Concrete & Reinforcement

Rebar Size Chart

Diameter, area and weight for every US bar from #3 to #18, the soft-metric names for the same bars, and the Canadian CSA bars that are genuinely different.

The bar number is the diameter in eighths of an inch — but only through #8. A #4 is 0.500 in (12.7 mm), area 0.20 in², weighing 0.668 lb/ft. From #9 up the rule changes and the numbers stop matching eighths.

US rebar sizes: nominal diameter, cross-sectional area and unit weight, #3 through #18
						Soft-metric
#3	0.375 in / mm	0.11	71	0.376	0.559	10M
#4	0.500 in / mm	0.20	127	0.668	0.994	13M
#5	0.625 in / mm	0.31	198	1.044	1.554	16M
#6	0.750 in / mm	0.44	285	1.503	2.237	19M
#7	0.875 in / mm	0.60	388	2.046	3.045	22M
#8	1.000 in / mm	0.79	507	2.673	3.978	25M
#9	1.128 in / mm	1.00	645	3.401	5.061	29M
#10	1.270 in / mm	1.27	817	4.311	6.416	32M
#11	1.410 in / mm	1.56	1007	5.314	7.908	36M
#14	1.693 in / mm	2.25	1452	7.661	11.401	43M
#18	2.257 in / mm	4.00	2581	13.616	20.262	57M

Nominal dimensions per ASTM A615. Deformations (the ribs) are not counted — the nominal diameter is defined as the plain round bar of the same weight per foot.

Where the numbering breaks

Through #8 the rule is simple and everybody knows it: the number is eighths of an inch, so #3 is 3/8 in and #8 is 1 in. Past that, the numbers are no longer diameters at all. #9 through #18 are named for the square bar of equal cross-section they replaced:

#9 — equal-area round of a 1 in square bar (1 in² → d = side × 2/√π), giving 1.128 in
#10 — equal-area round of a 1.125 in square bar (1.266 in² → d = side × 2/√π), giving 1.270 in
#11 — equal-area round of a 1.25 in square bar (1.562 in² → d = side × 2/√π), giving 1.410 in
#14 — equal-area round of a 1.5 in square bar (2.25 in² → d = side × 2/√π), giving 1.693 in
#18 — equal-area round of a 2 in square bar (4 in² → d = side × 2/√π), giving 2.257 in

Which is why guessing costs you. Read #10 as "ten eighths" and you get 1.250 in; the bar is 1.270 in. That is 1.6% on diameter — but steel is sold and stressed by area, and area goes as the square, so it is 3.2% on area, in the unconservative direction. #11 reads 2.5% small on diameter and 5.2% on area the same way; #14 goes 3.3% and 6.4% the other way.

#9 and #18 are the ones that lull you: 9/8 = 1.125 against an actual 1.128, and 18/8 = 2.250 against 2.257 — both within 0.3% on diameter. The standard picked #9 as the changeover precisely because the two rules very nearly meet there.

There is also no #12, #13, #15, #16, #17. The gaps are not sizes anyone dropped — those numbers were never assigned.

"10M" means two different bars

This is the one that costs money. The US soft-metric designations in the table above are not new bars — #10M is just #3 renamed, the number being the diameter in millimetres. Canada's CSA G30.18 bars are a genuinely separate series that happens to use the same style of name.

Bar designations that appear in both the US and Canadian systems
Designation	US (ASTM A615M)	Canada (CSA G30.18)	Area difference	Same bar?
10M	#3 — 9.5 mm, 71 mm²	11.3 mm, 100 mm²	+40.8%	No
25M	#8 — 25.4 mm, 507 mm²	25.2 mm, 500 mm²	-1.4%	Yes, within 2%

A Canadian 10M is 40.8% more steel than a US 10M. On a drawing that crossed the border, substituting one for the other is a real structural change, not a rounding difference. 25M is the opposite case — the two systems land within 1.4%, close enough to be the same bar.

Canadian CSA bars

Worth knowing how these are built, because it is backwards from the US system. CSA defines the area — a round 100, 200, 300 mm² and so on — and the diameter falls out of it as √(4A/π). That is why the areas are suspiciously tidy and the diameters are not.

Canadian CSA G30.18 rebar sizes: defined area with derived diameter and mass

10M	100	in / 11.3 mm	0.785	0.527
15M	200	in / 16.0 mm	1.570	1.055
20M	300	in / 19.5 mm	2.355	1.582
25M	500	in / 25.2 mm	3.925	2.637
30M	700	in / 29.9 mm	5.495	3.692
35M	1000	in / 35.7 mm	7.850	5.275
45M	1500	in / 43.7 mm	11.775	7.912
55M	2500	in / 56.4 mm	19.625	13.187

Area is the defined quantity in CSA G30.18; diameter is derived from it. Mass uses 7850 kg/m³, which reproduces the published values exactly.

