Rebar size designations are the only product code in construction where the number on the bar and the number in the buyer's head can differ by a factor of three and nobody notices until the bars arrive. Six beliefs cause almost all of it. Each one below has cost somebody a container.
A rebar size designation is a label for a nominal cross-section, not a measured diameter. That single sentence resolves five of the six myths that follow.
Myth 1: "#4 rebar is 4 mm"
It is 12.7 mm.
US bar sizes count eighths of an inch. #4 is four eighths — half an inch — which is 12.7 mm. #8 is eight eighths, one inch, 25.4 mm. Above #8 the pattern breaks: #9, #10 and #11 are sized by area rather than by a simple fraction, and #14 and #18 are their own case.
| US size | Nominal diameter | Soft-metric name | Mass |
|---|---|---|---|
| #3 | 9.5 mm | #10 | 0.560 kg/m |
| #4 | 12.7 mm | #13 | 0.994 kg/m |
| #5 | 15.9 mm | #16 | 1.552 kg/m |
| #6 | 19.1 mm | #19 | 2.235 kg/m |
| #7 | 22.2 mm | #22 | 3.042 kg/m |
| #8 | 25.4 mm | #25 | 3.973 kg/m |
| #9 | 28.7 mm | #29 | 5.060 kg/m |
| #10 | 32.3 mm | #32 | 6.404 kg/m |
| #11 | 35.8 mm | #36 | 7.907 kg/m |
Anyone who reads "#4" as a millimetre value orders bar that is a third of the diameter and a ninth of the area they needed. It happens most often on small trial orders placed by email without a drawing.
Myth 2: "#13 is a different bar from #4"
It is the same bar with a metric name. The soft-metric designation system renames US bars by their approximate diameter in millimetres — #4 becomes #13, #8 becomes #25 — without changing the bar, the rolling, or the marking system. CRSI's reference on bar sizes and markings lists both names against the same product.
The trap is the opposite direction: a buyer in a metric market who asks for "13 mm bar" may genuinely want a 13 mm bar rolled to a metric standard, not a 12.7 mm US bar under a metric alias. Ask which standard governs before you quote. The standard name settles it; the number does not.
Myth 3: "Grade 60, B500B and HRB400 are interchangeable"
They are three different minimum yield strengths.
| Designation | Standard | Minimum yield |
|---|---|---|
| Grade 60 | ASTM A615/A615M | 420 MPa (60,000 psi) |
| Grade 80 | ASTM A615/A615M | 550 MPa |
| B500A / B500B / B500C | BS 4449 | 500 MPa characteristic |
| HRB400 | GB 1499.2 | 400 MPa |
| HRB500 | GB 1499.2 | 500 MPa |
Substituting HRB400 where the design called for Grade 60 gives up 20 MPa of yield. It is often described as "equivalent" in trading conversations, and it is not equivalent — it is a substitution that a structural engineer has to approve, in writing, before it goes into a slab. The BS 4449 grades differ from each other too: A, B and C describe ductility class, not strength, and a designer who specified B500C did so for a reason.
Ductility, bend performance and fatigue requirements also vary between these standards. ASTM A615/A615M and GB 1499.2 are both public documents; quote the clause, not a rule of thumb.
Myth 4: "The diameter is whatever the caliper reads"
Put a caliper across a deformed bar and you will read something larger than the nominal diameter, because you are measuring across the ribs. Nominal diameter is derived from the bar's mass per unit length, back-calculated as if the bar were a plain round of the same steel.
That is why mass per metre is the acceptance measurement, not a caliper reading. It is also why a bar can pass a caliper check and fail an incoming inspection: the ribs are within tolerance while the underlying section is light.
Practical consequence for a supplier: publish the nominal diameter, the nominal mass per metre and the permitted mass deviation together. Publishing a single number invites the buyer to check it the wrong way.
