← AskMachinist · Guides · Calculator · 🔎 Search
How to Use a Depth Micrometer
Guide · Updated October 28, 2026
A depth micrometer measures the depth of holes, slots, steps and counterbores from a flat reference face. It reads like an outside mic, but the sleeve numbers run the other way, which trips up a lot of people.
Verify it first
Check the zero and each rod on a surface plate with gage blocks. Check it with a mic you've verified against gage blocks; the free Gage Block Calculator builds the stack.
Step by step
- Clean the base: Wipe the base and the reference face; a chip under the base shows up directly as depth error.
- Pick the rod: Fit the rod for the depth range, seat it fully and snug the rod nut.
- Check zero: Stand the base on a surface plate or a gage block pair and confirm the reading matches.
- Seat the base: Hold the base flat with firm, even pressure from one hand, spanning the feature.
- Run the spindle down: Turn the thimble until the rod nears the bottom, then finish with the ratchet or friction thimble.
- Read it: Read the sleeve and thimble, remembering the numbers increase as the rod extends.
- Repeat: Lift, reseat and measure again; readings should agree within a tenth or two.
Worked example
Checking a 0.375 in deep counterbore with the 0–1 in rod: the thimble edge covers the 0.3 mark and the third 0.025 in line past it, so the sleeve reads 0.375 in. The thimble line at the index is 2, adding 0.002 in, for 0.377 in. That is 0.002 in deep, so the next pass comes up 0.002 in on Z.
Why the scale runs backwards
On an outside mic the spindle backs away as numbers go up. On a depth mic the rod reaches further out of the base as numbers go up, so the sleeve markings are hidden by the thimble, not exposed. Read the last line the thimble edge has covered.
Common errors
Rocking the base, measuring on a burr or radius at the bottom, a rod not fully seated, and over-tightening the thimble all give wrong depths. For small or narrow features, a depth base on a height gage or a gage block stack set on the surface plate can be more reliable.
Tips
- Verify it: checking a depth micrometer with gage blocks.
- Reading practice: reading an inch micrometer.
- Metric version: reading a metric micrometer.
- Work on a clean surface plate.
FAQ
Why is a depth micrometer scale backwards?
The numbers rise as the rod extends, so the thimble covers the markings instead of uncovering them.
How accurate is a depth micrometer?
Usually 0.0001 in resolution, with accuracy about 0.0001–0.0002 in per rod when it is set correctly.
How do I zero a depth micrometer?
Stand it on a surface plate for the 0–1 rod, or on gage blocks for longer rods, and adjust the sleeve.
Do depth micrometer rods need checking?
Yes, check each rod at a known depth with gage blocks.
Depth mic or caliper depth rod?
A depth mic is much more accurate and has a wide, stable base.
Related guides and calculators
- How to Check a Depth Micrometer with Gage Blocks
- How to Read an Inch Micrometer
- How to Read a Metric Micrometer
- How to Use a Surface Plate
- Countersink and Counterbore Basics
- How to Use a Height Gage
- How to Spotface a Hole
- How to Counterbore on a Mill
- How to Use a Depth Gage
- How to Use a Blade Micrometer
- How to Use a Tube Micrometer for Wall Thickness
- All free machinist calculators
Build your stack in seconds
The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.
Questions? Ask a machinist.
General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.