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Inch and Metric Conversion for Machinists

Guide · Updated October 28, 2026

An inch is exactly 25.4 mm, by definition. Converting sizes is easy; the mistakes happen when tolerances get rounded, when TPI is confused with pitch, or when a converted number gets more decimal places than the gage can read.

Build metric or inch stacks

The calculator builds stacks in either unit from your set. The free Gage Block Calculator builds the stack.

Step by step

Inch to mm: Multiply by 25.4.
Mm to inch: Divide by 25.4.
Convert the limits, not the nominal: Convert the upper and lower limits separately.
Round inward: Round the converted limits toward the middle of the tolerance so you never accept a bad part.
Threads: Pitch (mm) = 25.4 ÷ TPI. TPI = 25.4 ÷ pitch.
Keep resolution sensible: Inch to 0.0001 in, metric to 0.001 mm for gage work.
Label units: Write units on every number on the traveler.

Worked example

A bore is 20.000 +0.021/−0.000 mm (H7). Lower limit 20.000 ÷ 25.4 = 0.78740 in; upper 20.021 ÷ 25.4 = 0.78823 in. Rounded inward to four places: 0.7874 to 0.7882 in. A 13 TPI thread has a pitch of 25.4 ÷ 13 = 1.954 mm, close to but not 2.0 mm, so 1/2-13 and M12×2 are not interchangeable.

Machine dials

Many manual mills read inch dials. With a DRO you can switch units; without one, convert targets to inch and plan moves. A 0.100 in dial turn is 2.54 mm.

Common problems

Rounding a tolerance outward accepts bad parts. Mixing TPI and pitch makes wrong threads. Using 25 or 2.54 cm shortcuts adds error.

Tips

Metric stacks: metric gage block stacks.
Mics: read a metric micrometer.
Threads: thread pitch gages.

Quick shop check

1 in = 25.4 mm exactly. 0.001 in = 0.0254 mm, and 0.01 mm ≈ 0.0004 in. A 12 mm bore = 0.4724 in; a 3/8 in bolt = 9.525 mm. For thread pitch, TPI = 25.4 ÷ pitch: M8 × 1.25 = 20.32 TPI, so a 5/16-18 nut will not fit M8.

FAQ

Is 25.4 exact?

Yes, 1 inch = 25.4 mm exactly.

How do I convert TPI to pitch?

Pitch in mm = 25.4 ÷ TPI.

How do I round tolerances?

Round the limits inward.

How many decimals?

Inch 4 places, metric 3 for precision work.

Can I use inch gage blocks for metric parts?

Yes with conversion, but a metric set is easier.

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