How to Measure Stuff with Calipers

By CTD Mike published on in How To

Here at Cheaper Than Dirt! we try to include accurate measurements of the critical dimensions of our gun parts and accessories. For example, let’s say you want to put a quad rail-type gas block on your AR-15, and you like the looks of item number ARR-567. The description says that it fits barrels with a gas block diameter of .936, or almost a full inch. How can you tell if it will fit your barrel? Well, you can stick a ruler under your barrel, squint and guess, or you can grab a set of calipers and know for sure. Calipers are an affordable way to make very accurate measurements, so you can know if the parts you are ordering will fit before you click that big red “check out” button.

This dial caliper needs to be "zeroed."

This dial caliper needs to be "zeroed."

A caliper is basically a clamp with a finely tuned measuring device installed on it, measuring precisely the distance between the two “lips” of the clamp. Calipers are divided into two groups, mechanical and electric. Mechanical calipers are cheaper (and old school cool), with a ruler built into the body of the clamp to giving you the larger increments, and a dial that looks like a watch face giving you the smaller increments. You add the smaller increments to the larger ones to get the total readout. For example, in the photo to the left, the ruler part gives us how many inches and tenths of inches that the lips have traveled since “zero.” The ruler is reading .9 inches, or nine tenths of one inch. For each tenth of an inch of travel, the dial will move all the way around a full circle, from “0” and all the way back to “0” again, so the big numbers on the dial are thousandths of an inch, and the small marks on the dial are ten-thousandths of an inch.  Since the dial is reading “54”, the total measurement is .954 inches. Pretty accurate huh? Here’s the part where I remind you not to toss your calipers around—the more precise an instrument is, easier it is to damage.

Dial caliper

.954 inches, old school style

Electronic calipers

.952 inches using the electronic calipers











An electronic caliper displays the measurement results as an easy to read number on the LED screen. Wow, that’s so easy, why would anyone still use a dial caliper? Well, the dial caliper doesn’t have an electronic screen to break, and it doesn’t have batteries to be dead at exactly the wrong time. No matter which version you choose, its important to remember that before you begin any measurement you need to “zero” your calipers. With the lips of the clamp completely together, the dial on a manual caliper should read exactly “0”. If the needle isn’t pointing to “0”, loosen the thumb screw mounted on the caliper and turn the dial itself until it does, then re-tighten the thumb screw. Electronic calipers will have a button to push that will set them to “0”. Make sure your calipers are zeroed or the measurements you obtain from them will be useless—if you didn’t start at zero, the number you are reading now doesn’t actually show the total amount of distance the caliper’s lips have moved.

To get a correct measurement of a gun part, for example the diameter of that AR-15 barrel, you want to measure the shorted possible distance between the caliper’s lips. That means placing those lips at an exact right angle to the shape of the part you wish to measure. The image on the left is an exaggerated example of what not to do. Because the tool is not square to the barrel, the caliper lips are not as close together as they should be and the measurement taken makes the barrel diameter seem wider than it actually is. Measured this way, the barrel seems to be nearly an inch wide, and the gas block ARR-567 seems like it would work. The image on the right shows the correct way to take a measurement, with the calipers as close together as possible because the tool is at a right angle to the barrel. At this point the barrel actually measured .747 inches. Item number ARR-576, a gas block for .750 inch barrels, is the correct choice. The three hundred-thousandths of an inch difference between the measured diameter and the specification of the gas block is so tiny that it just means the gas block will slip over the barrel without having to use a rubber mallet to whack it into place.

Caliper fail

Measurement FAIL. You are doing it wrong.

Measurement win

Measurement WIN. You are doing it right!












Remember to treat your calipers with care, zero them before use, and use the smallest measurement you can obtain as your “correct” measurement. If you have to take a measurement five times, write down the results each time, and take the smallest of your measurements as the most accurate, that’s perfectly ok. If you have any questions about fitment of gun parts and what will and won’t work on your gun, you can call our tech support guys at 800-421-8047. They are right here in Texas (just a few feet away from me) and get paid to be all-American gun nuts and help folks with their technical questions!


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The mission of Cheaper Than Dirt!'s blog, "The Shooter's Log," is to provide information-not opinions-to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (1)

  • MMSDave


    What about the good oldfationed calipers with no dial or battery to go bad? Any BEM can read those two that you showed.


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