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How to Check a Depth Micrometer with Gage Blocks
Guide · Updated October 27, 2026
A depth micrometer can't close on a gage block the way an outside mic does. The trick is to build two equal stacks, bridge them with the base, and measure down to the surface plate between them. Here are the test sizes and stacks.
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Test sizes (81-piece inch set)
| Size | Stack A | Stack B |
|---|---|---|
| 0.250 in | 0.250 | 0.150 + 0.100 |
| 0.500 in | 0.500 | 0.350 + 0.150 |
| 0.750 in | 0.750 | 0.450 + 0.300 |
| 1.000 in | 1.000 | 0.950 + 0.050 |
Metric (88-piece set): 5 mm = 5 and 4 + 1; 10 mm = 10 and 9.5 + 0.5. Build each pair separately; the same block can't be in both stacks.
Step by step
- Clean and soak: Clean the plate, blocks and micrometer base, and let them reach room temperature.
- Build two equal stacks: Build two stacks of the test height from different blocks.
- Bridge the stacks: Rest the depth micrometer base across both stacks with the rod between them.
- Read at the plate: Run the rod down to the plate with the ratchet and read.
- Repeat across the range: Check several heights on each rod and compare with the spec.
Tips
- Hold the base down firmly and evenly. A rocking base reads long.
- Check zero first with the base flat on the plate.
- See also outside micrometers and height gages.
Worked example: checking the 0–1 in rod at 0.500 in
Clean the surface plate, the base of the depth micrometer, and the blocks, and let everything soak to room temperature. From an 81-piece set, build one 0.500 in stack from the 0.500 block and a second from 0.350 + 0.150. Wring each and set them a little wider apart than the rod. Rest the base across both stacks, run the rod down to the plate with the ratchet, and read. Expect 0.500 in within the mic's spec. Lift and reseat the base twice more; if the readings move, check for dirt under the base or a burr on the rod tip before adjusting the zero.
FAQ
How do you check a depth micrometer with gage blocks?
Set two stacks of the same height on a surface plate, bridge them with the depth micrometer base, and run the rod down to the plate. The reading should equal the stack height.
Why two stacks instead of one?
The base needs support on both sides of the rod. Two equal stacks give a stable bridge with the rod reaching the plate between them.
What test sizes should I use?
For a rod of 0 to 1 in: 0.250, 0.500, 0.750 and 1.000 in, each built twice from different blocks, for example 0.500 and 0.350 + 0.150 from an 81-piece set.
How do I check the extension rods?
Repeat the check at a size inside each rod's range, for example 1.500 in for the 1–2 in rod, because each rod has its own setting.
Is there a free calculator for the matching stacks?
Yes. The Gage Block Calculator at gageblockcalc.com builds stacks from inch and metric sets. Free, no login.
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Questions? Ask a machinist.
General shop information only — follow your gage manufacturer's instructions and your quality system.