Notice where the number zero is located on your ruler. Some rulers have zero at the very tip. Others will print zero about 1/4 inch into the ruler. You must take your measurement from zero or it will be inaccurate. Each printed number on your ruler denotes the number of inches it is from zero. The smaller increments are divided by 1/4 inch and the smallest increments between the numbers are 1/8 inch.
Lay your label on a flat surface. Lay your ruler on top.
Line up the left edge of the label with zero on the ruler. Write down the number on your ruler that marks the right edge of the label. Not all labels are exactly multiples of whole inches in length. If your label's edge is marked by an unnumbered increment, do the following: Write down the nearest number to the edge and count the increments until you are at the end of the label. If the number is 4 and you count to the second biggest increment, the length of your label is 4 and 1/4 inches.
Place your ruler on top of the label. Line up the zero with the bottom edge. Write down the number that marks the top edge of your label. Count the increments as outlined in Step 3 if necessary. This measurement is the width of your label.