Grades

The grade number is the minimum yield strength in ksi — Grade 60 yields at 60 ksi. Nothing to look up there. What does need looking up is the tensile minimum, and the fact that A706 caps yield from above as well as below, since a bar that is too strong fails the ductility the seismic detailing assumes.

Rebar grades with minimum yield and tensile strength
Spec	Grade	Min yield	Min tensile	Notes
A615	40	40 ksi / 280 MPa	60 ksi / 420 MPa	Legacy. Still seen in older drawings and small residential work.
A615	60	60 ksi / 420 MPa	90 ksi / 620 MPa	The default for North American construction.
A615	80	80 ksi / 550 MPa	105 ksi / 725 MPa	High-strength; ACI 318 limits where it may be used.
A615	100	100 ksi / 690 MPa	115 ksi / 790 MPa	Newest addition; restricted applications.
A706	60	60 ksi / 420 MPa	80 ksi / 550 MPa	Weldable / seismic. Yield is capped as well as floored (60–78 ksi), and tensile must exceed 1.25× the actual yield.
A706	80	80 ksi / 550 MPa	100 ksi / 690 MPa	Weldable high-strength. Yield capped at 98 ksi.

Reading the marks on the bar

Every bar carries rolled-in marks: the producing mill, the size number, and a letter for what it is made of.

Steel type letters rolled into rebar
Letter	Specification	What it means
S	ASTM A615	Carbon steel. The ordinary bar.
W	ASTM A706	Low-alloy, weldable, controlled ductility. Required where the design calls for welding or seismic detailing.
SW	A615 + A706	Dual-certified — meets both. Can be substituted for either.
R	ASTM A996	Rail steel.
A	ASTM A996	Axle steel.
CS	ASTM A1035	Low-carbon chromium, high strength.

Grade is also marked, either as a number or as continuous longitudinal lines, but the line convention has changed between editions of A615 and is easy to misread from memory. Check it against the mill's own marking sheet rather than a chart.

No cover, development length, lap splices or bend diameters here — on purpose. Those are ACI 318 design provisions, not properties of the bar. They depend on concrete strength, exposure class, coating, bar spacing and which member you are in, so any single number reprinted out of that context is wrong more often than right. Take them from the structural drawings and ACI 318 itself.

How the weights are calculated

#3–#8   d = n/8 in
#9–#18  d = (equal-area square side) × 2/√π
A = π/4 × d²
w = A × ρ    ρ = 7850 kg/m³ = 0.28360 lb/in³

The ribs do not enter into it. Nominal diameter is defined as the plain round bar having the same weight per foot as the deformed bar, so the deformations are already accounted for in the number you started with.

Take the table with you — CSV · JSON

Data source and method

Diameter: Computed from the two rules above. Ten of the eleven US bars reproduce the ASTM A615 nominal diameter exactly at three decimals; #10 is the exception — geometry gives 1.2694 in and the standard fixes it at 1.270 in. The table carries the standard's value, since that is the bar that gets delivered.
Weight: Area × density, ρ = 7850 kg/m³ (0.28360 lb/in³). The same constant drives both unit systems, so lb/ft and kg/m cannot drift apart.
Verification: All 11 US bars checked against the published ASTM A615 nominal diameters, areas and weights; maximum weight deviation 0.19% (#10, from the 1.2694 → 1.270 rounding), every other bar within 0.15%. The Canadian masses reproduce the CSA G30.18 published values exactly — 10M = 0.785, 25M = 3.925, 55M = 19.625 kg/m — because CSA defines them with this same density.
Canadian bars: Areas are the defined quantities from CSA G30.18; diameters derived as √(4A/π).
Grades: Yield minimum is the grade number by definition. Tensile minimums and the A706 yield ceiling are from ASTM A615/A706.
Deliberately absent: ACI 318 design provisions, and the grade line-marking convention. See the notices above.
Not design: These are bar properties. Reinforcement layout, quantity and detailing come from the structural engineer of record.

Related charts

Thread size chart — tensile stress area, the same "what is the steel actually carrying" question for fasteners
Bolt torque chart — anchor bolts and hold-downs, where the concrete work meets the steel
Pipe size chart — another system where the name stopped matching the dimension a century ago