What the bar number measures, what a caliper reads, and what the yard actually bills
← Back to the calculatorthe bar number measures SIZE — #4 means 4 eighths of an inch, 0.500 in or 12.7 mm nominal; the stock length measures LENGTH — 20 ft and 40 ft in the US, 12 m and 6 m in metric countries; and the yard measures WEIGHT — a 20 ft #4 bar is 13.4 lb, and the invoice is pounds, not bars.
The three are tied together by one number, the unit weight: 0.668 lb per foot for #4. Multiply it by a length and you have weight; multiply by the bar count and you have the order. What follows is what each measurement means, what you actually read with a tape, a caliper and a scale, and where the shortcuts stop being exact.
In the US a bar’s size is not a millimetre reading — it is a count of eighths of an inch. The number after the hash is the nominal diameter in eighths: #4 is four eighths (4/8 in = 1/2 in = 0.500 in), #6 is six eighths (6/8 in = 3/4 in). That is the whole system, and it holds exactly from #3 through #8 — every one of those sizes lands on its own fraction to the third decimal.
Above #8 the rule starts to drift, because the large sizes were defined to land on round metric equivalents. #10 is not exactly 10/8 in (1.250 in) — it is 1.270 in, or 32.3 mm. #11 is 1.410 in, not 1.375. #14 is 1.693 in, not 1.750. So “the number is eighths” is exact for the bars most jobs use, and an approximation for the big sizes. Read the table, not the fraction, once you are past #8:
| Bar | The number means | Nominal dia. (in) | Nominal dia. (mm) | Area (in²) | Weight (lb/ft) |
|---|---|---|---|---|---|
| #3 | 3/8 in — exact | 0.375 | 9.5 | 0.11 | 0.376 |
| #4 | 4/8 in = 1/2 in — exact | 0.500 | 12.7 | 0.20 | 0.668 |
| #5 | 5/8 in — exact | 0.625 | 15.9 | 0.31 | 1.043 |
| #6 | 6/8 in = 3/4 in — exact | 0.750 | 19.1 | 0.44 | 1.502 |
| #7 | 7/8 in — exact | 0.875 | 22.2 | 0.60 | 2.044 |
| #8 | 8/8 in = 1 in — exact | 1.000 | 25.4 | 0.79 | 2.670 |
| #9 | 9/8 in — nominal is 1.128 | 1.128 | 28.7 | 1.00 | 3.400 |
| #10 | 10/8 in — nominal is 1.270 | 1.270 | 32.3 | 1.27 | 4.303 |
| #11 | 11/8 in — nominal is 1.410 | 1.410 | 35.8 | 1.56 | 5.313 |
| #14 | 14/8 in — nominal is 1.693 | 1.693 | 43.0 | 2.25 | 7.650 |
| #18 | 18/8 in — nominal is 2.257 | 2.257 | 57.3 | 4.00 | 13.600 |
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The same drift shows up in the mental shortcuts, and for the same reason. The estimate “unit weight ≈ bar number squared ÷ 24” is within 0.3% from #3 to #8 (4² ÷ 24 = 0.667, essentially #4’s 0.668 lb/ft) and then walks off: #10 reads 4.17 against a nominal 4.303 (−3.2%), and #14 reads 8.17 against 7.650 (+6.8%). One root cause — the big sizes were reset to metric equivalents — so one fix: past #8, stop shortcutting and read the chart.
The nominal diameter is a designation, not a reading. Put a caliper on a bar and you measure the actual bar — deformations included — and it reads larger than nominal. The size designation, the design area and the unit weight are all built on the nominal, so a caliper tells you the bar is oversize; it does not tell you what size it is called.
One pair shows why this matters. A #5 bar is 25% bigger in diameter than a #4 (0.625 in vs 0.500 in) but 56% heavier (1.043 vs 0.668 lb per foot), because weight follows area and area is d². Two bars an eighth of an inch apart are nearly impossible to tell apart on a pallet — and trivial to separate on a scale. The deformations rolled into the surface add the bond to concrete on top of the nominal area, which is why a design is never written from a caliper reading.
