How to Read a Micrometer Caliper

  • Written By Eric Crouch on March 5, 2020
    Last Updated: January 31, 2021

Reloading is an excellent way to create your own ammunition for personalized bullets or save money in the long run if you take your gun to the range almost every weekend. But you’ll need a key tool in order to make sure that your brass casings are the right size all the way around and are uniformly thick. For these instances, you need a micrometer caliper.

But too few gun hobbyists know how to read a micrometer caliper. In this guide, we’ll break down the various components of a standard micrometer caliper and show you how to manipulate its simple mechanisms and obtain accurate measurements for your brass casings.

Let’s get started


Before we get into how to read a micrometer caliper, we should go over the various components and parts of the caliper so you know what we’re referring to.

For starters, a “micrometer caliper” is a bit of a misnomer. Micrometers allow you to capture greater precision than standard calipers but are limited to smaller ranges of lengths as a result. So while their designs are similar to calipers, they have a few major differences in the construction.

To begin, imagine or hold a micrometer so that the longer or thinner end is positioned to the right, with the U-shaped part going to the left and down. The rightmost part of the micrometer is called the ratchet stop. To the left of that is the thimble and sleeve, which will be turned in order to calibrate the micrometer. The sleeve will have a series of numbers and markings along the cylindrical shape.

Right after the markings is the lock lever, which some micrometers may or may not have. This locks the micrometer into place to prevent you from accidentally shifting your measurement as you move it around. To the left of that is the spindle: the long bar of metal with a small gap between it and a smaller bit of metal called the anvil.

Finally, the U-shaped piece is called the frame, while some micrometers have thermal insulators or markings dictating the range of measurement for that particular tool. Some micrometers are longer or more generous than others, but for reloading purposes, you want a micrometer as precise and short as possible.

Using and Caring for a Micrometer

For reloading, micrometers are most commonly used for measuring the interior of a brass casing. They can be used to ensure that the thickness of a brass casing around the neck is consistent all the way around. This is important since it ensures casing integrity and prevents you from ruining your brass casings when you move on to the press portion of the reloading process. That’s why our tutorial will go over inside micrometers rather than ones calibrated for outside measurements.

Regardless of micrometer type, all micrometers require that you unlock the locking lever before trying to rotate the thimble. The thimble, when rotated, causes the spindle to approach the anvil and close the gap between them. The distance is measured by the sleeve, which you can then translate into actionable distances or measurements in regard to your casing interior.

Before you get started using a micrometer, you should clean both of the measuring faces with a clean cloth. This means wiping down both the spindle and the anvil carefully. This prevents contaminants from being drawn into the sleeve, which can potentially compromise your efforts. The thimble also usually has a textured grip, which allows you to tighten or loosen the thimble to a greater degree of precision. Use this portion when turning the thimble to avoid going too far.

Micrometers have thermal insulators on their bottom handle because the heat from your hand can actually influence the measurement of your micrometer. You’ll always want to hold the tool from this thermal insulator; it usually looks like a black or blue cover for the handle.

Similarly, make sure to never leave your micrometer exposed to the sun. Always let it cool down before using it to measure the interior of a brass casing.

Calibrating a Micrometer

The vast majority of reloading micrometers will come with an adjustable wrench or tool; this might look like an oddly shaped pin or key. This is used to help calibrate your micrometer, which is necessary before beginning any measurement attempt.

To zero a micrometer, you’ll adjust the index line with the “0” mark on the thimble scale. The index line is located directly to the left of the thimble scale or just before the locking lever.

  1. Set the anvil and spindle closed by turning the thimble appropriately. Use the ratchet if necessary to make sure the pressure is consistent and the two pieces stay together.
  2. Lock your spindle using the locking lever.
  3. Take your adjustment wrench and use the bigger side; it should be very easy to tell which is which.
  4. There’s a small hole on the sleeve barrel in which you can place the end of the wrench or jaw. This will allow you to rotate the wrench.
  5. Rotate the wrench carefully until the index line is lined up with the “0” mark on the thimble scale.
  6. Unlock the locking lever.
  7. Next, check your micrometer several times to make sure that the zero doesn’t slip from the index line. This is as simple as measuring a few objects or even brass casings. If this works, your micrometer is now ready to use.

Using the Micrometer

After zeroing your micrometer and making sure that all the components are working and that the tool is not too hot, you can begin to use it to measure the interior neck consistency of a brass casing.

If you’ve never used a micrometer before, take this opportunity to try out several different brass casings and get a feel for the tool before attempting to accurately measure neck thickness. You’ll definitely want to become practiced at using this tool before you take its measurements at face value. 

