The rule everyone prints, and the five sizes it does not fit

Open any rebar chart and you get the same sentence: the bar number is the diameter in eighths of an inch. Then, directly underneath, a table that contradicts it.

The rule is true, and it is exact, for #3 through #8. Three eighths, four eighths, up to eight eighths, which is why a #8 bar is exactly one inch. Nobody disputes that part.

Then #9 arrives at 1.128 inches when nine eighths is 1.125. #10 is 1.270 where the rule wants 1.250. #11 is 1.410 against 1.375. #14 goes the other way entirely, coming in at 1.693 when the rule predicts 1.750 — almost a sixteenth of an inch short. And #18 is 2.257 against 2.250.

Five sizes, five misses, in both directions and by wildly different amounts. Charts print them and move on, as if the numbers were arbitrary.

They are not arbitrary, and the standard is also being more careful than the charts that quote it. ASTM's own footnote does not say the number equals the eighths. It says bar numbers are "based on the number of eighths of an inch included in the nominal diameter" — which is a floor, not an equality. Read it that way and #9, #10, #11 and #18 stop being failures: nine whole eighths really do fit inside 1.128 inches, and eighteen fit inside 2.257.

That leaves exactly one bar that breaks even ASTM's own wording: #14. Only thirteen eighths fit inside 1.693 inches, and the bar is called #14. It is the single genuine anomaly in the system, and the explanation below is what puts it there.

The answer is square bars

Those five sizes were never designed in eighths. They are the round bars that replaced the old square ones, and each carries exactly the cross-section of the square it replaced.

Take a 1 inch square bar. Its area is 1.00 square inches. Now ask what round bar has that same area: diameter equals the side times the square root of 4/π, which is 1.1284 inches.

The standard prints 1.128.

Do it four more times and the pattern does not wobble:

• 1-1/8 inch square → area 1.27 sq in → round equivalent 1.269 — the standard says 1.270
• 1-1/4 inch square → area 1.56 sq in → 1.410 — the standard says 1.410
• 1-1/2 inch square → area 2.25 sq in → 1.693 — the standard says 1.693
• 2 inch square → area 4.00 sq in → 2.257 — the standard says 2.257

Five for five, to the thousandth of an inch. The second table above runs both tests on every size, and the five that are not named straight off the eighths are precisely the five that match a square bar.

And it settles #14. A 1-1/2 inch square holds 2.25 square inches, whose round equivalent is 1.693 — comfortably short of the 1.750 that fourteen eighths would need. The bar kept the number of the square it replaced rather than the number its own diameter earns, which is why it is the only size where the eighths reading fails outright.

Which is why their areas are the clean numbers

Once you see it, the whole table reads differently.

Look at the areas of the awkward sizes: 1.00, 1.27, 1.56, 2.25 and 4.00. Those are the tidy figures. Meanwhile #3 through #8 have areas like 0.11, 0.31 and 0.79 — the leftovers of a round number diameter.

That is the giveaway. For the small bars the diameter was the design target and the area fell out of it. For the big five it was the reverse: the area had to match a square bar that engineers were already specifying, and the diameter is simply what that area required. Two different design decisions, forty years apart, sharing one numbering system.

It also explains the direction of the errors. #14 comes in under its fourteen-eighths prediction because a 1-1/2 inch square holds less steel than a 1.75 inch round bar would; the others come in over, which is why they survive the floor reading and #14 does not.

And the eighths rule comes back at #20

There is a coda that most charts miss because most charts stop at #18.

ASTM A615 also lists a #20, and its nominal diameter is 2.500 inches — which is twenty eighths, exactly. The old rule returns the moment the standard stops matching square bars and simply names a new size.

It is in the first table above.

The traffic goes the other way too. Plenty of charts still list a #2 bar at a quarter inch. It is not in ASTM A615 as a deformed size, so it is not in the table here — if you meet one, you are looking at plain round stock or at a chart older than the standard it claims to follow.

The size is defined by weight, not by what it measures

Here is the second thing the standard says that nobody repeats, and it is the more useful of the two:

The nominal dimensions of a deformed bar are equivalent to those of a plain round bar having the same weight per foot as the deformed bar.