Because steel is sold and certified by mass, the measurement that identifies a bar’s size is its weight per length. Tape the length, weigh it, divide, and compare with the nominal unit weight. A 20 ft stick that weighs 13.4 lb is #4; the same length at 20.9 lb is #5. That is a two-minute check on delivery day:
| Bar | lb/ft | 5 ft piece (lb) | 10 ft piece (lb) | 20 ft bar (lb) |
|---|---|---|---|---|
| #3 | 0.376 | 1.9 | 3.8 | 7.5 |
| #4 | 0.668 | 3.3 | 6.7 | 13.4 |
| #5 | 1.043 | 5.2 | 10.4 | 20.9 |
| #6 | 1.502 | 7.5 | 15.0 | 30.0 |
| #7 | 2.044 | 10.2 | 20.4 | 40.9 |
| #8 | 2.670 | 13.3 | 26.7 | 53.4 |
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Rebar is measured in two lengths, and they are never the same number: the stock length you buy and the line length you place. US yards stock 20 ft and 40 ft bars (60 ft by special order); metric countries stock 12 m and 6 m. A line that fits inside one stock bar is one bar and no splice. A line longer than the stock bar is two bars plus a lap splice, and the lap is steel you buy on top of the line. A line much shorter than the stock bar leaves an offcut — and two short pieces cannot be joined into one long line.
That whole chain — pieces per bar as floor(stock ÷ line), the waste table for a given line length, and what each stock length does to your order — is worked through line by line on the how long are rebar bars page. This page stays with the measurement itself: the tape reads 20 ft, and the bar is billed by what those 20 ft weigh.
Quantity measurement is a chain with one direction — count, then feet, then pounds, then tons — and the unit weight is the link at every step:
| Order (#4, 20 ft stock) | Feet | Pounds | Short tons |
|---|---|---|---|
| 10 bars | 200 | 133.6 | 0.067 |
| 40 bars | 800 | 534.4 | 0.267 |
| 149 bars — one ton | 2,980 | 1,990.6 | 0.995 |
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Read the bottom row both ways: a ton of #4 is 149 twenty-foot bars or 74 forty-foot bars — the same 2,000 lb, double the footage. Suppliers ship whole stock bars and bill by the weight on the scale ticket, so the measurement of record is pounds, and the bar count is how you ask for it. The estimator page works the takeoff into an order, the tonnage page carries the pounds-to-tons conversions, and the price page folds the same weights into per-pound and per-foot quotes.
Metric countries name a bar straight out by its diameter in millimetres — 10 mm, 12 mm, 16 mm and so on — and measure its weight in kilograms per metre from the same steel density: kg/m = d² ÷ 162.2, so a 16 mm bar is 1.578 kg/m. The two systems do not convert exactly, so engineers pair them by closest nominal diameter:
| Metric size | In inches | kg/m | Area (mm²) | Closest US bar | Off by |
|---|---|---|---|---|---|
| 10 mm | 0.394 | 0.617 | 79 | #3 (0.375 in) | 4.8% |
| 12 mm | 0.472 | 0.888 | 113 | #4 (0.500 in) | 5.9% |
| 16 mm | 0.630 | 1.578 | 201 | #5 (0.625 in) | 0.8% |
| 20 mm | 0.787 | 2.466 | 314 | #6 (0.750 in) | 4.7% |
| 25 mm | 0.984 | 3.853 | 491 | #8 (1.000 in) | 1.6% |
| 28 mm | 1.102 | 4.834 | 616 | #9 (1.128 in) | 2.4% |
| 32 mm | 1.260 | 6.313 | 804 | #10 (1.270 in) | 0.8% |
| 40 mm | 1.575 | 9.865 | 1,257 | #14 (1.693 in) | 7.5% |
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16 mm pairs with #5 and 32 mm with #10 within 1%, but 40 mm sits closer to #14 than to #11 — the pairings are similar, not equivalent, and a substitution is the drawing engineer’s call. One more measurement trap crosses the border: the metric tonne is 2,204.6 lb, 10.2% heavier than the US short ton, so a “ton” quoted across systems is not the same number of pounds.
