FC

Rebar Calculator

Get bar counts, total linear footage, and lap splices for slabs, footings, and walls. Enter your area or run, pick a spacing, and the calculator lays out the grid the way it actually gets tied in the field.

Rebar Calculator

The live calculator embeds here. Enter your dimensions to get cubic yards, bag counts, and cost in seconds.

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The basics

How the calculator works

  • 1

    Enter the area or run

    Give it the slab dimensions or the footing length. The calculator figures the grid in both directions for a mat, or a single line for a footing.

  • 2

    Pick the on-center spacing

    Type the spacing off the print — 12", 16", or 18" are common. It divides the dimension by the spacing and adds one for the starting bar.

  • 3

    Set the lap length

    Any run longer than a 20 ft stock bar needs a lap splice. The default is 40 bar diameters; override it if the engineer specifies otherwise.

  • 4

    Total the footage

    Bars × length in both directions, plus laps, gives total linear feet. Convert to weight if your supplier sells by the ton.

Show the math

The formula

Bars per direction = (Dimension ÷ Spacing) + 1

Rebar is priced by the linear foot or the ton but installed on a grid. Divide each slab dimension by the on-center spacing in feet, add one for the starting bar, then multiply the bar count by the run length in both directions. Add lap splices for any run over the 20 ft stock length.

Worked example

A 20 ft × 20 ft slab with a single mat of #4 bar at 16 inches on center each way.

  1. 1.16 in ÷ 12 = 1.33 ft spacing
  2. 2.20 ft ÷ 1.33 + 1 = 16 bars each direction
  3. 3.16 bars × 20 ft = 320 ft per direction
  4. 4.320 × 2 directions = 640 ft
  5. 5.Add laps and waste ≈ 700 ft (35 sticks of 20 ft)

Order about 35 sticks of 20 ft #4 rebar.

20+ years in the field

Real contractor tips

Remember the stock length

Standard rebar is 20 feet. Any run longer than that needs a lap splice, and those laps add up fast across a big mat — never order off raw run length alone.

Defer to the engineer on laps

The 40-bar-diameter rule of thumb is a starting point. If the drawings call out a specific lap length or class, use theirs — it is what gets inspected.

Do not forget ties and chairs

Tie wire and chairs are cheap and easy to leave off the order. Add them by the roll or box so the bar shows up with everything needed to set it.

Round up to full sticks

You buy whole 20 ft sticks, not partial. Round your total footage up to the next full stick count so the crew is not splicing scraps.

Avoid these

Common mistakes contractors make

  • Forgetting to add one bar for the starting edge of the grid.
  • Ignoring lap splices on runs longer than 20 feet.
  • Using inch spacing without converting to feet.
  • Ordering by eyeballing the plan instead of counting the grid.
  • Leaving tie wire and chairs off the material order.
  • Counting only one direction on a two-way mat.

Versus typical concrete calculators

  • Counts bars on a grid instead of just totaling footage.
  • Adds lap splices for runs over the stock length automatically.
  • Works for slabs, footings, and walls in one tool.
  • Converts on-center spacing to bar counts for you.
  • Saves the rebar takeoff alongside the concrete for the same job.

Versus heavy takeoff software

  • No desktop install or training to lay out a mat.
  • Runs on your phone right at the rebar pile.
  • Free to start with no per-seat pricing.
  • Focused on concrete and rebar, not a full suite.
  • Open it and order today.

Frequently asked questions

Standard stock length is 20 feet. It also comes in 40 and 60 ft for big jobs, but residential yards almost always carry 20 ft. Any run longer than that needs a lap splice.

Stop guessing yardage on the back of a napkin

Run your next pour through the calculator, save the job, and keep every area in one place. Free to start.

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