To determine what size staples to buy for your stapler, check the bottom of the unit or the underside of the staple carrier. If it is not stated there, check the stapler packaging or manual if available. You can also search for the stapler online to find staple sizes.
The most commonly used format for staple sizes has two numbers: gauge and shank length. The first number, the gauge, is a measure of how thick the staple wire is; the higher the number, the thinner the wire. The second number is the shank length in millimeters. Many staplers can take multiple shank lengths, and some recommend choosing the longest shank length to maximize the potential uses for your stapling needs.