Rebar reinforces concrete in tension. Bar sizes are numbered in eighths of an inch — a #4 bar is 4/8, or 1/2 inch, in diameter. This chart covers the common sizes and uses.
| Bar size | Diameter | Typical use |
|---|---|---|
| #3 | 3/8 in | Slabs, sidewalks, patios |
| #4 | 1/2 in | Walls, footings, driveways |
| #5 | 5/8 in | Foundations, columns |
| #6 | 3/4 in | Heavy footings, columns |
| #7 | 7/8 in | Structural members |
| #8 | 1 in | Heavy structural work |
The number is the size. Bar number divided by 8 gives the diameter in inches. For most DIY slabs and footings, #3 and #4 cover the job.
See Concrete Cure Times and How to Pour Concrete.