The key to comparing staple sizes is realising that staples in the United States are identified using the "imperial" or customary standard, whereas the rest of the world uses the metric system. You can find certain types of staples (mostly for home and office staplers) for sale in the U.S. that are identified with using metric system measurements, such as "22/6," which refers to the wire's thickness and length in millimetres. However, most commercial and industrial staples are still measured in inches, so comparing or even buying staples often is a challenge.
Check the bottom of your stapler. Most manufacturers imprint the staple size that the product accepts on the bottom. The size is usually expressed in the metric, two-number form, such as "23/8." This means that the gauge or thickness of the staple is 23 (the higher the number, the thinner the staple) and the length of the shanks is 8mm. Thus, when shopping for staples, look for the size "23/8." Most home and office staplers adhere to the metric system of measurement, so this usually isn't a problem.
Convert this number into inches. One inch equals 25.4 mm, which often is rounded to 25, meaning that a 23/8 staple is a little less than a third of an inch long. The first number---the thickness of the wire---is the same in both the imperial and metric systems. Therefore, in the U.S. you can see staples identified as 22 gauge and 1 inch. This is especially true with industrial staples used in construction and commercial applications. If you have an old office stapler, the number on the bottom may be expressed in inches, such as "1/4 inch." You need to convert from imperial standards to metric in order to buy the appropriate-sized staple (something like a 23/6).
Find the imperial equivalent of a metric-identified staple by simply converting the other way. For instance, if an industrial staple produced in the U.S. is 16 gauge and 1 1/4 inches long, the metric equivalent in length would be 31.75 mm. This is particularly helpful if you're working outside the United States and need staples for your U.S.-produced staple gun. Fortunately, most staple guns and even office staplers today are manufactured with tolerances that allow for inexact conversion replacement staples to be used. Most staplers and staple guns even accept multiple sizes of staples.
Use staples that best approximate the size of staple your stapler requires, but err on the smaller size. For example, if the staple gun calls for 16 gauge staples with a 1-inch length, and your metric options are between a 22/25 and a 22/28 staple, use the 22/25 staple. It is slightly shorter than one inch and generally will work better than a 22/28, which is longer than one inch. Trial and error may, unfortunately, be required.
One exception to the entire staple measurement system is the No. 10 staple, which is internationally recognised and is used in small or "mini" staplers. Common home- and office-use staple sizes include 26/6, 24/6, 24/8, 13/6 and 13/8, along with No. 10 for mini staplers. Widely used heavy-duty staple sizes include 23/8, 23/12, 23/15, 23/20, 23/24, 13/10, and 13/14. Heavy-duty staplers are the type that usually have a punch handle for driving through thick stacks of paper.