Drill bits are sized, or measured, by the diameter of the solid metal shaft at the base of the drill bit's cutting flutes. For example, a 1/4 inch drill bit is milled from a piece of 1/4 inch diameter high speed steel. This size drill bit is measured by four different scales: English or SAE, metric, wire gauge and decimal.
Measure the diameter of a drill bits shaft with a dial or digital caliper. The caliper is an extremely accurate measuring tool. It can take measurements down to a few hundredths of an inch.
Convert the digital measurement into other measurement systems with a conversion table. The digital measurements can be converted to English measurements, metric system or wired gauge conversion tables, like the one on http://www.csgnetwork.com/drillsizeconvert.html.
Convert digital measurements into English or metric systems of measurements using the following simple equation. Digital measurements are in hundredths of an inch, so a measurement of .25 is the fraction 25/100, which is 1/4 of an inch. If using metric measurements, 1 inch = 2.54cm. So convert a digital measurement of .25 with the following equation: 25/100 inch * 2.54cm / inch = .635cm.
Drill bit size gauge cards are another simple tool for measuring drill bit sizes. Drill bit gauge cards are available at any hardware store or home centre, and are about the size of a 3 x 5 index card. The gauge card has multiple holes in different sizes. You can size drill bits accurately by putting the bit into the holes until the bit matches with the proper size.