Read that carefully. The nominal diameter of a #5 bar is not a measurement of a #5 bar. It is the diameter of a smooth bar that would weigh the same. The deformations — the lugs and the ribs — sit on top of that nominal, adding steel and adding thickness.

The same footnote block gives the other half: steel is taken as 490 pounds per cubic foot. Put those together and the weight column is not measured either, it is derived. Area in square inches times 490/144 — about 3.403 — gives pounds per foot, and working from the exact circle area lands within 0.2 percent of all twelve published weights.

⚠️ One trap in that. Use the rounded area printed in the chart instead of the exact one and the error grows to nearly 2 percent. On a #4 that is 0.681 pounds per foot instead of 0.668. Nobody notices on one bar; on a few tons of steel it is real money in the wrong direction.

So weigh it, do not caliper it

That definition turns straight into a method, and it is the practical half of this page.

If you have a cut-off and want to know what it is, cut a clean 12 inch length and put it on a kitchen scale. The third table gives the answer for every size. It works because the sizes are far apart by weight: neighboring bars differ by 27 to 78 percent, and no scale is going to confuse a 0.668 pound foot of #4 with a 1.043 pound foot of #5.

Calipers are the worse tool, and it is worth being precise about how much worse. Measuring across the lugs, using the minimum deformation heights the standard permits, reads 8 to 10 percent high — a #8 comes out at 1.100 inches rather than 1.000. That is not enough to make it look like the next size up; the gap to #9 at 1.128 survives. But it is more than enough to make you distrust a correct answer and start second-guessing. If you must measure, measure the barrel between the lugs.

The number rolled on the bar might not be the number you think

Every bar is marked, and one of those marks is the size. It is also the mark most likely to mislead you, because American rebar has been rolled with metric designations as well as inch-pound ones.

The metric designation is the nominal diameter in millimeters, so a #4 becomes a 13, a #8 becomes a 25, and a #11 becomes a 36. Most of those are unmistakable. One is not.

A bar rolled "10" is either an inch-pound #10 at 1.270 inches, or a metric #10 — which is the metric name for a #3, at 0.375 inches. That is 3.4 times the diameter and 11.5 times the cross-sectional steel between the two readings of the same stamp. Of the twelve inch-pound sizes and their twelve metric designations, that is the only number that appears in both lists — and it is the worst possible one for it to happen to. (A615 also carries an annex of alternate metric sizes, invoked only when a purchase order calls for it, which adds further numbers to watch for.)

This is not a curiosity someone noticed on the internet. CRSI states that its Board, through the Engineering Practice Committee, has been encouraging producers to revert to inch-pound marking on all sizes and grades, with the stated intention of reducing confusion and the chance of errors and delays. The industry body agrees there is a problem.

Reading the rest of the markings

The full sequence rolled into a bar, per CRSI, runs: the mill's letter or symbol, then the bar size, then the type of steel — usually S for carbon steel to ASTM A615, or W for low-alloy steel to A706 — and finally the grade.

Grade shows up one of two ways. Either the number itself (60, 75, 80, 100, 120) or a set of longitudinal lines: one line is Grade 60, two lines is Grade 75, three lines covers 80 and 100, four lines is 120. The marking has to run at least five deformations long to count.

Grade is yield strength in thousands of pounds per square inch, so Grade 60 steel yields at 60,000 psi. It is a completely separate property from size, marked separately, and the two get confused often enough to be worth saying: a #4 Grade 60 and a #4 Grade 80 are the same bar dimensionally and different structurally.

What this page will not do for you

It will not tell you what size to use. Choosing a bar size, its spacing, its cover or its lap length is structural design, governed by ACI 318 and by whichever code your building is permitted under, with residential foundation and wall reinforcement prescribed in the IRC. Where a bar carries load, that is an engineer's decision, and no chart — including this one — substitutes for it.

And these are nominal values. Real bar carries manufacturing tolerances and the standard permits an underweight allowance, so a weighed sample will land near the table rather than on it. Near is enough to identify a size. It is not enough to argue about a mill certificate.