A rebar number is a diameter, not a strength: for #3 through #8 it is the nominal diameter in eighths of an inch, so a #4 bar is 4/8 = 0.500 in and a #8 is 1.000 in. Above #8, sizes are set by cross-sectional area instead. Weight follows from the area — one square inch of bar weighs 3.4 lb per foot.
The size chart
These are the ASTM standard reinforcing bar values, as tabulated in Caltrans Bridge Design Details Table 13.1.1. Every US supplier works from the same numbers, because they come from ASTM A615 and A706.
| Bar | Nominal diameter | Deformation diameter | Area | Weight | 20 ft stick | Metric mark |
|---|---|---|---|---|---|---|
| #3 | 0.375 in | 0.44 in | 0.11 in² | 0.376 lb/ft | 7.5 lb | #10 |
| #4 | 0.500 in | 0.56 in | 0.20 in² | 0.668 lb/ft | 13.4 lb | #13 |
| #5 | 0.625 in | 0.69 in | 0.31 in² | 1.043 lb/ft | 20.9 lb | #16 |
| #6 | 0.750 in | 0.88 in | 0.44 in² | 1.502 lb/ft | 30.0 lb | #19 |
| #7 | 0.875 in | 1.00 in | 0.60 in² | 2.044 lb/ft | 40.9 lb | #22 |
| #8 | 1.000 in | 1.13 in | 0.79 in² | 2.670 lb/ft | 53.4 lb | #25 |
| #9 | 1.128 in | 1.25 in | 1.00 in² | 3.400 lb/ft | 68.0 lb | #29 |
| #10 | 1.270 in | 1.44 in | 1.27 in² | 4.303 lb/ft | 86.1 lb | #32 |
| #11 | 1.410 in | 1.63 in | 1.56 in² | 5.313 lb/ft | 106.3 lb | #36 |
| #14 | 1.693 in | 1.88 in | 2.25 in² | 7.650 lb/ft | 153.0 lb | #43 |
| #18 | 2.257 in | 2.50 in | 4.00 in² | 13.600 lb/ft | 272.0 lb | #57 |
The deformation diameter is the one to use for clearances, as Caltrans notes in its own table: the ribs stand proud of the nominal diameter, and a #11 bar is 1.63 in across the deformations, not 1.41 in. The 20 ft column is the stick weight, which is what you feel carrying it and what shows up on the freight ticket.
Where the numbering breaks its own rule
Eighths of an inch works cleanly up to #8 — 3/8, 4/8, 5/8, 6/8, 7/8, 8/8. Then it stops:
- #9 is 1.128 in, not 1.125. That is because a #9 is defined by an area of exactly 1.00 in², and the diameter follows from the area.
- #10 is 1.270 in for an area of 1.27 in², #11 is 1.410 in for 1.56 in².
- #14 and #18 continue the area logic at 2.25 in² and 4.00 in² — the round numbers are 1½ and 2 times a #9's inch-square section. There is no #12, #13, #15, #16 or #17 in the inch-pound system.
So the inch rule is a field shortcut that happens to be exact for the sizes most crews touch, and approximate above them.
The weight math you can do in your head
Multiply any nominal area by 3.4 and you land on the published weight:
- #5: 0.31 in² × 3.4 = 1.05 lb/ft (published 1.043)
- #8: 0.79 in² × 3.4 = 2.69 lb/ft (published 2.670)
- #9: 1.00 in² × 3.4 = 3.40 lb/ft — exactly the published value
- #11: 1.56 in² × 3.4 = 5.31 lb/ft (published 5.313)
The other direction is just as useful for a delivery check: tonnage divided by the weight per foot gives you the footage you should have received. The rebar calculator does the whole loop — bars, footage, stock lengths and tons — from the grid on your drawings.
Reading the marks: mill, size, steel, grade
Every bar carries its own paperwork rolled into the steel. CRSI lists the sequence:
- The mill mark — a letter or symbol identifying the producing mill.
- The bar size. CRSI notes that its Board is encouraging producers to go back to inch-pound marking for all sizes and grades, which is why you still meet both systems in the same yard.
- The type of steel — usually S for carbon steel to ASTM A615, or W for low-alloy steel to ASTM A706.
- The grade — either the number (60, 75, 80, 100, 120) or rolled lines: one line for Grade 60, two for 75, three for 80 and 100, four for 120.
Grade is a yield strength in ksi: Grade 60 means a specified minimum yield of 60,000 psi. It is the number the designer used, and it is not interchangeable — substituting a different grade or a different ASTM specification for what the drawings call out is an engineering change, not a purchasing one.
A615 versus A706: when the letter matters
ASTM A615 — marked S
Deformed and plain carbon-steel bars. The default for most reinforced concrete. Chemistry is not controlled for weldability, so welding it requires a procedure that accounts for the actual carbon equivalent.
ASTM A706 — marked W
Low-alloy steel with controlled chemistry and tighter mechanical requirements. Specified where bars get welded or where a structure needs the ductility that seismic detailing relies on. Caltrans works from A706 Grade 60 for bridge reinforcement.
Both come in the same sizes and weigh the same per foot. The difference is in the steel, and it is the drawings that decide which one belongs in the wall.
Metric marks are the same bars
A bar marked 13, 16, 19 or 25 is not a different product. The metric designation is the nominal diameter in millimetres, rounded: 0.500 in is 12.7 mm and gets marked 13; 0.625 in is 15.9 mm and gets 16; 1.000 in is 25.4 mm and gets 25. These are "soft metric" designations — the bar itself never changed size, only the number rolled onto it. Match by diameter or by weight per foot, never by assuming a number in one system means anything in the other.
What a size chart cannot tell you
Nothing on this page says what size to use, how far apart to put it, how much cover to hold or how long a lap has to be. Those are design decisions, and they depend on loads, concrete strength, exposure, bar coating and the code your jurisdiction adopted. They arrive on drawings, from an engineer, or in a plan review comment — never from a chart.
What the chart is for: converting a drawing you already have into steel you can price, order and check on delivery. That is what our rebar calculator does, and what the concrete block calculator does for the vertical bars in a masonry wall.
Bottom line
- #3 to #8: the number is the diameter in eighths of an inch. #9 and up are defined by area.
- Weight per foot = nominal area × 3.4. A 20 ft #4 is 13.4 lb; a 20 ft #8 is 53.4 lb.
- Use the deformation diameter, not the nominal one, when you are checking clearances.
- The bar tells you what it is: mill, size, S or W, then the grade number or lines.
- Size and spacing come from the drawings. Quantities and tonnage come from the calculator.