A depth micrometer (or depth mic) is a measuring tool commonly used to check precise measurements of slots, keyways, grooves and various other locations. They are a very accurate measuring device.
Depth micrometers are commonly used to take measurements to an accuracy of .001″ or .0001″ in inches. Measurements in millimeters can be made down to .01mm or .001mm.
How to use a depth micrometer
Depth mics can be used to measure many different types of part characteristics. I will explain how to check a hole depth.
Before using your micrometer, ensure that the measuring tool and surface to be measured are free of dirt, debris, chips, etc. The micrometer thimble should spin freely. Place the tool on part over the hole. Spin the micrometer thimble until the rod extends to the bottom of the hole. Use the ratchet or friction stop if available on your tool.
Note: The depth mic should be checked for accuracy whenever a rod is changed to measure a different size. It can be easy for something to get contamination in between the micrometer and the depth rod where they come together.
Keeping things as clean as possible will help with this problem.
How to read a depth micrometer
I recommend a digital depth micrometer for ease of measurement especially if the measurer will only occasionally be taking readings with their micrometer. Unfortunately the price of a digital depth mic can be quite high so if you must use an analog micrometer then please keep reading.
The most common varieties of depth micrometers read in increments of one thousandth of an inch (.001″) or one ten-thousandth of an inch (.0001″). The process of reading a measurement from either type is similar. Along the sleeve of the depth micrometer will be graduations similar to a ruler.
The graduations at every fourth interval are most often numbered 0, 1, 2 and so forth. These numbers represent .100″ or one hundred thousandths of an inch. If using a depth micrometer with a 1-2″ rod, the graduation marked 6 would correspond to a measurement of 1.600″. The graduations between the numbers are each .025″ or twenty five thousandths of an inch. If we were to use a depth micrometer with a 4-5″ rod and obtained a measurement at the 3rd graduation after the .200″ mark, then our reading would be 4.275″. This would be the reading if the 0 on the thimble lined up exactly with the 3rd graduation after the .200 mark on the reading line.
If instead the number ten lined up with the reading line and we could still see the 3rd graduation after the .200″ mark, then our measurement would be 4.285″. For micrometers that read to .0001″ we would additionally rotate the micrometer without turning the spindle to determine which numbers line up on the sleeve and thimble. If a number lines up on the thimble with the number 7 on the sleeve, our reading would now be 4.2857″.
Formula for depth micrometer readings
Base depth micrometer rod size + (.100″ x largest visible number) + (.025″ x graduations visible after the largest number) + (.001″ x reading from thimble) +(.0001″ x reading from sleeve for .0001″ micrometers)
Example for a depth micrometer with a 1-2″ rod
1.000″ + (.100″ x 4) + (.025″ x 2) + (.001″ x 3) + (.0001″ x 8) =
1.000″+ .400″ + .050″ + .003″ + .0008″ = 1.4538″
When to use a depth micrometer
Depth micrometers while very accurate have one downfall. Depth micrometers like most standard micrometers are most commonly found in 1″ measuring range increments (3-4″, 4-5″, etc.). For a depth micrometer, this means that multiple sized rods are needed to be capable of covering the measurer’s measurement needs. Because of this depth micrometers are commonly sold in sets.
A 0-3″ micrometer set will cover the needs of most applications while a 0-12″ set is more than most people, especially hobbyists will need.
What makes a good depth micrometer
A good depth micrometer needs two things: precision and accuracy. Some adjustments can be made with most depth micrometers to account for small errors in accuracy but nothing can be done to fix a tool that isn’t precise.
Quality depth micrometers will turn smoothly without any drag. This is the telltale sign of a good tool. If your depth micrometer ever feels like it is rubbing internally, disassemble the micrometer and clean per the manufacturers instructions to eliminate any possible contamination that may be causing the issue.
Where to buy depth micrometers
Depth micrometers are available from a number of online retailers. For a more in depth guide of which depth micrometer is best for your situation, please see our reviews section.
Some general advice, as usual for most products Amazon has a number of good options available. Walmart sells depth micrometers but we do not recommend any that they currently offer.
Are cheaper depth micrometers as good as expensive ones?
While some of the cheaper (made in China) type depth micrometers have gotten much better than they were in years past, they are nowhere near the same quality that you will see in a depth micrometer from one of the tried and true manufacturers such as Starrett or Mitutoyo.
A depth micrometer is the type of tool that is best to purchase once. In most cases it can be more beneficial to search for a used option on Craigslist or Facebook marketplace. Ebay can also be a good alternative. For more information on the best depth micrometers for your application, see our Best Depth Micrometers article.
How to calibrate a 0-1" depth micrometer
- Verify that the micrometer is clean.
- Visually examine the micrometer for any condition that could cause errors in the calibration.
- Whenever necessary to disassemble for adjustment, use care and cleanliness to assure no damage to the internal threads of the tool.
- Spin the thimble until the depth rod is inside the tool.
- Place the tool on a surface plate and spin the thimble to extend the depth rod to the zero position. Use the ratchet or friction stop if available.
- Repeat the process by placing the depth micrometer on gage blocks and overhanging the tool to allow the depth rod to extend down to the surface plate.
- Check accuracy of the micrometer at various locations within the tool’s measuring range. Gage blocks which have been calibrated themselves should be use for this operation. Block sizes which are used should test the micrometer at different positions of the thimble and not only increments of .025″. This ensures the scale on the thimble is accurate.
- Adjustments can be made at this step as needed. Different depth micrometers have different procedures for adjustment. Consult manufacturer documentation for instructions regarding the adjustment of your micrometer if needed. If adjustments are made, the calibration procedure should be started over to verify the adjustments were adequate.
- Calibration results are commonly recorded in a register or database for traceability of measurement history.