Most flexible copper refrigerator water lines have an outside diameter of 1/4 inch. Most flexible plastic water lines for refrigerators come with outside diameters of either 1/4 inch or 5/16 inch, reveals Darrins Plumbing Tips.

What size are most water lines?

3/4 inches The main pipes, which come off the main water supply, typically measure 3/4 inches in diameter. The branch lines, which feed individual fixtures, typically measure 1/2 inch in diameter.