Visually inspect the ruler to identify whether the ruler is divided into eighths or sixteenths. Rulers that have sixteenth of an inch markings will have line markings of five different lengths. Rulers that only measure to eighths of an inch will have four different lengths for the line markings.
Label the longest line markings on the ruler with whole inches, if these marking are not already labeled. Leave the first line unlabelled or label it with 0. The second long line is labeled as 1, the third line as 2. Continue to label the inch markings to the end of the ruler. The last long marking at the end of a standard foot-length ruler will be 12.
Label the second longest line markings on the ruler as half inches. There will be 12 of this category of line markings on a standard foot-length ruler. Label each marking with 1/2.
Label the third longest line markings as quarter inches. There will be two of this category of marking between every two whole-inch markings. With the lower-number inches end of the ruler held in your left hand, the quarter marking that lies to the left of the 1/2-inch line marking found between every two whole-inch markings should be labeled as 1/4. The quarter marking that lies to the right of each 1/2-inch marking should be labeled as 3/4. Repeat these labels for each inch along the length of the ruler.
Label the fourth longest line marking as eighth-inch markings. There will be four of this category of marking between every two whole-inch markings. The first of these markings after a whole-inch marking should be labeled as 1/8. The second is labeled as 3/8, the third as 5/8 and the last of the four is labeled 7/8. Repeat these labels for each inch along the length of the ruler.
Label the fifth longest line marking as sixteenth-inch markings. There will be eight of this category of marking between every two whole-inch markings. The first of these markings after a whole-inch marking should be labeled as 1/16. The second is labeled as 3/16, the third as 5/16, the fourth as 7/16, the fifth as 9/16, the sixth as 11/16, the seventh as 13/16 and the last of the eight markings is labeled 15/16. Repeat these labels for each inch along the length of the ruler.