Rebar Size Chart & Rebar Diameter Chart

Reading the rebar diameter chart is arithmetic: the bar number is the diameter in eighths of an inch. #3 rebar diameter is 3/8 in (0.375 in, 9.5 mm), the number 4 rebar diameter is 1/2 in (0.500 in, 12.7 mm), and #5 is 5/8 in. Some suppliers still call this a rebar gauge chart, but rebar has no gauge system — unlike wire, where a larger number means a thinner conductor, a larger rebar number always means a thicker bar.

Steel rebar sizes are paired with a grade that states the yield strength in ksi. Grade 60 is the default for most concrete reinforcement in North America: 60,000 psi yield strength, with tensile strength around 90,000 psi. Grade 40 survives in light residential work, and Grade 75 or 80 appears in heavily loaded columns and mats. The grade is rolled into the bar along with the mill mark and bar number, so a bar can be identified on site without paperwork.

The common types of rebar differ by coating rather than size. Plain carbon steel bar is the baseline. Epoxy coated rebar adds a bonded green film for bridge decks, parking structures and anywhere de-icing salt reaches the steel. Galvanised and stainless bars go into marine and long-service work, and glass-fibre bar is used where the reinforcement must be non-conductive or non-magnetic. All of them use the same designation numbers and the same nominal diameters.

On a construction project, size selection follows the required steel area, not habit. A load bearing footing or slab is specified as square inches of steel per foot of section, which is why the spacing table above matters more than the bar list: #4 at 12 in on centre and #5 at 18 in on centre deliver almost the same area. Substituting a smaller bar at the same spacing reduces area and can compromise structural integrity, so any change belongs with the engineer of record.