Drill Chart: Bit Sizes [ Decimal, Number, Letter, Metric ]

drill chart bit sizes decimal metric letter number

It’s very convenient to have a drill chart around with bit sizes, so we thought we’d put one here for you.  It covers decimal, fractions, numbered drill bits, letter drill sizes, metric drill bits, and probably a few more things I forgot to mention.

For more on twist drills in general, see our twist drill article for all the different types and tips for holemaking with them.

Metric Drill Bit Sizes

Standard metric lengths (from standards like BS 32*) define metric drill bit sizes using this system:

For 0.2 to 0.98mm, sizes use N from 2 through 9:

  • N – 0.1 mm.  So for N = 2, it’s 2 – 0.1, etc.
  • N – 0.1 + 0.02 mm
  • N – 0.1 + 0.05 mm
  • N – 0.1 + 0.08 mm

From 1 through 2.95mm, using N from 10 through 29, we get:

  • N – 0.1 mm
  • N – 0.1 + 0.05 mm

For 3 through 13.9mm, with N from 30 to 139:

  • N – 0.1 mm

For 14 through 25mm, and N from 14 through 25:

  • N – 1 mm
  • N – 1 + 0.25 mm
  • N – 1 + 0.5 mm
  • N – 1 + 0.75 mm

There are also Reynard Series of preferred metric drill sizes.

Fractional-Inch Drill Bit Sizes

Fractional-inch sizes are in common use in the US and generally run from 1/64 inch up to 1 3/4 inch in 1/64″ increments.  After that they run in 1/32″ increments up to 2 1/4″, then in 1/16″ increments to 3″, 1/8″ increments to 3 1/4 inches, then there’s a 3 1/2″ size.

Decimal Equivalence Chart

A good drill size chart can also be used as a handy decimal equivalence chart as well as to convert decimal sizes to metric.

Number Drill Bit Sizes & Letter Drill Bit Sizes

The ASME B94.11M twist drill standard establishes number drill bit sizes from size 1 to 97.  In practice, you will seldom see number sizes past #80.

The numbered sizes are based on but unfortunately are not identical to the Stubs Steel Wire Gauge, which originated in 19th century Great Britain.

Letter Sizes are Imperial drill bit sizes designated A (smallest) to Z (largest).

Number and Letter Sizes are in common use in the US, and to a lesser extent the UK (rapidly moving to metric), but are no longer in use for most of the rest of the world which has gone fully metric.

Why So Many Sizes?

While it may seem logical that most holes are nice round numbers in size, certain operations benefit from a lot of intermediate sizes.  Specifically, reaming and tapping.  Don’t just pick the first tap drill size you come across, not even the one in our Drill Chart below.  Check out our article on tap drill sizes to find a much better way.

Drill Chart

drill chart bit sizes decimal letter number metric

Drill Chart PDF Download

 

 

Drill Bit Size Calculator

The thing about charts, especially wall charts, is they’re convenient for some things, but unwieldy for others.  I prefer software because it can search, copy values to where they’re needed, and generally save you effort.  That’s why I built a Drill Size Chart into my G-Wizard software:

drill chart bit sizes

With the checkboxes at the top, you can limit what you’re seeing to exactly what you want.  Just want end mill sizes?  No problem!

Plus, the box at top lets you quickly search to sizes near that one.

Here’s the best news: you get the Drill Bit Size Calculator free for life when you sign up for the 30-day free trial of our G-Wizard Machinists Calculator.  Matter of fact, you get a BUNCH of free calculators!

 

Like what you read on CNCCookbook?

Join 100,000+ CNC'ers!  Get our latest blog posts delivered straight to your email inbox once a week for free. Plus, we’ll give you access to some great CNC reference materials including:

  • Our Big List of over 200 CNC Tips and Techniques
  • Our Free GCode Programming Basics Course
  • And more!

Just enter your name and email address below:

Full Name
Email *
100% Privacy: We will never Spam you!