Five-eighths inch on a ruler is a mark past 1/2 inch and before 1 inch. If the ruler divides the inches into eighths, 5/8 inch is the fifth mark on the ruler from the left side. If the ruler divides the inches into 16ths, it is the 10th mark.
Count the number of marks between an inch mark and the next inch mark to determine whether the ruler marks 8ths or 16ths. Inches are generally marked with a number and a longer line. Half-inches, 1/4 inches, 1/8 inches and 1/16 inches are marked with increasingly shorter lines. One-half inch is midway between the left end of the ruler and the 1-inch mark. On a ruler dividing the inches into eighths, 5/8 inch is the next line. On a ruler dividing inches into 16ths, it is two lines from the half-inch mark.