How to Use and Read an Outside Micrometer

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Bolts in hand (Photo: @kinek00 via Twenty20)
Outside micrometers or “metric micrometers” are used to measure the thickness or outside diameter of small parts. That means, if you need to figure out a precise measurement (we’re talking within 1/1000th of an inch) an outside micrometer is the tool for the job.
You don’t need to be a professional to use an outside micrometer. In fact, they will probably come in handy when you’re trying to do odd jobs around the house. For example, outside micrometers are a great way to identify replacement parts. Let’s say your garbage disposal has a broken screw, you can measure the diameter of the broken screw with a micrometer to determine the proper replacement size.
Larger outside micrometers are used to get accurate measurements of the outside diameters of pipes and fittings, which can be a lifesaver during plumbing repairs.
Are you thinking of adding an outside micrometer to your toolbox? Here’s everything you need to know about using and reading one.

How to use an outside micrometer

An outside micrometer resembles a C-clamp. The C-shaped part is called an anvil, and the retractable prong is a spindle.
To use an outside micrometer, position the object you want to measure between the anvil and the spindle. To open and close the spindle, rotate the dial (or ratchet) at the base at the handle.
Tighten the spindle until the object is grasped between the spindle and anvil; however, don’t make it so tight that you can’t remove the object from the micrometer with relative ease. Lock the spindle in place by turning the lock lever (which resembles a flathead screw top and is located at the base of the spindle). Pull out the object and set it aside.

How to read an outside micrometer

At this point, the micrometer should be locked into a measurement. First, determine which scale you should use. For household use, a 0.001-inch scale is standard. If that scale is too big, you might require a vernier micrometer that can provide measurements to one ten-thousandth of an inch.
For a 0.001-inch scaled tool, which measures to 1/100th of an inch, you will obtain three numbers and add them together. Two of those numbers are located on the sleeve, which has numbers that go along the handle, closest to the spindle. The last number is located on the thimble, which is the larger part that wraps around the micrometer handle.
To calculate the measurement, read the large number that is closest to the sleeve. If that number is 2, then the first numeral in your addition equation is 0.2 inches. Then, look to any notches that follow the large number you just noted. Each notch is equivalent to 0.25 inches. If there are three notches that follow the large number, then the second numeral in your equation is 0.075 inches. Finally, look to the thimble, and pinpoint the number that aligns with the horizontal line along the length of the sleeve. If that number is 4, then the third number in your equation is 0.004 inches.
Each number you read on the micrometer is separated by a tenth of a place. Add your three numbers together for a final micrometer reading. In this example, our reading is 0.2 + 0.075 + 0.004 = 0.279 inches. If you decide a standard micrometer is too much trouble (ugh, math!), there are digital micrometers available that will complete the equations for you.