Updated July 18, 2026
Standard rebar sizes and spacing
This calculator assumes standard 20-foot rebar sticks when it turns your linear footage into a bar count. You set your own grid spacing, but if you are not sure what to pick, the table below is the convention most residential jobs land on.
| Use | Typical spacing | Common size |
|---|---|---|
| Residential slab | 18 in on center | #3 or #4 |
| Patio or sidewalk | 16 to 18 in on center | #3 |
| Driveway | 12 to 16 in on center | #4 |
| Footing or structural slab | per engineer | #4 or #5 |
A #4 bar is half an inch in diameter and weighs 0.668 pounds per foot, so 430 linear feet of it is roughly 287 pounds of steel. Residential grids commonly run 12 to 24 inches on center, with 18 inches a frequent choice for slabs and tighter 12 to 16 inch spacing under driveways that carry vehicles.
Sources: Inch Calculator rebar data, Omni rebar reference.
How it works
The calculator places bars across the width at the specified spacing, plus one bar at each edge. Bars run lengthwise are spaced across the slab width the same way. Total linear feet equals bars running along the length (each as long as the slab length) plus bars running across (each as long as the slab width). Divide by 20 and round up to get the number of standard 20-foot rebar sticks.
The spacing and bar size numbers above are checked against rebar manufacturer data and construction references, the same standard we hold every calculator on this site to; details are on the editorial standards page.
The math in one line
Rebar is a grid, so you count bars in each direction and add their lengths. Pick a spacing, lay bars that way across the width, lay another set across the length, and the two sets cross to form the mat. Standard rebar comes in 20-foot lengths, so the last step is dividing total linear feet by 20 and rounding up.
Tips from the field
Lap splices where bars meet, overlapping by about 40 bar diameters so the steel acts as one continuous run, and account for that lap when you buy. Keep the mat in the lower third of the slab, around 1.5 to 2 inches off the ground for a 4-inch pour, on chairs or dobies rather than pulled up by hand during the pour. Tie the intersections so the grid does not shift when concrete hits it. Order a stick or two extra; offcuts at the edges rarely line up perfectly with 20-foot lengths.
A worked example
Take a 20 by 10 foot slab on a 12-inch grid. Across the 10-foot width: 10 feet is 120 inches, divided by 12-inch spacing is 10, plus one bar for the far edge is 11 bars running the length. Across the 20-foot length: 240 inches divided by 12 is 20, plus one is 21 bars running the width. Linear feet is (11 bars x 20 feet) plus (21 bars x 10 feet), which is 220 plus 210, or 430 linear feet. Divide by 20 and round up: 22 sticks. The calculator does this as you type.