Units and Conversions
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This technical bulletin addresses an important aspect of the language of measurement – the units used when reporting or discussing measured values. The dimensioning and tolerancing practices used on engineering drawings and related product specifications use either decimal inch (in) or millimeter (mm) units. Dimensional measurements are therefore usually reported in either of these units, but there are a number of variations and conversions that must be understood. Measurement accuracy, equipment specifications, measured deviations, and errors are typically very small numbers, and therefore a more practical spoken language of units has grown out of manufacturing and precision measurement practice.
Metric System
In the metric system (SI or International System of Units), the fundamental unit of length is the meter (m). Engineering drawings and measurement systems use the millimeter (mm), which is one thousandths of a meter (1 mm = 0.001 m). In general practice, however, the common spoken unit is the “micron”, which is slang for the micrometer (μm), one millionth of a meter (1 μm = 0.001 mm = 0.000001 m). In more rare cases, the nanometer (nm) is used, which is one billionth of a meter. A summary of this language is shown below:
| millimiters | Common Language and Conversions |
| 1 mm | 1 mm = 0.001 m = one millimeter (don’t say one mil) |
| 0.001 mm | 1 μm = 0.001 mm = one micrometer = one micron (common spoken slang) |
| 0.000001 mm | 1 nm = one nanometer = 0.001 μm |
• The word “micron”, while commonly spoken, should not be used in writing or when reporting values.
• The micrometer unit is pronounced with the emphasis on the first syllable and the long “o” (mī′krō-mē′tər); however, the common dimensional measuring tool is pronounced without the emphasis (mī-krŏm′ĭ-tər)
Inch System
In the inch system, the fundamental spoken unit in manufacturing practice is not the inch, but is 0.001 inches or one thousandth of an inch. In spoken practice all other numbers follow from this, and traditional rules from “math” may be broken, e.g. 0.123 inchesis pronounced one hundred twenty-three thousandths. Thissame spoken practice extendsto smaller numbers where 0.0001 inches, one tenth-thousandths, is typically called “one tenth”. When numbers get really small, the fundamental unit may switch over to millionths of an inch, 0.000001 inches. This unit is common, for example, in surface roughness or in the calibration of gage blocks. A summary of this language, including the common slang, is shown below:
| Inches | Common Spoken Language with Base of One Thousandths of an Inch (0.001 in) |
| 0.100 in | one hundred thousandths |
| 0.010 in | ten thousandths |
| 0.001 in | one thousandths = one thou = one mil (mil is specific to certain industries) |
| 0.0001 in | one ten-thousandths = one tenth |
| 0.00001 in | ten millionths = ten microinches = 10 μinches = 10 μin |
| 0.000001 in | one millionth = one microinch = 1 μinch = 1 μin |
• In writing or reporting values, the acceptable inch units are the inch, μinch and mil.
• The μinch and mil are based on the application of metric prefixes to inch units.
• In general, use decimal units, avoid fractions, and don’t use feet or yards.
• Avoid using 1” (1 inch) or 1’ (1 foot) as those can get confused with angular units of minutes (1’) and seconds (1”).
• The inch abbreviation (in) is not well-standardized. Other options include IN (capitalized) and in. (with the period).
Inch - Metric Conversions
All conversions between inch and metric units can be done using the extremely important and officially exact relationship of 1 inch = 25.4 mm. The following approximate conversion factors are useful for quick conversions particularly when talking with others who are not bilingual in both inch and metric units:
0.0001 in ≈ 2.5 μm
0.001 in ≈ 25 μm
1 μm ≈ 40 μin
1 μin ≈ 25 nm
Metric Prefixes
In manufacturing and dimensional measurement practice, metric use is usually restricted to meters (m), millimeters (mm), micrometers (μm), and in rare cases nanometers (nm). Some industries use centimeters (cm). Metric units are well standardized. The complete list of metric prefixes is shown below:
| Prefix | Symbol | Numerical Multiplier | Exponential |
| yotta | Y | 1,000,000,000,000,000,000,000,000 | 1024 |
| zetta | Z | 1,000,000,000,000,000,000,000 | 1021 |
| exa | E | 1,000,000,000,000,000,000 | 1018 |
| peta | P | 1,000,000,000,000,000 | 1015 |
| tera | T | 1,000,000,000,000 | 1012 |
| giga | G | 1,000,000,000 | 109 |
| mega | M | 1,000,000 | 106 |
| kilo | k | 1,000 | 103 |
| hecto | h | 100 | 102 |
| deka | da | 10 | 101 |