Convert Microinch to Millimeter
Convert microinches to millimeters instantly. 1 microinch = 2.54e-5 millimeter — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Millimeter to Microinch converter for the reverse conversion.
Units explained
Microinch
A microinch is an Imperial unit of length equal to one millionth of an inch (2.54×10⁻⁸ m). It is used almost exclusively in surface-finish specifications for precision machining and optical components.
The microinch is derived from the inch using the SI-style prefix micro- to denote one millionth. Standardised through the 1959 International Yard and Pound Agreement, which fixed the inch at exactly 25.4 mm.
Microinches express surface roughness (Ra values) in US-based precision manufacturing — typical machined surfaces are 16–125 µin. Optical components and bearing races are often specified to 1–4 µin Ra.
Used in US precision-manufacturing literature throughout the 20th century; value became exact in 1959 when the inch was fixed at 25.4 mm via the International Yard and Pound Agreement.
Millimeter
A millimeter is a metric unit of length equal to one thousandth of a meter (1/1000 m). It is the standard small-scale unit in the SI system and is widely used wherever sub-centimeter precision matters.
The millimeter was defined alongside the meter when France adopted the metric system in 1795, using the standard SI prefix milli- (from Latin mille, "thousand") to denote one-thousandth.
Millimeters are used in engineering drawings, machining, manufacturing, paper sizes, rainfall measurement, and any context where dimensions below one centimeter need to be specified clearly. Most rulers worldwide are marked in millimeters.
Adopted in 1795 in France as part of the original metric system; standardized internationally by the Metre Convention in 1875 and confirmed in the SI in 1960.
Microinch to Millimeter conversion formula
The relationship between microinches and millimeters:
To convert microinches to millimeters, multiply the value in microinches by 2.54e-5. To reverse, multiply millimeters by 39370.0787401575.
How to use this converter
Type a value into the calculator. The result in millimeters updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Millimeter to Microinch converter for the reverse direction.
Step-by-step: convert microinches to millimeters
- Write down the value in microinches (µin).
- Multiply that value by the factor 2.54e-5.
- The product is the equivalent value in millimeters (mm).
- To reverse, multiply the millimeter value by 39370.0787401575.
Worked examples
Example 1 — Convert 1 µin to mm:
1 × 2.54e-5 = 2.54e-5 mm
Example 2 — Convert 100 µin to mm:
100 × 2.54e-5 = 0.00254 mm
Real-world example — Sub-micron to millimeter
500,000 microinches converts to a small everyday quantity in millimeters — useful when relating optical-wavelength specifications to physical mounting hardware.
500000 µin × 2.54e-5 = 12.7 mm
Real-world example — From wavelengths to millimeter-scale objects
A value of one million microinches sounds vast at the atomic scale but converts to a small everyday quantity in millimeters — useful when relating optical-wavelength specifications to physical mounting hardware.
1000000 µin × 2.54e-5 = 25.4 mm
Microinch to Millimeter conversion table
Standard reference values for converting microinches to millimeters:
| Microinch [µin] | Millimeter [mm] |
|---|---|
| 0.01 | 2.54e-7 |
| 0.1 | 2.54e-6 |
| 1 | 2.54e-5 |
| 2 | 5.08e-5 |
| 3 | 7.62e-5 |
| 4 | 0.0001016 |
| 5 | 0.000127 |
| 10 | 0.000254 |
| 20 | 0.000508 |
| 30 | 0.000762 |
| 40 | 0.001016 |
| 50 | 0.00127 |
| 100 | 0.00254 |
| 500 | 0.0127 |
| 1000 | 0.0254 |
Frequently asked questions
How many millimeters is 1 microinch?
How do I convert microinches to millimeters?
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How many millimeters is 100 microinches?
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Sources & references
Conversion factor (1 µin = 2.54e-5 mm) verified against the following authoritative sources:
- BIPM — The International System of Units (SI Brochure 9th ed.)
Official BIPM publication defining the seven SI base units (including the meter) and the rules for their use. The global authority on units of measurement.
- NIST — Guide to the SI
US National Institute of Standards and Technology reference covering the SI base and derived units with definitions and usage rules for US technical practice.
- NIST Special Publication 811 — Guide for the Use of the International System of Units
Detailed NIST guide covering exact conversion factors between SI and US customary units along with formatting and rounding conventions.
- NIST — Refinement of values for the yard and pound (Federal Register 1959)
The treaty (signed by US
- International Astronomical Union — System of Astronomical Constants
The IAU defines astronomical units including the AU (149597870700 m exactly) light-year and parsec used in astronomy and astrophysics.