Rebar sizes in the United States are written as a number: #3, #4, #5 and so on. For bars up to #8 the number is simply the diameter in eighths of an inch, so a #4 bar is 4/8 in, or half an inch. Above #8 the rule changes, and that is where most of the confusion starts.
Rebar Size Chart
Standard deformed bar sizes to ASTM A615, with the soft metric designation used on the same bars. Weight depends only on the nominal diameter, not on the grade, so a Grade 60 #5 and a Grade 80 #5 weigh exactly the same.
| Bar size | Metric | Diameter (in) | Area (in²) | Weight (lb/ft) | Weight (kg/m) |
|---|---|---|---|---|---|
| #3 | #10M | 0.375 | 0.11 | 0.376 | 0.560 |
| #4 | #13M | 0.500 | 0.20 | 0.668 | 0.994 |
| #5 | #16M | 0.625 | 0.31 | 1.043 | 1.552 |
| #6 | #19M | 0.750 | 0.44 | 1.502 | 2.235 |
| #7 | #22M | 0.875 | 0.60 | 2.044 | 3.042 |
| #8 | #25M | 1.000 | 0.79 | 2.670 | 3.973 |
| #9 | #29M | 1.128 | 1.00 | 3.400 | 5.060 |
| #10 | #32M | 1.270 | 1.27 | 4.303 | 6.404 |
| #11 | #36M | 1.410 | 1.56 | 5.313 | 7.907 |
| #14 | #43M | 1.693 | 2.25 | 7.650 | 11.384 |
| #18 | #57M | 2.257 | 4.00 | 13.600 | 20.239 |
There is no #12, #13, #15, #16 or #17. The jump from #11 straight to #14 and #18 is a historical artifact of the sizes the mills settled on, not an oversight.
Why #9 Breaks the Eighths Rule
A #9 bar is not 9/8 in, which would be 1.125 in. It is 1.128 in. That looks like a rounding error and it is not.
The large bars were originally square, and when they became round the mills kept the same cross-sectional area so that designs did not have to change. A 1 in square bar has an area of exactly 1.00 in², so the round bar that replaced it needed a diameter of 1.128 in to match. The same logic gives the next two sizes:
| Bar | Old square bar | Square area (in²) | Round diameter to match |
|---|---|---|---|
| #9 | 1 in | 1.00 | 1.128 in |
| #10 | 1-1/8 in | 1.27 | 1.270 in |
| #11 | 1-1/4 in | 1.56 | 1.410 in |
So from #9 up, the number is not the diameter in eighths. It is a label inherited from a square bar that no longer exists. The round numbers in the area column are the giveaway: 1.00, 1.27 and 1.56 are too tidy to be coincidence.
Where the Weights Come From
You can check every weight in the chart yourself. Steel weighs about 3.4028 lb per foot for each square inch of cross-section, so:
weight (lb/ft) = 0.7854 × d² × 3.4028
For a #8 bar, d = 1.000 in, so the weight is 0.7854 × 1 × 3.4028 = 2.672 lb/ft, against the published 2.670. Note that the weights come from the exact diameter, while the areas in the chart are rounded for design use. That is why 0.20 in² × 3.4028 gives 0.681 rather than the listed 0.668 for a #4. Use the published weight for ordering, not the rounded area.
The same relationship drives every steel section, which is why the steel beam weight calculator works the way it does, and why a beam’s weight per foot tells you its area at a glance.
Grades Are a Separate Question
Size tells you the geometry. Grade tells you the strength, and it is stamped on the bar as a separate mark.
| Grade | Minimum yield strength | Typical use |
|---|---|---|
| 40 | 40,000 psi | Small bars, light residential work |
| 60 | 60,000 psi | The general-purpose default in the US |
| 80 | 80,000 psi | Where reinforcement is congested |
| 100 | 100,000 psi | Heavy columns and specialist work |
Substituting a higher grade is not automatically safe. Higher grades mean fewer bars for the same force, but development length, crack control and ductility all change with grade, and the code limits which grades may be used where. The Concrete Reinforcing Steel Institute, which develops consensus standards and technical resources for steel-reinforced concrete, publishes the reference material designers work from (checked September 2026).
Reading the Marks on a Bar
Every deformed bar carries rolled-in marks, read along its length: the producing mill, the bar size number, a letter for the type of steel, and the grade. Grade 60 is marked either with the number 60 or with a single continuous longitudinal line; Grade 40 carries no grade mark at all.
That last point matters on site. An unmarked bar is not necessarily mystery steel, but it is not Grade 60 either, and mixing the two in the same mat is how a slab ends up with less capacity than the drawings assume.
Epoxy, Galvanized and Black Bar
Plain bar is called black. Where chlorides are a risk, from de-icing salt or seawater, the bar gets a coating: epoxy, or hot-dip galvanizing.
Galvanizing behaves differently from a barrier coating like epoxy. The American Galvanizers Association describes the zinc as providing barrier and cathodic protection plus a zinc patina, and notes it commonly gives over 70 years of protection with minimal maintenance when specified to the ASTM standards (checked September 2026). Epoxy stops working where it is chipped; zinc keeps protecting the steel next to a scratch. Our article on whether galvanized steel rusts goes into how that works.
Frequently Asked Questions
What size rebar do I need for a slab or footing?
It depends on the span, the loads, the soil and the concrete strength, so it is a design output rather than a lookup. #4 and #5 are the common sizes in residential slabs and footings, but the spacing matters as much as the size and both come from the engineer.
How much does a stick of rebar weigh?
Multiply the weight per foot in the chart by the length. Rebar is normally stocked in 20 ft and 40 ft lengths, so a 20 ft #5 bar is 20 × 1.043 = 20.9 lb.
Is #4 rebar the same as 1/2 inch rebar?
Yes. #4 means 4/8 in, which is 0.500 in. The same holds up to #8: #3 is 3/8 in, #6 is 3/4 in, #8 is 1 in.
What is the metric equivalent of #5 rebar?
#16M. The soft metric numbers are the nominal diameter in millimeters rounded to a whole number, so #16M is the same physical bar as #5 at 0.625 in, or 15.9 mm.
Can rebar be welded?
Only if the steel is suitable and the procedure is right. Standard A615 bar has no controlled carbon equivalent, so it is not intended for welding; A706 low-alloy bar is the weldable specification. Welding the wrong bar makes it brittle exactly where it was meant to stretch.
The Short Version
Up to #8, the bar number is eighths of an inch. From #9 up it is a label inherited from the square bars these sizes replaced, which is why #9 has an area of exactly 1.00 in². Weight follows from diameter alone and is independent of grade. Size, spacing and grade all come from the design, not from a rule of thumb, and a licensed structural engineer specifies them.
Reference information only, not engineering advice. Disclaimer.