Machining Tolerance Conversion Table — Inch, mm & µm
Convert tolerance half-widths between inches, millimetres, and micrometres, with total tolerance-zone widths and clear mil versus micron definitions.
Updated September 6, 2026
A tolerance is easy to misread during a unit change. The number after ± is a half-width, while the full permissible zone is twice that number. This table keeps both visible: ±0.001 inch corresponds to ±0.0254 mm, and its total zone spans 0.0508 mm. The first column lists positive half-width magnitudes; keep the ± sign when applying a row to a symmetric dimension.
The international inch is exactly 25.4 mm. A micrometre is 0.001 mm, while an inch-based mil is 0.001 inch or 25.4 micrometres. In machine-shop speech, a tenth can mean one ten-thousandth of an inch; write the unit and decimal explicitly rather than relying on that shorthand. None of the example magnitudes below assigns a tolerance to a part, implies a machine capability, or supplies an ISO fit class. The drawing and design requirements determine those choices.
Inch tolerance half-width to metric
| ± half-width (in) | ± half-width (mm) | ± half-width (µm) | Total zone (mm) |
|---|---|---|---|
| 0.0001 | 0.00254 | 2.54 | 0.00508 |
| 0.0002 | 0.00508 | 5.08 | 0.01016 |
| 0.0005 | 0.01270 | 12.70 | 0.02540 |
| 0.0010 | 0.02540 | 25.40 | 0.05080 |
| 0.0020 | 0.05080 | 50.80 | 0.10160 |
| 0.0050 | 0.12700 | 127.00 | 0.25400 |
| 0.0100 | 0.25400 | 254.00 | 0.50800 |
The ± half-width is entered in the stack-up calculator. The total-zone column is twice that number and must not be entered as a half-width.
Micrometre tolerance half-width to inches
| ± half-width (µm) | ± half-width (mm) | ± half-width (in) |
|---|---|---|
| 1 | 0.001 | 0.0000394 |
| 2 | 0.002 | 0.0000787 |
| 5 | 0.005 | 0.0001969 |
| 10 | 0.010 | 0.0003937 |
| 20 | 0.020 | 0.0007874 |
| 25 | 0.025 | 0.0009843 |
| 50 | 0.050 | 0.0019685 |
| 100 | 0.100 | 0.0039370 |
| 200 | 0.200 | 0.0078740 |
Inch values are rounded to seven decimal places. Conversion precision does not imply measurement accuracy.
Sources & Further Reading
- NIST SI Units — Length: international inch is exactly 25.4 mm — https://www.nist.gov/pml/owm/si-units-length
- SI prefix micro = 10⁻⁶; milli = 10⁻³. Zone width for a symmetric ±t dimension is (nominal + t) − (nominal − t) = 2t.
Frequently Asked Questions
Is a mil the same size as a micrometre?
No. An inch-based mil is 0.001 inch, exactly 25.4 micrometres. A micrometre is 0.001 millimetre. Write out the unit when moving between shop measurements and metric drawings.
Does ±0.01 mm mean a 0.01 mm-wide acceptable range?
No. It permits 0.01 mm above nominal and 0.01 mm below nominal, for a total range of 0.02 mm. A unilateral tolerance or limit dimension needs its own interval interpretation.
Do decimal places determine the tolerance I should apply?
Not on their own. Some drawings define general tolerances by decimal place in a title block, but that is a drawing-specific rule. This chart converts stated tolerances; it does not invent a default tolerance when none is specified.
Try the Calculators
Machining Tolerance Conversion Table — Inch, mm & µm — reuven.tools/reference/machining-tolerance-conversions — verified against: NIST SI Units — Length: international inch is exactly 25.4 mm — https://www.nist.gov/pml/owm/si-units-length; SI prefix micro = 10⁻⁶; milli = 10⁻³. Zone width for a symmetric ±t dimension is (nominal + t) − (nominal − t) = 2t.