No, the listed diameter of a rebar does not include the ribs. A rebar's size is its nominal diameter, which the RSIC Manual of Standard Practice defines as the diameter of a plain round bar with the same mass per metre. A 10M bar is 11.3 mm nominal, but measured across the ribs it reads more, and measured across the core between ribs it reads a little less. Nominal sizes are what drawings, bar lists and design calculations use; the overall size over the ribs only matters for clearances.
This post explains what nominal diameter really means, why bars are ribbed at all, what the ribs are made of in terms of geometry, and the few situations where the actual outside size matters.
Nominal diameter: what the number on the bar means
A deformed bar is not a perfect cylinder, so there is no single "diameter" to measure. Standards solve that by defining size through mass. Steel has a known density, so a bar's mass per metre tells you its cross-sectional area, and the area tells you the diameter of a smooth round bar that would weigh the same. That is the nominal diameter.
For Canadian bar, CSA G30.18 sets these nominal values:
| Bar size | Nominal diameter | Nominal area | Nominal mass |
|---|---|---|---|
| 10M | 11.3 mm | 100 mm² | 0.785 kg/m |
| 15M | 16.0 mm | 200 mm² | 1.570 kg/m |
| 20M | 19.5 mm | 300 mm² | 2.355 kg/m |
| 25M | 25.2 mm | 500 mm² | 3.925 kg/m |
| 30M | 29.9 mm | 700 mm² | 5.495 kg/m |
| 35M | 35.7 mm | 1000 mm² | 7.850 kg/m |
| 45M | 43.7 mm | 1500 mm² | 11.775 kg/m |
| 55M | 56.4 mm | 2500 mm² | 19.625 kg/m |
The masses work out exactly from the areas at a steel density of 7,850 kg/m³: 100 mm² of steel weighs 0.785 kg per metre. The "M" number is a rounded version of the nominal diameter; the full size chart is in our rebar sizes in Canada guide.
US bars use the same idea. A #4 bar is "1/2 inch" because its nominal diameter is 0.500 in, based on its nominal weight per foot, not because it measures 1/2 inch over the ribs.
Nominal vs actual diameter: what a caliper shows
Put a caliper on a real bar and you get two different readings depending on where you measure:
- Over the ribs (outside diameter): larger than nominal. This is the envelope the bar needs to pass through a hole.
- Across the core between ribs: slightly smaller than nominal, because some of the bar's steel is in the ribs rather than the core.
The nominal diameter sits between the two. How far the outside diameter exceeds nominal depends on rib height and pattern, which vary between mills and bar sizes. That is why you will not find an official "10M rebar OD" in the standards: the outside diameter is a by-product of the rib design, not a controlled dimension.
When the overall size does matter, the RSIC manual gives approximate hole diameters for passing a bar through formwork or structural steel. They show how much room the ribs take up:
| Bar size | Nominal diameter | Approximate hole to pass the bar (RSIC) |
|---|---|---|
| 10M | 11.3 mm | 15 mm |
| 15M | 16.0 mm | 20 mm |
| 20M | 19.5 mm | 25 mm |
| 25M | 25.2 mm | 30 mm |
| 30M | 29.9 mm | 40 mm |
| 35M | 35.7 mm | 45 mm |
| 45M | 43.7 mm | 55 mm |
| 55M | 56.4 mm | 70 mm |
Those are clearance holes with some room to spare, not outside diameters, but they are a practical guide for form holes and steel plates.
Weighing beats measuring
If you need to confirm the size of an unmarked bar, weigh a measured length and divide by the length. Compare the result with the mass column above. Because nominal size is defined by mass, this is more reliable than a caliper, especially for telling apart close sizes such as 15M and US #5. Our guide on how to identify rebar size on site walks through markings, measuring and weighing.
Why rebar is ribbed
Reinforced concrete works because the steel and concrete act together. When the concrete cracks under load, the force it can no longer carry in tension has to pass into the steel, and that only happens if the bar cannot slip.
A smooth bar relies on adhesion and friction, which are limited and unreliable. The ribs, properly called deformations, give the bar a mechanical key. The concrete fills the spaces between the ribs, and each rib bears against the concrete like a small shear key. That interlock is what lets a bar develop its full strength over a predictable length.
