Reinforcement
Rebar Sizes Explained
The number is the diameter in eighths of an inch. Full size chart and grades.
Reviewed July 2026
Rebar sizes follow a simple code once you know the trick: for the common bars, the number is the diameter in eighths of an inch. A #4 bar is four-eighths, or half an inch; a #8 bar is eight-eighths, or one inch. That single rule covers most of the rebar you will ever handle, and the full chart below fills in the rest, along with what the grade numbers mean.
The eighths-of-an-inch rule
US and imperial rebar sizes give the nominal diameter in units of one-eighth inch for bar sizes #2 through #8, so #8 equals 8/8 inch, a one-inch diameter. There are no fractional sizes in this system, and tradespeople often just say the number: a "four bar" is four-eighths, a half inch. The nominal cross-sectional area of a bar works out to roughly (bar number / 9) squared, in square inches, which is why a #9 bar happens to have almost exactly a 1.00 square-inch area.
US rebar size chart
| Bar | Dia (in) | Dia (mm) | Area (in^2) | lb/ft |
|---|---|---|---|---|
| #3 | 3/8 (0.375) | 9.5 | 0.11 | 0.376 |
| #4 | 1/2 (0.500) | 12.7 | 0.20 | 0.668 |
| #5 | 5/8 (0.625) | 15.9 | 0.31 | 1.043 |
| #6 | 3/4 (0.750) | 19.1 | 0.44 | 1.502 |
| #7 | 7/8 (0.875) | 22.2 | 0.60 | 2.044 |
| #8 | 1 (1.000) | 25.4 | 0.79 | 2.670 |
| #9 | 1.128 | 28.7 | 1.00 | 3.400 |
| #10 | 1.270 | 32.3 | 1.27 | 4.303 |
| #11 | 1.410 | 35.8 | 1.56 | 5.313 |
| #14 | 1.693 | 43.0 | 2.25 | 7.650 |
| #18 | 2.257 | 57.3 | 4.00 | 13.60 |
Sizes above #8 stop following the eighths rule cleanly and skip some numbers for historical reasons: #9, #10, #11, #14, and #18 correspond to the cross-sections of older one-inch and larger square bars.
What the grades mean
A bar's size is only half the specification; the other half is its grade, which describes strength, not dimensions. In US use, the grade number is the minimum yield strength in ksi (thousands of psi). Grade 60 rebar has a minimum yield strength of 60,000 psi and is by far the most common grade in modern US construction. Rebar is also made in grades 40, 75, 80, 100, and higher for special uses. Two bars of the same #5 size can be different grades, so drawings always call out both, such as "#5 Grade 60."
Why the ribs and the standards
Those raised ribs, lugs, or deformations on the surface are not decoration. They mechanically lock the bar into the surrounding concrete so it cannot pull out under load, and their spacing and height are set by standards such as ASTM A615 (plain carbon-steel bar) and ASTM A706 (a weldable low-alloy bar). Steel and concrete also expand at nearly the same rate with temperature, which is part of why the two materials work together so well; if they did not, temperature swings would tear the bond apart.
Choosing a size
Bar size, grade, spacing, and cover are engineering decisions tied to the loads a structure carries, and they belong in a design by a qualified engineer rather than a rule of thumb. As orientation only, light residential slabs and footings often use #3 or #4 bar, driveways and walls step up to #4 or #5, and heavier structural work uses #6 and larger. For a corrosion-proof alternative to steel in wet or coastal work, see fiberglass rebar vs steel.
Frequently asked questions
What do rebar numbers mean?
For bars #3 through #8, the number is the diameter in eighths of an inch. So #4 rebar is 4/8, or 1/2 inch, and #8 is 8/8, or 1 inch. Above #8 the simple rule breaks down and the sizes match older square-bar areas.
What size is #4 rebar?
#4 rebar is 1/2 inch (0.5 in, 12.7 mm) in nominal diameter, with a cross-sectional area of 0.20 square inches and a weight of about 0.668 pounds per foot. It is one of the most common sizes in residential work.
What does grade 60 rebar mean?
In US practice the grade is the minimum yield strength in thousands of psi (ksi). Grade 60 rebar has a minimum yield strength of 60,000 psi and is the most widely used grade in modern US construction. Grades 40, 75, 80, and 100 also exist.
More on reinforcement
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