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Flatness and Parallelism Checks with Gage Blocks
Guide · Updated October 27, 2026
Flatness and parallelism are two of the most common GD&T callouts, and both can be checked in the shop with a surface plate, an indicator and a few gage blocks. The difference is in how you support the part.
Zero on the nominal size
Build the zero stack for any part height with the free Gage Block Calculator. Free, no login.
Parallelism vs flatness
| Parallelism | Flatness | |
|---|---|---|
| Reference | A datum face | None (the surface itself) |
| Part support | Datum face down on the plate | Three points, leveled |
| Result | Spread of top-face readings | Spread after leveling |
Step by step
- Clean everything: Clean the plate, part and blocks, and let them soak to room temperature.
- Build the zero stack: Build a stack at the nominal part height and zero the indicator on it.
- Parallelism: sweep the top: Rest the datum face on the plate, sweep the top face, and note the high and low readings.
- Flatness: support on three points: Support the part on three equal stacks or jacks and adjust until three corners read the same.
- Sweep and record: Sweep the whole surface. Total spread = flatness (or parallelism) error.
Worked example
A 1.2345 in block is checked for parallelism. Zero on 1.000 + 0.134 + 0.1005 and sweep the top: readings from −0.0002 to +0.0004 in. Parallelism error is 0.0006 in, and the part is up to 0.0004 in over size at its thickest point. See the 1.2345 in stack.
Tips
- A worn plate adds error to both checks: surface plate care and checking.
- Keep the test indicator stylus near parallel to the surface: cosine error.
- Thin parts distort under clamping or uneven support, so support them gently.
FAQ
How do you check parallelism with gage blocks?
Put the part's datum face on a surface plate, zero an indicator on a gage block stack equal to the nominal height, then sweep the top face. The spread between the highest and lowest readings is the parallelism error.
How do you check flatness on a surface plate?
Support the part on three equal stacks or adjustable jacks, level the surface by adjusting until three points read the same, then sweep the whole surface. The total indicator spread approximates flatness.
Why not just lay the part on the plate to check flatness?
A bowed part rocks or rests on its high spots, so sweeping the top measures parallelism and thickness variation, not flatness of the bottom face.
What height should the zero stack be?
The nominal part height, so readings show deviation from size as well as parallelism. Build it free with the Gage Block Calculator at gageblockcalc.com.
Is this the same as a GD&T check?
It's a practical shop check that matches the idea of the GD&T tolerance zones. For acceptance on critical parts, follow your inspection plan or use a CMM.
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