exposed — slabs, walls and joists, #11 and smaller	¾ in
Not exposed — beams and columns (primary reinforcement, ties, stirrups)	1½ in

Frequently Asked Questions

How do you calculate rebar spacing?

Spacing = (width − 2 × cover − bar diameter) ÷ (number of bars − 1), center to center. For a 36 in wide footing with 3 in cover and five #5 bars (0.625 in): (36 − 6 − 0.625) ÷ 4 = 7.34 in.

What is the minimum clear spacing between bars?

ACI 318 (25.2.1) requires the larger of 1 in, one bar diameter, and 4/3 of the maximum aggregate size. With ¾ in aggregate that is 1 in for bars up to #8.

What is the maximum rebar spacing in a slab?

ACI 318 limits main flexural reinforcement in slabs to the smaller of 3 × slab thickness and 18 in, and shrinkage and temperature steel to the smaller of 5 × thickness and 18 in.

What sizes are #3 to #8 bars?

US bar numbers are the diameter in eighths of an inch: #3 = 0.375 in (10 mm), #4 = 0.5 in (13 mm), #5 = 0.625 in (16 mm), #6 = 0.75 in (19 mm), #8 = 1.0 in (25 mm).

How much concrete cover do I need?

ACI 318 gives 3 in for concrete cast against earth, 1½–2 in for concrete exposed to weather, and ¾ in for slabs not exposed to weather. Eurocode 2 sets cover by exposure class, typically 25–50 mm.