In a nutshell, the way a micrometer works is that the thimble is turned by the operator, which travels it down toward the bottom of the sleeve. This hides values on the scale, which are clearly marked in a horizontal fashion on the sleeve surface. Think of it like a screw turning downward over time. 

There are several different types of micrometer calipers available. For reloading, ball micrometers are commonly used to measure the interior of casing neck thickness. But standard micrometer calipers can also be used to measure the diameter of brass casings or the thickness of their exteriors to some extent. The procedure for using a caliper works the same way for each with only minor adjustments.

  1. If using a ball micrometer caliper, place your brass casing on the anvil by sliding the neck around the measurement surface. Think of it like the anvil sticking to the inside of the brass casing neck. Alternatively, if you are using a standard micrometer caliper, you’ll place the brass casing or whatever other object you want to measure between the anvil and the spindle. The basic concept is the same; the micrometer uses the distance between the two measuring surfaces to give you a number.
  2. Turn the thimble so that the spindle presses against the outside of the case neck. Turn the thimble until you can’t any longer or meet resistance.
  3. At this point, the measurement surface is on the sleeve should tell you a particular value or number. This is how you gather measurements. 

Outside vs. Inside Micrometer

This is where things get a little tricky. Both outside and inside micrometers (including ball micrometers) use main scales and thimble scales graduated by very small increments. In other words, the pitch of a micrometer screw thread is 40 threads per inch. Revolving the thimble once will advance the spindle face forward or away from the anvil about 1/40 of an inch or 0.025”. This also means that every fourth line you turn the thimble is a 0.1” increment. This is usually indicated by the 0.1” lines being longer than the others.

With inside micrometers, the numbers on the scale are in reverse order, going from high to low instead of low to high. But the basic principle is the same.

  1. As the thimble is turned, the thimble travels toward the bottom of the sleeve and hides values beneath its surface. This allows you to read the micrometer.
  2. You look at the value shown on the main scale, or the first number hidden by the thimble. In other words, look at the number closest to the thimble on the sleeve surface.
  3. This will cause one of the numbers on the thimble to align with the index – the long line that splits down the middle of the sleeve all the way from the beginning. Remember, you zeroed this at first.
  4. For instance, if you turn the micrometer thimble until the first number hidden is 0.4 inches, then you would look at where the index line arrived on the thimble scale. Say, for instance, that it lined up with 0.020”.
  5. You would then combine the two values for your total measurement, or 0.420”.

That’s it! Using and reading a micrometer is really quite simple once you get the hang of it. For reloading purposes, measuring the interior of a brass casing will have you turning the thimble until it stops, then reading the values on both the thimble scale and the sleeve scale. When combined, this gives you the total thickness of the case neck.

For Reloading

That’s all well and good, but how do you make sure that neck thickness is consistent for your brass casing?

  1. When the brass casing is either placed between the anvil and spindle or placed on the ball anvil, keep the pressure on the spindle consistent as you try to turn the casing with your fingers. You may need to use a special mount or holding tool in order to properly do this.
  2. As you turn the casing, it may or may not actually spin based on the neck thickness. For instance, if the casing is not uniformly thick all the way around, then it should spin somewhat and require you to turn the thimble even tighter in order to lock it into place once again. You can record these values on a piece of paper or calculator.
  3. Doing this will give you a rough estimate of the general thickness of your casing’s neck. 
  4. What you’re looking for are brass case necks that are uniformly thick all the way around. 

When you measure the interior of a brass casing neck, discrepancies of around 1000th of an inch usually mark that casing as too compromised to be reloaded at that time. You’ll need to trim the casing down because you can’t add new brass matter to a case. In other words, you can’t ever make a case thicker. You can only trim down a case neck so that all the sides are as thin as the thinnest section.

Of course, this may cause problems depending on the type of ammunition you want to reload and whether that neck thickness is already too thin. Basically, if the casing is already pretty thin at its thinnest point, it may not be workable to shear it down to an even thinner consistency all the way around. Such a casing would be compromised and may not be a good candidate for reloading because it might crack when fired.

However, using these tools is a great way to avoid these mishaps and ensure that the brass casings you put through the reloading process will hold up under duress.


Ultimately, that’s all there is to reading a micrometer caliper. It seems a lot more complex than it actually is and it becomes very easy and quick once you get the hang of things. Before you know it, you’ll be able to measure brass casings and their neck thicknesses rapid-fire, one after the other. Good luck!

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