Letter drill bit sizes are a system used to label drill bits from A (the smallest) to Z (the largest). They cover a range from about 1/4 inch to about 1/2 inch in diameter. Number drill bit gauge sizes range from size 80 (the smallest) to size 1 (the largest), and then letter gauge sizes pick up from A to Z after that.
So the lettered bits are basically larger than the numbered ones
Letter Drill Bit Sizes and Uses
The numbered sizes are based on but not identical to the Stubs Steel Wire Gauge, which came from 19th century Great Britain. Instead of continuing with numbers past size 1, the system started using letters. If letters had not been used, an “A” drill would be “0” gauge, “B” would be “00” gauge, and so on.
A letter “J” drill would therefore be a “0000000000” gauge drill. That would be a mess, so letters made things much simpler.
There is no consistent dimension increment between sizes. In fact, the “F” drill and 1/4 inch are basically the same size. Letter drill bits offer additional intermediate diameters for those who need to be precise.
They are most often used for drilling holes in metal. The more precise the drilling requirement, the more important it is to pick the right size category, whether that is number, letter, fractional, decimal, or metric.
A chart showing letter drill sizes lists the diameters. For example, A is 0.234 inches (5.94 mm), B is 0.238 inches (6.05 mm), E is 0.250 inches (6.35 mm), F is 0.257 inches (6.53 mm), and the letter Q size drill bit is 8.43 mm in diameter. A letter O drill is 0.316 inches.
Gauge bit sizes run from 107 to 1 and A to Z, where Z equals 0.413 inches (10.49 mm).
When a project calls for a gauge bit size, it is generally fine to use an imperial or metric size instead. Online charts can help with conversions. Metric sizes fit in between number, fractional, and letter bits on the chart, but they are not the same size and are not interchangeable.
Letter drill bits come in different materials. Drill Bit Warehouse carries letter sizes in High Speed Steel, Cobalt, and Solid Micro-grain Carbide. HSS means high speed steel, which makes good general-purpose drills.
High production grade M42 Cobalt drills are made with 8% cobalt and ground from a solid blank. This type is used in high tensile material such as stainless steel and titanium. Sizes 1/16 inch and larger have a 135-degree split point.
Letter drill bit sets also feature products with 135-degree split points. F drill bit is the tap size for 5/16-18 thread and the clearance size for 1/4 inch.
