Here’s another comment we hear a lot ……

“Okay … I understand the concept and progression of fractional drill bits … but what in the world are numbered bits and lettered bits?”

Simply put, there is a whole world of sizes that exist tucked in between the fractional sizes. For example …. The normal drill index goes from 1/8 to 9/64 to 5/32.

There are actually 8 more sizes of drill bits that are readily available between the 1/8 and 5/32.

Numbered bits are related to wire size, so the larger the number, the smaller the size. The numbering sequence starts with the smallest drill size #80 which is at a decimal size of .0135. as the numbers decrease, the drill bit size increases till it reaches a number 1 drill bit which is equal to .2280.

Lettered drill bits start with a letter A which is equal to a .2340 decimal size. They progress in size till they hit the letter Z which is a decimal size .4130

If you go to our website and click on the technical chart tab, then click on the Decimal / Metric tab, you can see exactly how how all the numbered, fractional, metric, and letter sizes interact. 

Posted by Greg Corby