To learn how to measure perforation you must first understand what stamp perforation is and why it is used. Great Britain was the first country to issue stamps for postage. It all started with the Queen Victoria One Penny Black postage stamp in 1840. It was printed in sheets of 240 stamps and became very popular as a way to pay for postage. But the only problem was that a person needed a scissors or a knife to separate the stamps. It soon became apparent after a while that there must be an easier way to separate the new stamps. It was decided in 1854 to cut tiny holes around the stamps so they could be separated by hand without damaging the stamp. These tiny holes are called perforation and can be measure by a stamp collector. The number of perforation counted within a 2cm (0.78") length is the perforation size. For example, if there are 11 perforation in the 2cm length then the perforation size is 11.
Many gauges are available to help you make this type of measurement. An example of one is shown below. This perforation gauge has pins that are space evenly across a 2 cm area. Line up the stamp perforation holes with the gauge pins and this is the stamp perforation size. Easy huh! But wait, many stamps produced today have perforation sizes in decimal. For example, 11.2 could be a perforation size for a stamp. Now I'm sure your thinking how do I get an exact measurement using the pin gauge shown below. Well the only way would be to estimate the perforation between two pin sizes. Or you could purchase another gauge that can do it for you. Click this link Linns Multigauge to view the Linns Multigauge. It can measures perforation sizes in .1 increments.
Another way to measure perforation is by using a 2 cm rule and counting the perforation yourself. Click this link Centimeter Gauge to see a gauge that has a 2cm gauge. Remember that when you measure perforation size, start from the center of the first tab before the perforation hole and count to the center of the last tab after the last perforation hole using the 2 cm length as your reference.
Now that you know how to measure perforation. You will be able to identify your stamps in stamp catalogs more accurately when a stamp has been printed with multiple perforation sizes. Perforation size is a very important specification when identifying a stamp. Some stamps have more then one perforation size and are called compound perforation. For example, a stamp with a Compound Perforation size would be listed in a catalog as 11 x 12. The first number 11 represent the perforation size for the top and bottom of the stamp, and the second number 12 represents the perforation size of the right and left sides of the stamp. That's it, you are now ready to start your own perforation measurements. And if you need a perforation gauge just click this link Linn's Muti-Gauge