Rebar Spacing and Weight Calculator: How Much Steel Does a Concrete Slab Need?

Rebar is easy to underestimate. You calculate the concrete volume, order the bags or the ready-mix, and then realize you never figured out how much steel you need. Or you estimate it too loosely and end up with half a bundle left over — or worse, short on a pour day when the supplier is two hours away.

This guide walks through the standard method for calculating rebar quantity and weight for the most common concrete applications. For weight calculations on specific bar sizes and lengths, use our .

Rebar designation: what the numbers mean

Rebar is designated by number — #3, #4, #5, and so on. The number represents the diameter in eighths of an inch. A #4 bar is 4/8 = 1/2 inch in diameter. A #5 bar is 5/8 inch.

Bar size	Diameter	Weight (lbs/ft)	Common uses
#3	3/8"	0.376	Driveways, light slabs, ties
#4	1/2"	0.668	Residential slabs, footings, walls
#5	5/8"	1.043	Heavy slabs, columns, structural footings
#6	3/4"	1.502	Commercial, heavy structural
#8	1"	2.670	Columns, heavy structural members

Standard spacing for residential slabs

The American Concrete Institute (ACI 318) sets minimum requirements for reinforcement. For typical residential applications:

• 4-inch slab (driveway, patio, sidewalk): #3 or #4 at 18" on center each way, or welded wire mesh (WWM) 6x6 W1.4xW1.4
• 6-inch slab (garage floor, light industrial): #4 at 12-18" on center each way
• Continuous footing: Minimum 2 bars running lengthwise; additional transverse bars per engineer spec
• Spread footing (column pad): #5 at 12" each way is common for residential column footings

Local building codes and soil conditions can require more reinforcement than these minimums. Always check with your local building department and verify your design with a structural engineer for anything beyond a simple residential slab.

How to calculate rebar quantity for a slab

The method depends on the spacing pattern — typically bars run in both directions forming a grid.

Step 1: Calculate the number of bars in each direction

For bars running in the X direction (across the width):

Number of bars = (Length / Spacing) + 1

For bars running in the Y direction (along the length):

Number of bars = (Width / Spacing) + 1

Worked example: 20 x 30 ft driveway, #4 at 18" OC each way

Convert spacing to feet: 18" = 1.5 ft

X-direction bars (run the 30 ft length, spaced across the 20 ft width):

Number = (20 / 1.5) + 1 = 13.3 + 1 = 15 bars (round up)

Length each = 30 ft

Total X length = 15 x 30 = 450 linear feet

Y-direction bars (run the 20 ft width, spaced along the 30 ft length):

Number = (30 / 1.5) + 1 = 20 + 1 = 21 bars

Length each = 20 ft

Total Y length = 21 x 20 = 420 linear feet

Total rebar: 450 + 420 = 870 linear feet

Step 2: Convert to weight

#4 bar weighs 0.668 lbs/ft:

870 ft x 0.668 = 581 lbs of rebar

Add 10-15% for lap splices (where bars overlap at joints):

581 x 1.12 = 651 lbs — order about 660 lbs or 0.33 tons

Lap splice lengths

When rebar runs are shorter than the full dimension of the slab, bars must overlap at a minimum length called the lap splice. ACI 318 requires lap lengths of at least 40 bar diameters for tension splices in Class B conditions, the most common case in slabs and footings.

Bar size	Minimum lap splice (Class B)	Typical rounding
#3	18"	2 ft
#4	24"	2 ft
#5	30"	3 ft
#6	36"	3 ft

Standard rebar comes in 20-ft and 40-ft lengths. For a 30-ft slab, you will need one lap splice per bar. Budget 2 extra feet per bar to account for the overlap.

Concrete cover requirements

Rebar must be positioned with a minimum concrete cover — the distance from the bar surface to the concrete edge. ACI 318 minimums for cast-in-place concrete:

Condition	Minimum cover
Slabs and walls not exposed to weather	3/4"
Slabs exposed to weather (#5 and smaller)	1-1/2"
Footings cast against earth	3"
Columns and beams	1-1/2" to 2"

Use plastic bar chairs or rebar supports to maintain cover. Rebar resting directly on the soil provides no corrosion protection and does not meet code.

Worked example: continuous footing

A continuous perimeter footing: 120 linear feet, 12" wide x 18" deep, with 2 #5 bars running lengthwise and #3 ties at 24" OC.

Longitudinal bars (2 x 120 ft = 240 ft of #5):

240 ft x 1.043 lbs/ft = 250 lbs

Transverse ties (#3 at 24" = 2 ft spacing):

Number of ties = 120 / 2 = 60 ties

Length per tie = perimeter of 12" x 18" cross section with hooks = approximately 5 ft

60 x 5 = 300 ft of #3 x 0.376 lbs/ft = 113 lbs

Total: 363 lbs, plus 12% for splices = ~407 lbs

Use the  for any bar size and length combination, or the concrete calculator to estimate the volume of your pour alongside the rebar quantity.