How to Calculate Amount of Rebar

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If you're getting ready to install a concrete slab flooring in your home or on your patio, you'll want to include a rebar grid to reinforce the tensile strength of that slab. In order to do this, you'll need to know how to estimate how much rebar you'll need for the slab.

How to Calculate Amount of Rebar
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How Rebar Is Classified

Rebar carries numbered classifications that are associated with specific diameter measurements. Rebar diameters vary widely and give homeowners and contractors greater flexibility for various projects.

For example, the smallest rebar has a quarter-inch diameter and is labeled #2. This size of rebar is best for light-duty construction. At the other end of the spectrum is #18 rebar, which measures 2.257 inches in diameter. Each integer increase in the classification label represents a corresponding diameter increase of one-eighth inch.

A rebar classification number also indicates weight. Rebar number charts, available online, provide an indication of the diameter and corresponding weight. For example, a #8 rebar, with a diameter of 1 inch, weighs approximately two and two-thirds pounds per foot of length.

Layout of Rebar for Concrete Slabs

To reinforce concrete slabs, you'll need to estimate the needed amount of rebar for your two-dimensional shape. No matter what the specific layout may be, the process is roughly the same.

To demonstrate that process, assume there is a straightforward rectangular area to be measured. This slab will have equal length and equal width.

The basic rebar layout is a grid shape. Designate two of the intersecting sides to track your measurements. Traditionally, contractors designate the longer side as longitude and the shorter one as latitude. Measure your longitude and latitude carefully and note the exact figures.

Next, determine how much space you will need to leave between the rebars in both directions. Since you'll be forming a grid shape with your rebar, your longer bars will run one way, for example, north to south, while the shorter ones will intersect them at right angles, or east to west.

Divide your longitude length measurement by the space you intend to leave between the latitude rebar. Add one rebar to the end number. The result of this calculation tells you how many rebars you'll need for the latitudinal layout.

An example may help clarify how to perform this calculation. Assume your longitude is 15 feet and there will be 14 inches between latitude rebar.

  1. Convert your longitude measurement into inches: 15 x 12 = 180.
  2. Divide your result by the spacing measurement: 180 / 14 = 12.87 (round up to 13)
  3. Add one rebar to your result: 13 + 1 = 14

You'll therefore need 14 lengths of rebar.

To calculate how many pieces of rebar you'll need for the longitudinal placement, the calculation is the same, except you'll switch latitude for longitude throughout the calculation. In other words, begin by measuring the latitude length of your area. Divide that result by the space to be left between longitudinal bars, then add one.

To determine the total linear rebar footage required, multiply the number of rebar you calculated for each side by the length measurement. Add those numbers together for the total number of linear feet of rebar you'll need.

Impact of Building Regulations on Estimated Rebar Need

In some cases, your local municipality's building regulations may require an offset in rebar length. Many such regulatory provisions mandate that rebar must not be placed too close to the edge of form work.

For example, many building codes require a minimum of two inches clearance between rebar and form work. Thus, for a rectangular concrete slab, you'll need to clear a total of four inches off a piece of rebar in order to provide the required two-inch clearance on each end.


Annie Sisk is a freelance writer who lives in upstate New York. She has written extensively for publications and websites in the home repair and decor fields, among other topics. A homebody by nature, Annie particularly enjoys Scandinavian and French Country design, and learning how complicated things are put together.

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