A tape gives you the length and a scale gives you the weight, but the bar itself carries a third measurement rolled into the steel. ASTM A615 §19.3 fixes the order of the marks: the mill, the bar size, the letter S, and the grade — where the grade is a set of continuous longitudinal lines rather than a number.
That is the whole identification, and it is worth more than it looks: the line count tells you the grade without a folder, and the SW pair tells you the bar was made to A706 as well as A615 — which is the one detail that decides whether a welder can touch it. The chemistry behind those letters is on the what rebar is made of page; the sizes and unit weights are on the size chart.
The number is the bar’s nominal diameter in eighths of an inch: #4 is four eighths, 4/8 in, which is 0.500 in or 12.7 mm. The rule is exact from #3 through #8 — #3 is 3/8 in, #6 is 6/8 in (3/4 in), #8 is 8/8 in (1 in). Above #8 it drifts, because the large sizes were defined to round metric equivalents: #10 is 1.270 in, not exactly 10/8 in (1.250 in).
Three ways, for three different questions. Size is measured by the bar number, which counts eighths of an inch of nominal diameter. Length is measured in stock lengths — 20 ft and 40 ft in the US, 12 m and 6 m in metric countries. Quantity is measured by weight: bars times length times pounds per foot, invoiced in pounds or short tons. A 20 ft #4 bar ties all three together — the number #4, the 20 ft length and the 13.4 lb weight.
Yes — nominally. #4 is 0.500 in (12.7 mm) of nominal diameter. The sizes around it are the same fractions: #3 is 3/8 in (0.375), #5 is 5/8 in (0.625), #6 is 3/4 in (0.750) and #8 is 1 in. Nominal means it is a designation, not a caliper reading — measure a bar across its deformations and it reads larger than the nominal size.
A caliper reads the actual bar, deformations included, which comes out larger than the nominal diameter the designation and the design area are built on. So a caliper tells you the bar is oversize, not what size it is called. The measurement that identifies a bar is mass per length: weigh a tape-measured length, divide by the length, and compare with the nominal weight per foot — a 20 ft #4 is 13.4 lb and a 20 ft #5 is 20.9 lb.
Weight per foot times 20: #3 is 7.5 lb, #4 is 13.4 lb, #5 is 20.9 lb, #6 is 30.0 lb and #8 is 53.4 lb. A 40 ft bar is exactly double — #4 is 26.7 lb. These are nominal ASTM A615 / A706 weights; epoxy-coated and galvanised bars add weight on top, and the mill certificate governs anything critical.
Put a 20 ft length on a scale. #4 is 13.4 lb and #5 is 20.9 lb — 56 percent heavier, because weight follows area and area is diameter squared, while the diameter itself is only 25 percent bigger (0.625 in vs 0.500 in). The two sizes are nearly impossible to tell apart on a pallet by eye, and trivial to separate on a scale.
Count the bars, multiply by the stock length to get feet, multiply feet by the weight per foot to get pounds, and divide by 2,000 for short tons: 40 bars of 20 ft #4 is 800 ft, 534.4 lb and 0.267 short tons. Suppliers ship whole stock bars and bill by the weight on the scale ticket, so give the yard both the bar count and the weight.
16 mm is 0.630 in, and the closest US size is #5 at 0.625 in — within 0.8 percent. The other clean pairings are 32 mm to #10 (0.8 percent), 25 mm to #8 (1.6 percent) and 28 mm to #9 (2.4 percent), but 40 mm sits closer to #14 than #11 at 7.5 percent off. Metric and imperial sizes are similar, not equivalent — swap only on the drawing engineer’s call.