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February 28, 2010


Calipers can be the most important tool in your toolbox. Yes, they might cost $90-$125 for a good pair, but they will last you a lifetime. Shown here are my 6” Starrett dial calipers that were a gift from my father when I left home to go to college. That was 22 years ago, and they are still getting daily use in my workshop…

Calipers are used to measure things with some accuracy. Most sets have a range of 0-6”, but you can buy them up to 24” in length. Digital calipers are available from many sources; they have a battery and a readout just like a calculator. They can be converted from millimeters to inches at the touch of a button. I might someday buy modern calipers, but for now I just know that there are 25.4mm in an inch, and use simple division or multiplication to switch from one unit of measure to the other.

Mechanical calipers work by using a simple rack and pinion setup. There is a rack (long, straight gear) along the body of the calipers. Behind the dial is a pinion (small round gear). As the head is moved, the pinion must rotate, which spins the needle. Pretty simple. Modern digital calipers use linear encoders, which are slightly more accurate, and are protected from metal chips that can clog the gears.


One thing to keep in mind when using calipers: they are accurate to about .001” You may be tempted to use that digital readout that goes out to 4 digits, but the basic mechanical limits of calipers keep them useful and accurate only to one thou. If you need better accuracy or resolution, use a micrometer.

Calipers can be used to measure the outside dimensions of a part. In this photo, the fiber gasket is .802” in diameter.


By using the inner jaws of the caliper, you can measure inside dimensions. The ID of the washer is .552”


Another use for calipers is to measure depth. The back end of the caliper can be placed against a lip, while sliding the movable jaw. The height of this steel part (covered with blue Dykem for the photo) is about .388”



  • Don’t use those nice sharp points or edges to scribe lines on your workpiece.
  • Don’t abuse your calipers.  Keep them clean and stored inside their box when not in use.
  • Don’t use compressed air to blow chips out of the rack and pinion.  It will just force them in there tighter.
  • Don’t loan them to a friend, you won’t get them back.  🙂

Retailers that sell calipers:


· Rutland Airgas

· Enco

Manufacturers of quality calipers:

· Starrett

· Mitutoyo

· Brown and Sharp

Cheap plastic calipers are also available for $15-$20 from places like Harbor Freight. Buyer beware.

One Comment leave one →
  1. April 28, 2013 10:22 am

    Hi there, just wanted to say, I loved this post.

    It was helpful. Keep on posting!

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