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How to Check Squareness
Guide · Updated October 27, 2026
Squareness (perpendicularity) asks whether a face stands at exactly 90° to its base. In the shop you check it on the surface plate against a master square, with an indicator and gage blocks to set the heights.
Set exact heights
Build the stacks for your top and bottom reading heights with the free Gage Block Calculator. Free, no login.
Step by step
- Clean and soak: Clean the plate, part and master square, and let them reach room temperature.
- Zero on a master: Set the indicator against a cylinder square or master square at the bottom height and zero it.
- Read top and bottom: Read the master at the top height and note any error in the setup itself.
- Measure the part: Read the part at the same two heights.
- Calculate: Part error = (part top − part bottom) − (master top − master bottom), over that height.
Worked example
A 4 in tall part. Master square: bottom 0, top +0.0001 in. Part: bottom 0, top +0.0006 in. Part error = 0.0006 − 0.0001 = 0.0005 in over 4 in.
Gage block gap method (no master indicator stand)
Stand the part beside a master square on the plate. Slide equal gage blocks (for example 0.100 in) between them near the bottom and near the top. If one slides freely and the other is tight, try 0.1001, 0.1002 and so on until both feel the same. The difference is the squareness error over that height.
Tips
- A worn plate tilts everything: see surface plate care.
- For the matching flatness and parallelism checks, see flatness and parallelism.
- Keep the test indicator near parallel to the face: cosine error.
FAQ
How do you check squareness in the shop?
Stand the part on a surface plate, run an indicator on a height gage or square-check stand up the side, and compare the readings at the bottom and the top. The difference over that height is the squareness error.
What is a cylinder square?
A precision cylinder with ends square to its axis. Use it as a master to zero the indicator setup, so the stand's own error drops out.
How do gage blocks help with squareness?
Put equal stacks between the part and a master square at two heights, and any difference in feel or indicator reading shows the out-of-square amount. Blocks also set exact heights for the top and bottom readings.
How do I express squareness error?
As a total over the measured height, for example 0.0005 in over 4 in, or as a GD&T perpendicularity zone width on the drawing.
Is there a free gage block calculator?
Yes. The Gage Block Calculator at gageblockcalc.com builds any stack in inch or metric sets, free with no login.
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Questions? Ask a machinist.
General shop information only — follow your gage manufacturer's instructions and your quality system.