Rebar Calculator

Rebar (reinforcing bar) is embedded in concrete to handle tensile stress, the pulling forces concrete cannot resist on its own. Correct spacing and sizing prevents cracking, spalling, and structural failure.

Spacing formula: Number of Bars = ceiling(Span / Spacing) + 1

The +1 is the part people forget. Bars sit at both edges of the mat as well as at every interval between, the same way a 10-foot fence at 10-foot spacing still needs two posts. Drop the +1 and you are one bar short in each direction.

The ceiling is the other half, and it is the half that gets rounded the wrong way. A spacing figure is a maximum, not a target. Take a 20 ft span at 18 inch spacing: it comes to 13.33 spaces. Round down to 13 and each gap opens up to 18.5 inches, which is past the spec you just asked for. Round up to 14 spaces, 15 bars, and the gaps close to 17.1 inches, which is inside it. Steel is the cheap part of a slab, so err toward the extra bar every time.

The calculator lays out the mat across the full slab dimension. Concrete cover, the margin of concrete between the steel and the slab edge, shifts the outermost bars inward by an inch or two but does not change the count at normal spacings.

Weight calculation: Weight (lbs) = Length (ft) × Unit Weight (lbs/ft)

Rebar size and weight reference:

Worked example, a 20 × 30 ft driveway:

• Rebar: #4 bars at 18 inch spacing (1.5 ft), both directions
• Bars running the 20 ft direction: ceiling(30 / 1.5) + 1 = 21 bars, each 20 ft = 420 ft
• Bars running the 30 ft direction: ceiling(20 / 1.5) + 1 = 15 bars, each 30 ft = 450 ft
• Total: 36 bars, 870 ft of steel
• Add 10% for laps and cuts: 957 ft, and at 0.668 lbs/ft that is 639 lbs of #4 bar

The 30 ft direction divides evenly and lands on its nominal 18 inch spacing. The 20 ft direction does not, so its 15 bars sit at 17.1 inches instead. That is the ceiling doing its job.

At roughly 20 ft per stick, that is 48 sticks. Order whole sticks, not linear feet, or the yard will do the rounding for you.

Standard spacing guidelines:

• Residential slabs: 18 inch centers maximum
• Driveways: 12 to 18 inch centers, #4 bar minimum
• Foundation walls: 12 inch vertical, 18 inch horizontal
• Concrete cover: 3 inches wherever concrete is cast directly against earth, which includes the bottom of a slab-on-grade. Formed surfaces exposed to weather need 1.5 inches for #5 bar and smaller. Cover is what stops the steel rusting and splitting the slab from inside.