Rebar Calculator

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

The bar count includes calculated offcuts but no extra site margin. For large grids marked as a deterministic estimate, confirm an optimised cutting schedule with the supplier before ordering because the fast estimate can overcount.

Worked example

A 12 ft × 12 ft slab with #4 bar at 12 inches on center and 3 inches of edge cover. Inputs: Slab length 12 ft, Slab width 12 ft, Maximum bar spacing from drawings 12 in O.C., Edge cover 3 in, Rebar size 4 #, Required lap splice (bar diameters) 40. Result: 0 .

Rebar for common slab sizes

Calculated for #4 bar at 12 inches on center, 3 inches of edge cover and 20 ft stock. Tap a size to load it above.

Method & assumptions

Slab rebar is laid as a grid. The entered spacing is a maximum: divide the usable span by it, round the number of intervals up, then add one bar so both ends are present. This keeps the actual spacing at or below the value from the drawings. The usable span is the slab dimension minus twice the entered edge cover.

Bar weight uses published nominal mass per foot for each U.S. bar size and the physical steel installed, including the duplicated length at every lap. Rebar is stocked in fixed lengths, so the bars you buy are counted by a common cutting plan for both directions rather than by dividing total linear feet by stock length or rounding each direction separately. The exact state search is limited to 100,000 combinations. Above that limit, the calculator uses a deterministic longest-first cutting estimate that finishes promptly but can overcount a fully optimised nesting plan; send large schedules to the supplier or a cutting optimiser before ordering.

This is a takeoff for a grid already specified by a structural design. It does not decide whether a slab needs rebar, select bar size or spacing, determine cover, or calculate a code-compliant splice. Development and splice length vary with bar, concrete, coating, confinement, location and splice class. Copy those values from the project drawings before relying on the purchase count.

Pro tips and common mistakes

• Keep the steel off the ground. Rebar laying on the subgrade does almost nothing. Set it on chairs or dobies so it sits in the middle third of the slab, where it actually controls cracking.
• Respect the edge cover. Keep 2 to 3 inches of concrete between the bar and any face. Steel too close to the surface rusts, expands and spalls the concrete off.
• Enter the designed splice. Copy the lap from the structural drawings. A generic number of bar diameters is not a substitute for the required development and splice calculation.
• Tie the intersections. Use the grid ties figure to buy tie wire. Tying every intersection (or every other one) keeps the grid from shifting during the pour.
• Buy the stock length that fits your slab. A 12 foot piece wastes 8 feet of a 20 foot bar but nothing of a 30 foot one. If your yard carries 10, 30 or 40 foot bars, change the stock length and compare the bar count before you order.
• Match the bar to the job. Patios and walkways are fine with #3 to #4 and welded wire mesh. Driveways and structural slabs usually want #4 or #5 on a tighter grid, per code.

Frequently asked questions

Sources and references

These references and structured source records support the method and material data used on this page.

• Rebar (Wikipedia)
• ACI-hosted reinforcing-bar table: nominal diameter and weight per foot
• CRSI Reinforcing Basics

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