Understanding Drill Bit Sizes

Nothing could be simpler than drilling a nice clean hole in something. Why then, is it so hard to figure out drill bit sizes?

Well, one reason is that there are two different measuring systems in the world, U.S. and metric. A second reason is that some obsolete standards are still floating around out there like icebergs waiting to wreck your next project.

The only thing that you can count on is that the size of a drill bit will be stamped on the shank-the end of the bit that attaches to the drill. But what does that numbers mean?

United States (US) vs. Them
If you're purchasing drill bits in the United States, chances are they will be sized in fractions of an inch. Standard drill bits in the US start at 1/64-inch and increase in size by increments of 1/64 inch up to 1 inch in diameter.

One disadvantage of this system is that smaller bits have a large variation in size from one bit to the next. A 1/64-inch drill bit is half the size of a 1/32-inch drill bit. While this might not seem like a big difference, it makes precision drilling next to impossible.

Metric For Everybody Else
The British Standard introduced metric drill bit sizes in 1959. Metric drill bits range in size from 0.2 mm to 25 mm. Most metric drill bit sets come in increments of 0.5mm, but smaller bits may found with increments as small as 0.1 mm.

Number and Letter Gauges
A drill bit sizing scheme that is fading in popularity is gauge numbered drill bits. This system starts at 80 for the smallest drill bit (0.104 inch) up to 1 (0.228 inch). Larger drill bits use letters from A to Z. Converting gauged drill bits to standard diameters requires the use of a table or sizing chart.

To make things even more confusing, this scheme is similar to (but not to be confused with) standard wire gauges. It is a common mistake to refer to gauge-sized drill bits as "wire gauge" bits.

The Hole Truth
Unless you're a professional or have a specific need, your best bit is to purchase a drill bit set that contains a variety of sizes.

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