How to measure for rebar
Measure the length and width of the slab or footing in feet, then pick a grid spacing in inches. The calculator lays out a two-way grid of bars, counts the total linear feet of steel, converts that to 20-foot sticks, and estimates the tie points and tie wire you’ll need. Enter your numbers above to get quantities and an estimated material cost.
Recommended grid spacing
- Patios & light slabs: 16–18 inches on center
- Driveways & garage floors: 12–16 inches on center
- Footings & heavy slabs: 12 inches or tighter, per your engineer
Tighter spacing means more steel and more crack control. When in doubt, follow the spacing called out on your plans or by a local engineer — these are general guidelines, not a structural specification.
How the math works
Rebar in a slab forms a grid: one set of bars runs the length, another runs the width, and they cross at regular intervals.
The bars + 1 rule
A common mistake is dividing the slab width by the spacing and stopping there. That counts the gaps, not the bars. A 20-foot run at 12-inch spacing has 20 gaps but 21 bars, because you need a bar at both edges. The calculator adds that extra bar in each direction automatically:
- Bars (width-way) = floor(width × 12 ÷ spacing) + 1
- Bars (length-way) = floor(length × 12 ÷ spacing) + 1
- Total rebar = (width-way bars × length) + (length-way bars × width)
Sticks and overlap
Rebar is sold in sticks — most commonly 20-foot lengths of #4 (½-inch) bar. The calculator divides total linear feet by 20 and rounds up, since you can’t buy a partial stick.
Long runs need lap splices: where one bar ends, the next overlaps it. A typical lap is about 40 times the bar diameter — roughly 20 inches for #4 bar. The Overlap & waste field bumps the linear footage to cover those laps plus cuts and offcuts; 10% is a reasonable starting point.
Tie points and wire
Every place two bars cross is a tie point, and intersections are tied with wire to hold the grid together during the pour. The calculator multiplies the two bar counts to get tie points, then estimates rolls of tie wire at about 1.5 feet of wire per tie.
Estimating cost
The figure shown covers material only — sticks of rebar plus tie wire — at national-average prices adjusted by region. It does not include chairs or supports, delivery, labor, or equipment. Edit the unit price to match a local supplier quote, and remember that bar size and grade (Grade 40 vs. Grade 60) change the price per stick.