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How to determine the amount & size of rebar in a concrete slab

Updated February 21, 2017

Long, narrow and load-bearing concrete structures are often strengthened with pieces of reinforcing bar, often called rebar. Rebar is necessary for many concrete building projects and recommended for others, but the rebar itself is not sold with instructions on how much and what size to use. Fortunately, those tough decisions have already been made for you by the authors of your state and local building codes. To find out how much rebar and what size to use in your slab, you must find and refer to these documents, which are customised around the unique properties of your area's geography and climate.

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  1. Find your state's building code online. All state building codes are published on websites operated by their respective states. You can find links to all of the state building code websites in the directory linked to in the Resources section of this article.

  2. Find your city or county building codes (county building codes usually apply to construction projects in unincorporated areas). These may be available online as well, particularly if you live in a large city. But since not all local building codes are listed online, the best approach is usually to contact your local city hall or, if you live in an unincorporated area, your county administrator's office. When you call, inquire about who to contact to obtain copies of the local building code. They may give you a different contact to follow up with, so be ready to write a name and number down.

  3. Browse through both sets of building codes to find the concrete and rebar specifications of each. These may be listed under different headings or categories, as building code templates are not standardised across states and local municipalities. Every building code should have a table of contents, however, which should make the concrete building codes easy enough to find.

  4. Compare the two codes, which should contain formulas and requirements that tell you exactly how much and what size of rebar to use in your concrete slab. If they are the same, follow those recommendations. If they are different but not mutually exclusive, choose the larger amount or size of rebar and follow those guidelines. If they're different and mutually exclusive, contact a representative of both your state and local building code divisions. Point out the conflict and ask for further instructions.

  5. Tip

    If your state and local building codes dictate that you do not need any rebar in your slab, it does not mean that you can't use it. However, if rebar would be necessary to maintain the integrity of your slab in your climate and area, it would be in the code.

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