How to Identify Rebar Size on Site: Markings and Measuring

The quickest way to tell rebar size is to read the markings rolled onto the bar. Canadian bar made to CSA G30.18 carries the mill symbol, the bar size number (10, 15, 20 and so on) and the grade mark, repeated along one side about every 1 to 1.5 m. If the markings are missing, worn or on a short offcut, measure the diameter with a caliper or, most reliably, weigh a piece of known length and compare its kg per metre with the size chart. Paint colour is not a size code.

This guide walks through each method, the traps that catch people (especially US bars on Canadian sites), and what to check before a mixed-up bar ends up in the forms.

Read the rolled-on markings first

Every deformed bar made to CSA G30.18 is marked during rolling. Per the identification requirements published in the RSIC Manual of Standard Practice, the markings appear in this order:

1. Mill symbol, identifying the producing mill (on all grades).
2. Bar size, shown as the designation number (on all grades).
3. Grade mark: either the number (400 or 500) or offset lines (one line for 400, two lines for 500), with a W for weldable grades.

The markings repeat at intervals of about 1 to 1.5 m along the bar, or as close as 0.5 m on bar made from coil. Turn the bar until you find the side with the stamps, and wipe off mud or mill scale. On a short offcut you may only get one partial set, so check both ends of the piece you cut from.

What the size number means depends on where the bar was made:

That first row is the one that causes real mistakes. A bar rolled with "10" could be a Canadian 10M, a US soft-metric #3 that is noticeably smaller, or a US inch-size #10 with nearly three times the diameter. The mill symbol tells you which country the bar came from. Our guide to Canadian vs US rebar sizes covers the full conversion and the soft-metric naming trap.

For what the grade marks mean, see rebar grades in Canada.

Measure the diameter

When the markings are gone, a caliper settles most questions. The sizes that matter on most jobs are far enough apart that a careful reading tells them apart.

Canadian nominal sizes, from CSA G30.18:

The nominal diameter is the diameter of a plain round bar with the same mass per metre. A real deformed bar does not measure exactly that:

• Between the ribs, the core reads a little under nominal.
• Over the ribs, the reading is a little over nominal.

So take both readings. The nominal size of the bar sits between them. A 10M bar will never be confused with a 20M this way, but a 10M and a US #4 (12.7 mm), or a 15M and a US #5 (15.9 mm), can be too close to separate by caliper alone.

A tape measure or a ruler held across the end of the bar is not precise enough for anything beyond telling 10M from 20M. Sheared ends are often slightly deformed, so measure a few centimetres back from the cut.

Weigh a cut piece of known length

Weight is the most reliable check on a bar with no markings, because rebar is sold and specified by its mass per metre.

1. Cut or find a straight piece and measure its length accurately, for example 500 mm.
2. Weigh it on a scale that reads to at least 10 grams.
3. Divide the weight in kg by the length in metres.
4. Compare the result with the mass column above.

Example: a 500 mm offcut weighs 0.79 kg. That is 0.79 ÷ 0.5 = 1.58 kg/m, which matches 15M (1.570 kg/m). A 20M piece of the same length would weigh about 1.18 kg, and a 10M piece about 0.39 kg.

Rolling tolerances mean a sample will not match to the gram, but adjacent Canadian sizes differ by a wide margin, so the answer is usually obvious. The exception is telling Canadian bar from US bar of nearly the same size. A US #5 weighs about 1.55 kg/m against 1.570 for a 15M, which is too close to separate by weight. That is a job for the mill symbol, the delivery paperwork or the supplier.

For converting lengths to weight across a whole order, our rebar size chart has the full table to 55M.

Colour codes: what paint and coatings do and do not mean

People often ask what size "yellow rebar" or "red-ended rebar" is. In Canada there is no national colour code that ties a colour to a bar size. Colour on rebar usually means one of these:

• Paint on bar ends or bundles. Mills and fabricators paint ends to mark a heat, a grade, a length or an order. RSIC practice describes straight bars being bundled, tagged or painted before shipping. The meaning is set by that mill or shop, not by a standard, so the same colour can mean different things from different suppliers.
• Coloured tags. Fabricators may use different tag colours for different parts of a structure, such as footings, walls and slabs. The colour groups the bundles; the text on the tag gives the size.
• Coatings. Epoxy-coated bar is a manufacturer colour, often green for bar coated straight and purple or grey for bar bent before coating. Glass fibre (GFRP) bar comes in a range of colours depending on the maker and has its size printed on the bar instead of rolled on.
• Safety caps. Yellow, orange and other plastic caps on exposed bar ends are safety covers, not a size marker. Our post on rebar caps in Ontario covers when exposed bar has to be protected and why plain mushroom caps are not impalement protection.

If a colour is the only clue you have, ask the supplier what their marking means, then confirm with a caliper or a scale.

Tags, bar lists and mill certificates

On a job with fabricated bar, you should rarely have to identify a size from the steel itself. Each bundle from a fabricator carries a weather-resistant tag showing the bar mark, size, grade, number of pieces and length, and the tag matches a line on the bar list and the placing drawings.

When bars get separated from their tags:

• Match the shape and length to the bar list. A bent bar with a known shape code and dimensions is usually unique on the list.
• Check the mill test reports. Fabricators keep mill certificates traceable to the heat of steel, and the heat links back to size and grade. If a specification calls for 400W, the certificates are the proof.
• Ask the fabricator. They can tell you which bundle a loose bar most likely came from.

Our guide to reading rebar shop drawings explains how bar marks link the tag, the list and the drawing.

When it matters most

A wrong size is not a cosmetic problem. A 10M placed where the drawings show 15M has half the steel area, and a US #3 marked "10" has less area again. The places where mix-ups most often happen:

• Leftover bar from another job, especially offcuts used for dowels or extra bars.
• Imported or US-made bar with soft-metric numbers.
• Mixed bundles where tags have been cut off during placing.
• Renovation work, where an existing bar is exposed and someone needs to know what is in the wall or slab. In that case the engineer decides what testing, if any, is needed.

If you cannot confirm a size with confidence, do not place it in structural work. Set it aside and get a positive identification.

Getting the right bar on site

The simplest protection is bar that arrives cut, bent and tagged to a bar list, so every piece is identified before it reaches the forms. We detail, cut, bend and tag 10M to 55M rebar to your drawings and deliver across the GTA and Ontario. Send your drawings for a quote, or see our rebar products and size chart.