Drill bits come in various sizes, from 1/16 inch or smaller, all the way up to 1 inch. When manufactured, the bits are gauged by measuring the outside edges, also known as the outside diameter. After manufacturing, the bits are placed in clearly-marked slots in cases, but you may have found a bit that doesn't seem to belong. You may have purchased one or more individual bits and may not be sure what sizes they are when looking at them later. You will need to determine the sizes of such bits when specific-sized holes must be drilled.
Place the drill bit of unknown size into the drill, tightening the bit into the drill head tightly with a drill chuck, or until hand-tight if the drill contains a chuckless drill head.
Mount a vice to your flat work surface and place your small piece of scrap wood into it securely so that it doesn't move when pressing firmly upon the block of wood.
Plug in the drill and turn it on to your desired speed and drill a hole into the scrap wood. Depth of this hole doesn't matter, but be sure you are holding the drill steady in a straight vertical line as you are drilling so the resulting hole isn't widened significantly from movement.
Remove the block of wood from the vice and lay flat on your work surface with the hole facing upward.
Place a ruler across the hole so the flat edge stretches across the widest portion of the hole, from one side to the other. Measure the hole by counting the marks: sixteenths, eights, quarters, halves, inch.
Calculate the size. If you are counting sixteenths, for example, draw a line on paper and place the number 16 below the line and the number of sixteenths marks you count on top of the line. If the number 16 can be divided by the top number evenly, do so. If, say, you measured 2/16, divide the top by 2 and the bottom by 2, which would give you 1/8. Therefore, your drill bit would be 1/8 inch. Likewise, if you were measuring eighths and you counted three eighth marks, the drill bit is 3/8 inch since the top number cannot be divided equally into the bottom.
After you have measured and determined the size of your bit, consider placing your bit into a small bag with the measurement written on a piece of paper inserted along with it for easy identification next time. If you can afford it, and you have an understanding in how to convert decimal readings to fractions, purchase a quality micrometer for the most accurate and fastest means of measuring an unknown bit size.