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Rebar Calculator

Updated How we build and test

Enter the slab size, the bar spacing and the clearance at the edges. The calculator lays out a rebar grid in both directions and gives the number of bars, total linear feet, sticks to buy (with lap splices where a bar is longer than one stick) and the weight.

in o.c.
in

Distance from the slab edge to the first bar.

ft
in

Set by your engineer or plans. Used only where a bar is longer than one stick.

Linear feet of rebar

Enter the slab size and bar spacing.

Rebar grid formula

Bars in one direction = (width in inches − 2 × clearance) ÷ spacing, rounded down, + 1
Bar length = slab length − 2 × clearance
Pieces per bar = 1 if the bar fits on one stick, otherwise (length − lap) ÷ (stick − lap), rounded up
Weight = linear feet × unit weight (#3 = 0.376, #4 = 0.668, #5 = 1.043 lb per ft)

Worked example: a 20 × 20 ft slab, #4 at 16 in

  1. Bars each way: (240 − 6) ÷ 16 = 14.6, rounded down to 14, + 1 = 15.
  2. Each bar is 20 ft − 6 in = 19.5 ft, so it fits on one 20 ft stick.
  3. Total: 30 bars, 585 ft, 30 sticks.
  4. Weight: 585 × 0.668 = 390.8 lb.

Lap splice length

The Concrete Reinforcing Steel Institute explains that lap length varies with concrete strength, rebar grade, bar size and spacing, and that ACI 318 leaves it to the engineer to specify. Use the lap shown on your plans. The calculator only adds laps where a single bar is longer than one stick.

Tips

  • Support the grid on chairs or dobies so it sits in the slab, not on the ground.
  • Bars come in 20 ft lengths at most suppliers; some also stock 10 ft and 40 ft.
  • Bar size, spacing and whether a slab needs rebar at all depend on the design; follow your plans or local code.

Questions

How much rebar for a 10 × 10 slab?

At 16 in spacing with 3 in clearance, 16 bars and 152 ft of rebar.

How do I convert linear feet to weight?

Multiply by the unit weight: 0.668 lb per foot for #4 bar.

What does 16 in o.c. mean?

On center: 16 in from the middle of one bar to the middle of the next.

Can I use this for a footing or pad?

Yes. Enter the footing or pad length and width; the result lists the cut length of each bar, which is the length minus the clearance at both ends. For the concrete volume, use the concrete calculator.

Sources

  1. Lap Splices, Concrete Reinforcing Steel Institute.
    Used for: Lap length depends on concrete strength, grade, size and spacing and is specified by the engineer.
  2. NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
    Used for: Inch and foot definitions.

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