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How to Check a Micrometer with Gage Blocks

Guide · Updated October 27, 2026

A quick zero check tells you the anvils meet; it does not tell you whether the spindle thread reads true across the range. A short series of gage block stacks does. Below are the classic test sizes for a 0-1 in and a 0-25 mm micrometer, with the fewest-block stack for each from the same solver the free gage block stack calculator uses.

Why these odd sizes?

An inch micrometer spindle advances 0.025 in per turn; a metric one 0.5 mm. Round sizes like 0.250 or 0.500 in all stop at the zero line of the thimble, so they test the same few spots on the thread. Sizes such as 0.105, 0.210 and 0.315 in (or 2.5, 5.1, 7.7 mm) land at a different thimble position each time and spread the check over whole turns of the spindle, which is why they are used for micrometer calibration.

Before you start

• Let the micrometer and blocks sit together for a while so they reach the same temperature; handle blocks by the edges or with gloves. The temperature correction calculator shows how much a few degrees matter.
• Clean the anvil and spindle faces with a lint-free paper pulled between them.
• Check zero (or the 25 mm standard on larger mics) using the ratchet or friction thimble, and adjust per the maker's instructions if needed.
• Wring two-block stacks properly: see how to wring gage blocks.

0-1 in micrometer: test sizes and stacks (81-piece inch set)

Tap a size to open it in the calculator. Write the reading in the last column on a printout.

0-25 mm micrometer: test sizes and stacks (88-piece metric set)

Larger micrometers (1-2 in, 25-50 mm and up) use the same idea: add the low end of the range to each test size and build the stack in the calculator, which picks the fewest blocks from your set.

Reading the results

• Take each reading with the same light ratchet pressure, two or three times, and note the average.
• Compare the error at each size to your micrometer's stated accuracy and your shop's acceptance limit.
• A steady error at every size usually means zero is off; an error that grows or wanders through the range points to the spindle thread or worn faces, and that micrometer should go to a calibration service.
• Check the faces for flatness and parallelism too if your shop's procedure calls for it (an optical flat is the usual tool).

Not sure which blocks are good enough for this? Workshop grades are fine for most shop checks; see gage block grades explained. Checking calipers too? See how to check calipers with gage blocks or how to check a height gage, or a dial indicator. Buying a set? Read how to choose a gage block set or gage block set vs micrometer.

Tools that help

• 81-piece inch gage block sets: cover every inch test size above. Check grade and certificate.
• Micrometer checking block sets: small sets made just for this series.
• Optical flats for micrometer faces: for flatness and parallelism checks.

See also: How to zero a digital caliper and micrometer, How to use a dial bore gage.

See also: How to Use a Telescoping Gage.

See also: How to Use Small Hole Gages.

See also: How to Read an Inch Micrometer.

FAQ

Why not just check a micrometer at 0.250, 0.500, 0.750 and 1.000 in?

Those sizes land on the same few spots of each spindle turn (0.025 in per turn), so thimble and thread errors at other positions go unseen. Sizes such as 0.105, 0.210 and 0.315 in land at different thimble positions every time.

Which gage blocks do I need to check a 0-1 inch micrometer?

An 81-piece inch set covers the whole test series with one or two blocks per size: for example 0.210 in = 0.110 + 0.100 and 0.815 in = 0.700 + 0.115.

How often should a micrometer be checked?

Check zero every time you use it and whenever it is dropped. Run the full gage block series on a schedule your shop sets, often every few months for daily-use mics, and after any adjustment.

As an Amazon Associate I earn from qualifying purchases; links marked "sponsored" may earn a commission at no cost to you. Prices and availability change; check each listing before buying. This guide is general information; follow your shop's calibration procedure and the micrometer maker's instructions.

Need a stack for another size? Use the free gage block stack calculator (no login). Have a shop question? Ask a machinist or browse all guides.