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How to Use a Depth Gage
Guide · Updated October 28, 2026
Dial and digital depth gages have a wide base and a sliding rod, so they are quick for holes, steps and pockets. They are good to about 0.001 in; for tighter tolerances use a depth micrometer or a height gage on a surface plate.
Verify zero and span
Zero on a surface plate, then check against a gage block stack of known height. The free Gage Block Calculator builds the stack.
Step by step
- Clean the base: Wipe the base and the part face; any chip adds to the reading.
- Zero it: Stand the base on a surface plate and zero the gage with the rod touching the plate.
- Check span: Set the base across two equal gage blocks and confirm the reading matches their height.
- Seat the base: Hold the base flat across the feature with steady pressure.
- Lower the rod: Slide the rod down gently to the bottom, without forcing it.
- Read it: Read the dial or display while the gage is still seated.
- Repeat: Measure two or three times; readings should agree within 0.001 in.
Worked example
A pocket is specified 0.500 ±0.005 in deep. Zero the digital depth gage on the plate, check it on a 0.500 in gage block pair (reads 0.500 in), then measure the pocket in three spots: 0.497, 0.498 and 0.497 in. It is in tolerance; if the drawing were 0.500 ±0.001 in, finish with a depth micrometer.
Caliper depth rods
A caliper’s depth rod works in a pinch, but the narrow end rocks easily and reads high if it tilts. Hold the caliper body square to the face and use it for rough checks only.
Depth gage vs depth micrometer
The depth gage is faster and covers more range at once. The depth micrometer is more accurate and repeatable, with interchangeable rods for long ranges.
Tips
- More accuracy: using a depth micrometer.
- Calipers: using a dial caliper.
- Zeroing: zeroing a digital caliper and micrometer.
FAQ
How accurate is a dial depth gage?
Typically about 0.001 in.
How do I zero a depth gage?
On a clean surface plate with the rod just touching the plate.
Can I use calipers for depth?
For rough checks; the rod tilts easily.
When should I use a depth micrometer?
When the tolerance is tighter than about ±0.002 in.
Why do my depth readings vary?
A rocking base, a chip, or a burr at the edge of the feature.
Related guides and calculators
- How to Use a Depth Micrometer
- How to Use a Dial Caliper
- How to Zero a Digital Caliper and Micrometer
- How to Check a Depth Micrometer with Gage Blocks
- How to Use a Surface Plate
- How to Measure a Groove or Recess with Gage Blocks
- How to Hone a Bore
- All free machinist calculators
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