Rebar Size Converter

Rebar (reinforcing bar) sizes use a numbering system based on diameter.

US bar sizes (imperial):

• Bar number = diameter in 1/8 inch increments
• Example: #4 bar = 4/8 inch = 0.5 inches diameter

Common rebar sizes (ASTM A615, imperial):

The elegance of the imperial system is that the number is the size: a #4 is 4/8 inch, a #8 is a full inch, so you never need a chart to know the diameter. It breaks down above #8, where the bars go to round areas instead: a #9 is exactly 1.00 in², a #11 is 1.56 in², and their diameters stop landing on eighths.

“Metric rebar” means two completely different things, and this is where projects go wrong.

Canadian M bars (CSA G30.18):

So a 10M bar is not 10 mm across. It is 11.3 mm, and the 10 refers to its 100 mm² area. Reading that number as a diameter is the single most common rebar mistake on a cross-border job, and it goes the wrong way: you assume the bar is thinner than it really is.

The area is what actually carries load, so that is what has to match when you substitute. A #4 is 129 mm² and a 10M is 100 mm², which sound like near neighbours and are 22% apart in steel area. Meanwhile a few pairs line up almost exactly and are genuinely interchangeable on area: #5 against 15M (200 mm² both, to the millimetre), and #11 against 35M (1006 against 1000). Reinforced concrete is engineered around the total steel area in a section, so a near-size swap quietly changes the strength of the structure. When in doubt follow the engineer’s spec, not what happens to be on the truck.