Rebar Size Numbering Explained (#3, #4, #5)

What does the number on a piece of rebar tell you? For the small bars it is a fraction in disguise, and for the large bars it is a size code. This guide explains the numbering and then gives the full rebar size chart, with diameter, area and weight per foot.

What Does the Number on Rebar Mean?

For bars up to and including #8, the number is the nominal diameter in eighths of an inch. The WSDOT Bridge Design Manual states this rule, and the published tables agree with it.

So you can read a size in your head. A #3 is 3/8 in (0.375 in), a #4 is 4/8 in, or 1/2 in (0.500 in), and a #5 is 5/8 in (0.625 in). A #6 is 0.750 in, a #7 is 0.875 in, and a #8 is 1.000 in. These are nominal dimensions of a round section, which is how the tables list them.

WSDOT says bars are referred to by number, running from #3 to #18. To turn a bar size into pounds for a whole order, use the rebar weight calculator.

Which Bar Numbers Break the Eighths Rule?

Five sizes do: #9, #10, #11, #14 and #18. WSDOT says the diameters of #9, #10 and #11 are set so the bar areas match square bars. The published tables list #14 and #18 with diameters that also differ from eighths.

The gap is small for #9. Pure eighths would give 9/8 = 1.125 in, but the published diameter is 1.128 in, a difference of 0.003 in. For #11, 11/8 = 1.375 in, against 1.410 in published, a difference of 0.035 in. A #11 bar is about 1.41 in across, according to the Caltrans manual.

Our own check on the areas: the published 1.00, 1.27 and 1.56 sq in for #9, #10 and #11 fit squares that measure 1 in, 1 1/8 in and 1 1/4 in on a side. The #14 area of 2.25 sq in is 1.5 x 1.5, and the #18 area of 4.00 sq in is 2 x 2.

Rebar Size Chart From #3 to #18

The chart below lists every size in the Caltrans bar chart: #3 to #11, then #14 and #18. It has no #12, #13, #15, #16 or #17.

These are published nominal values. The Caltrans chart reproduces a Concrete Reinforcing Steel Institute (CRSI) chart dated May 2003, and a City of Corpus Christi spec gives the same figures. The one difference is that Corpus Christi prints the #14 weight as 7.6, a rounded form of 7.650.

The Caltrans chart is headed “ASTM Standard Reinforcing Bars” and notes that both A615 and A706 specifications are in use. So this table is not limited to one specification. NYSDOT’s table matches the diameters and areas for #4 to #11 and lists no weights.

Hand check on one area: a #3 circle is 3.1416 x 0.375 x 0.375 / 4 = 0.1104 sq in. The chart lists 0.11.

How Does Weight per Foot Grow With Bar Size?

Weight per foot climbs steeply, from 0.376 lb/ft for a #3 to 5.313 lb/ft for a #11 in the chart. The bars below are drawn at 100 px per lb/ft, so each bar length is its weight in pixels divided by 100.

Our own check, not a published rule: weight is about 3.40 times the area in sq in. For a #8, 0.7854 sq in (the unrounded circle area) x 3.40 = 2.670 lb/ft, the printed value. With the rounded 0.79 sq in, 0.79 x 3.40 = 2.69, which is within rounding of 2.670. For a #18, 4.00 x 3.40 = 13.60.

One short example from the weight tool: a 20 ft #4 bar is 20 x 0.668 = 13.36 lb, shown as 13.4 lb. Fifty of those bars come to 50 x 20 x 0.668 = 668 lb, or 303.0 kg (668 / 2.2046).

How Do You Read the Marks Stamped on a Bar?

A bar carries marks for its size, steel type, grade and producing mill. The Caltrans Reinforced Concrete Construction Manual says these marks are usually embossed, or rolled in, along one side of the bar.

• Size. In the US, a single number gives the size. The manual’s example shows “11” for a #11 bar.
• Steel type. A letter names the standard: S for A615 and W for A706. A bar marked SW meets both.
• Grade. A separate number gives the grade, such as 60 for Grade 60, which has a minimum yield of 60,000 psi. No grade mark identifies Grade 40 or 50 steel.
• Mill. A separate mark identifies the producing mill. A legend of mill marks is in the CRSI Manual of Standard Practice.

This article does not give the order of the marks along a real bar. Check the bar in your hand against the plans.

What Are the Bracketed Metric Numbers?

Each US size in the Caltrans chart is paired with a bracketed metric number, called a soft-metric designation. Metric-marked bars carry that other number instead.

For example, #3 is [#10], #4 is [#13] and #5 is [#16]. This is why a chart can show a #10 beside a #3 bar, and why the two numbers must not be mixed up. That chart is the only source we read for these pairs.

Canadian M bars such as 10M are a different system. The weight tool lists them separately from US sizes, so do not swap sizes across systems by name. Metric conversion has its own article.

What About Older Bars, #2 Bars, and Choosing a Size?

Some older and special bars sit outside the main chart, and the plans set the size for your job. This article does not cover how to pick a size.

The Caltrans chart notes that the A305 specification existed from 1947 to 1968, and that older structures can contain square bars. One City of Corpus Christi spec also lists a #2 bar at 0.250 in, which is 2/8 in. It says smooth round bars are designated by size number through No. 4, and larger smooth bars by diameter in inches. We did not read the ASTM standard, so whether it covers #2 today is not verified here.

For layout, see the rebar calculator. Other bar-related tools sit under structural concrete calculators and the foundation and footing calculators.

FAQs About Rebar Sizes

Sources and Further Reading

Bar tables are nominal values, so use the sizes and grades on your plans. This article is a reference for reading bar numbers and does not choose a size for you. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-10-19.