Myth 5: "T12 means grade T, 12 mm"
In UK practice, the old "T" prefix marked a deformed high-yield bar and the number was the diameter in millimetres — so T12 was a 12 mm ribbed bar, not a grade. Current UK detailing practice under BS 8666 uses H for grade B500B or B500C ribbed bar, so the same bar is now H12 on a modern bar schedule.
If a bar bending schedule arrives with T-prefixed marks, it is either an old drawing or an office that has not updated its template. Either is fine — but confirm the grade explicitly rather than assuming the letter carries it, because the letter used to and now does not.
Myth 6: "Weight tolerance is a detail"
Rebar is bought by mass and used by section. The gap between those two is where margin disappears in both directions, which is why weight deviation limits sit inside the product standards rather than in a commercial annex.
This one changed recently and is worth knowing if you sell from or buy in China: GB 1499.2-2024 replaced the 2018 edition, took effect on 25 September 2024, and — importantly — moved from a recommended standard (GB/T) to a mandatory one (GB), with revised weight-deviation requirements and re-inspection provisions. A quotation still citing GB/T 1499.2-2018 is citing a superseded document.
What Actually Works
Put the whole identity of the bar on the paperwork and on the drawing, in this order:
- Standard and edition — ASTM A615/A615M-24, BS 4449:2005+A3:2016, GB 1499.2-2024
- Grade — Grade 60, B500B, HRB400
- Size — the designation used by that standard, plus nominal diameter in mm
- Nominal mass per metre and the permitted deviation
- Bar marks — mill, bar size, grade symbol, as rolled onto the bar
- Test certificate reference — heat number, mill certificate, EN 10204 3.1 if requested
Six lines removes the entire myth list. For labelling conventions across the wider building-materials catalogue, how to label building material dimensions covers the layout, the standard rebar sizes reference gives the full size table, and building material spec diagrams shows what a complete sheet looks like when it leaves the office.
Next Steps
- Audit your existing listings for bare designations. Any place a listing says "#4" or "12 mm" with no standard next to it is a future dispute.
- Add nominal mass per metre to every size row. It is the number the buyer's inspector will actually measure.
- Update citations to current standard editions — particularly anything still referencing GB/T 1499.2-2018.
- Put the size and grade on the product image, not only in the table. Bundles look identical in photographs; a spec image that carries the measured diameter, the grade and the standard on the bar itself is the difference between a catalogue picture and a document a contractor can order from. The measurement has to be real, which is the reason to use annotation that snaps to the actual edge and locks the measured value onto the image rather than an AI-generated illustration that prints a diameter nobody verified.
FAQ
What does #4 rebar mean?
US rebar sizes count eighths of an inch, so #4 is 4/8 in — 0.5 in, or 12.7 mm nominal diameter, with a nominal mass of 0.994 kg/m. Its soft-metric name is #13, which is the same bar.
What is the difference between #4 and #13 rebar?
Nothing physical. #13 is the soft-metric designation for the same US #4 bar, named for its approximate diameter in millimetres. If a buyer wants a genuinely metric 13 mm bar rolled to BS 4449 or GB 1499.2, that is a different product — ask which standard applies.
Is HRB400 equivalent to Grade 60?
Not exactly. HRB400 has a 400 MPa minimum yield under GB 1499.2; ASTM A615 Grade 60 requires 420 MPa. They are often traded as equivalents, but substituting one for the other is a design decision that needs the engineer's written approval.
How do I check rebar size on delivery?
Weigh a measured length and compare it against the nominal mass per metre, then check the rolled-in bar marks for mill, size and grade. A caliper across the ribs reads high and will not detect an underweight bar.
Which rebar standard should I quote for export?
Quote the standard the destination market's designer specified — ASTM A615/A615M for the US, BS 4449 for the UK and much of the Middle East, GB 1499.2 for domestic Chinese projects — and name the edition. Bars can be produced to more than one standard, but only if the certificate says so.