This is built into Canadian design rules. The development lengths and lap splices in CSA A23.3 are written for deformed bars, and the RSIC suggested specification calls for all reinforcing steel, including column spirals, to be deformed as defined in CSA G30.18. A smooth bar with the same area would need very different anchorage. Our post on what rebar does in concrete covers the bigger picture of how the two materials share load.
The ribs also carry the bar's identification. The mill symbol, bar size and grade marking are rolled into the bar between the deformations, which is why markings show up as raised characters along one side.
Rebar rib dimensions: what the standard controls
A typical deformed bar has two kinds of ribs:
- Transverse ribs (lugs): the raised bands that wrap around the bar at an angle. These do most of the work in bond.
- Longitudinal ribs: on many bars, continuous ribs running along opposite sides of the bar.
CSA G30.18 controls the deformations with requirements that include their spacing, their minimum height and the gaps between the ends of the transverse ribs, scaled to the bar size. Mills design their own rib patterns within those limits, which is why bars from different mills look slightly different.
The deformation requirements also matter on site. The RSIC manual notes that rust or mill scale on rebar is acceptable without cleaning, provided the minimum dimensions, including the height of the deformations, and the mass of a cleaned sample still meet G30.18. Light surface rust is normal; bar that has lost enough steel to fall below those limits is not.
Rebar vs deformed bar vs plain bar
- Rebar is the everyday term for reinforcing bar. On Canadian drawings it means deformed bar to CSA G30.18 unless something else is specified.
- Deformed bar is the technical name for the same product, ribbed for bond.
- Plain (smooth) bar has no deformations. It is used where the bar is meant to slide rather than bond, such as slip dowels across pavement and slab joints.
- Deformed wire is the welded wire reinforcement equivalent; in mesh designations, a "D" prefix denotes deformed wire, as in D18.
Coated and non-steel bars follow the same nominal logic. Epoxy-coated bar is detailed at the nominal size of the steel under the coating. GFRP bar has a nominal area under CSA S807 and a larger measured area that includes its sand or wrap surface, so a GFRP bar can look bigger than a steel bar with the same nominal area.
When the actual size matters
For design, detailing and ordering, use nominal sizes. The overall size over the ribs matters in a handful of places:
- Holes in forms, plates and steel members for bars to pass through. Use the RSIC clearance sizes above as a starting point.
- Drilled-in dowels. The adhesive maker's table sets the drill size, and it is always larger than nominal; see rebar dowel hole size.
- Congested joints. Where bars cross or lap in tight spaces, the extra size of the ribs eats into clear spacing and cover, which is one reason detailers check congestion carefully.
- Chair heights. Cover is measured to the outside of the bar, so a few millimetres of rib height can matter in thin slabs.
- Couplers and threaded splices, which are made for a specific bar size; follow the coupler maker's requirements.
Getting the right bar size
We detail, cut and bend CSA G30.18 rebar from 10M to 55M, tagged by size and mark so there is no guessing on site. Send us your drawings for a free take-off and quote, or see the size chart on our rebar products page.
Frequently asked questions
Does rebar diameter include the ribs?
No. The listed diameter is the nominal diameter: the diameter of a plain round bar with the same mass per metre. Measured over the ribs, a real bar is larger; measured between them, it is slightly smaller.
What is the outside diameter of 10M rebar?
There is no single published outside diameter, because rib height and pattern vary by mill. The nominal diameter is 11.3 mm. For passing a bar through formwork or steel, the RSIC manual suggests a hole of about 15 mm for 10M.
Why is rebar ribbed?
The ribs (deformations) lock the bar into the concrete mechanically, so force can pass between them without the bar slipping. Canadian design rules for development and lap lengths assume deformed bar.
Is rebar the same as deformed bar?
In everyday use, yes. Rebar in Canada is deformed bar made to CSA G30.18. Plain (smooth) bars exist for special uses such as slip dowels at pavement joints, but they are not used as main reinforcement.
How do I find the real size of a bar with no markings?
Weigh a measured length and compare its mass per metre with the CSA G30.18 table. Mass is what the nominal size is based on, so it is more reliable than a caliper.
