You can measure rebar diameter with a caliper, a flexible tape, a scale, or simply by reading the markings rolled into the bar itself. The right method depends on whether you’re checking new stock, verifying bars already tied in place, or identifying unmarked pieces on a jobsite. Each approach has trade-offs in speed and accuracy, so it helps to understand all four.
Read the Bar Markings First
Every standard reinforcing bar is manufactured with a series of raised symbols rolled directly into the steel. The first mark identifies the producing mill. The next mark is the bar size. In the U.S. system, the number corresponds to the bar’s diameter in eighths of an inch. A bar stamped “4” is 4/8 inch (half an inch, or about 12.7 mm) in diameter. A “6” is 6/8 inch (3/4 inch, roughly 19 mm). This is by far the fastest way to identify size if you can find the markings.
Metric rebar follows a simpler convention: the number is the nominal diameter in millimeters. Standard sizes under ISO 6935 are 6, 8, 10, 12, 14, 16, 20, 25, 28, 32, 40, and 50 mm. Bars larger than 50 mm exist but are special-order items agreed between the manufacturer and purchaser.
The catch is that markings can be hard to spot. Rust, concrete splatter, or a thick coat of epoxy can obscure them. If you’re working with bars already placed in formwork, you may not be able to rotate the bar to find the stamped face. That’s when you need a physical measurement.
Using a Caliper
A vernier or digital caliper gives the most precise reading. Open the jaws, place them across the bar, and close until snug. The complication with rebar is that it isn’t perfectly round. Deformed (ribbed) bars have raised lugs and longitudinal ribs that make the outer dimension larger than the nominal diameter. If you measure across the ribs, you’ll get an inflated number.
To get the nominal diameter, measure between the ribs in the smooth valley between deformations. Take readings at two perpendicular orientations and average them, because the cross-section can be slightly oval after rolling. A 16 mm bar, for example, will read close to 16 mm between the ribs but could read 17 mm or more across the tops of the deformations. The valley-to-valley average is what you compare against the standard size chart.
Digital calipers that read in both inches and millimeters make this especially convenient. For most construction verification, accuracy to 0.5 mm is plenty to distinguish one bar size from the next, since standard metric sizes jump by 2 to 8 mm between consecutive sizes.
Using a Flexible Tape or String
When you don’t have a caliper, you can wrap a flexible measuring tape, a strip of paper, or a piece of string around the bar and measure the circumference. Then divide by pi (3.14) to get the diameter.
For example, if the circumference reads 50 mm, the diameter is 50 ÷ 3.14 = 15.9 mm, which tells you it’s a 16 mm bar. This method works surprisingly well for a quick site check. A few tips to improve accuracy: wrap the tape in the smooth section between rib rows so the deformations don’t add extra length, pull it snug without stretching, and mark the overlap point precisely. Using a thin, non-stretch material like a cable tie or a strip of card stock gives a cleaner reading than a cloth tape.
The main weakness is that even a 1 mm error in the circumference translates to roughly 0.3 mm error in diameter. That’s usually fine for telling a 12 mm bar from a 16 mm bar, but it won’t reliably distinguish sizes that are only 2 mm apart (like 10 mm versus 12 mm) unless you’re very careful.
Estimating From Weight
If you have a scale and a tape measure, you can back-calculate the diameter from the bar’s weight per unit length. The standard formula used across the steel industry is:
Weight (kg/m) = D² ÷ 162
D is the diameter in millimeters, and 162 is a constant derived from the density of steel (7,850 kg/m³) and the cross-sectional area formula for a circle. To use this in reverse, weigh a known length of bar, calculate its weight per meter, then solve for D:
D = √(weight per meter × 162)
Say you cut a 1-meter piece and it weighs 1.58 kg. Multiply 1.58 by 162 to get 255.96, then take the square root: about 16. That’s a 16 mm bar. You don’t need to cut the bar, of course. Weigh any measured length and divide the total weight by that length in meters to get the per-meter figure.
This method is most useful when you have a bundle of unmarked bars and a platform scale. It’s overkill for checking a single bar, but it’s a reliable cross-check when markings are missing and you want to confirm what you measured with a caliper.
Matching Your Measurement to a Standard Size
Rebar is produced in discrete sizes, so your measurement should land close to one of them. In the U.S. system, common sizes and their nominal diameters are:
- #3: 3/8 inch (9.5 mm)
- #4: 1/2 inch (12.7 mm)
- #5: 5/8 inch (15.9 mm)
- #6: 3/4 inch (19.1 mm)
- #7: 7/8 inch (22.2 mm)
- #8: 1 inch (25.4 mm)
- #9: 1-1/8 inch (28.7 mm)
- #10: 1-1/4 inch (31.8 mm)
- #11: 1-3/8 inch (35.8 mm)
Metric sizes follow the ISO 6935 series listed earlier: 6, 8, 10, 12, 14, 16, 20, 25, 28, 32, 40, and 50 mm. If your reading falls between two standard sizes, round to the nearest one. A caliper reading of 15.7 mm is a 16 mm bar. A reading of 19.3 mm is a 20 mm bar. If you’re genuinely midway between two sizes and can’t tell, try a second method (circumference or weight) to confirm.
Practical Tips for Accurate Readings
Always measure in the valley between deformations, not across the raised ribs. This is the single most common source of error. On a heavily deformed bar, the ribs can add 1 to 2 mm to the apparent diameter, which is enough to make you misidentify the size.
Clean the bar before measuring. Scrape off rust scale, dried concrete, or mud with a wire brush or the edge of a trowel. Even a thin layer of rust can add a fraction of a millimeter, and dried concrete can add much more. If the bar is coated with epoxy (green-colored rebar), keep in mind that the coating is typically 0.18 to 0.30 mm thick on each side, adding up to 0.6 mm to a caliper reading. The nominal size refers to the steel core, not the coated outside.
Take multiple measurements. Check at least two spots along the bar’s length and two orientations at each spot. Manufacturing tolerances allow small variations, and averaging a few readings gives a more reliable number than trusting a single one.