Rebar size (bar number)
US rebar is numbered in eighths of an inch of diameter, so a #4 bar is half an inch and a #5 bar is five eighths.
What does rebar size (bar number) mean?
The numbering system is one of the few pieces of jobsite arithmetic that is genuinely convenient once learned. Residential footings and slabs typically use #3 through #5, while columns and heavy structural work go much larger. Bar size affects not only capacity but also minimum bend radius, spacing, and how much clearance the concrete needs to flow around it.
In practice
'Half-inch rebar' and '#4 bar' are the same thing, and suppliers use both terms interchangeably.