# Millimeters to Thou (mm to thou) Source: https://www.unitconvertercalculator.com/length/millimeters-to-thou/ **1 mm = 39.370078740157 thou** One millimeter equals approximately 39.37 thou (mils). This is the most important cross-system precision conversion in manufacturing: metric engineering (mm) meets American precision (thou). The ratio 1 mm = 39.37 thou (or 1 thou = 0.0254 mm) is memorized by every machinist working internationally. ## Formula Convert Millimeters to Thou ## Conversion Table | Millimeters (mm) | Thou (thou) | |---|---| | 0.01 mm | 0.39370078740157 thou | | 0.025 mm | 0.98425196850394 thou | | 0.05 mm | 1.9685039370079 thou | | 0.1 mm | 3.9370078740157 thou | | 0.25 mm | 9.8425196850394 thou | | 0.5 mm | 19.685039370079 thou | | 1 mm | 39.370078740157 thou | | 2 mm | 78.740157480315 thou | | 5 mm | 196.85039370079 thou | | 10 mm | 393.70078740157 thou | | 25 mm | 984.25196850394 thou | | 50 mm | 1968.5039370079 thou | | 100 mm | 3937.0078740157 thou | | 500 mm | 19685.039370079 thou | | 1000 mm | 39370.078740157 thou | ## Units ### Millimeter (mm) A metric unit of length equal to one thousandth of a meter. Used in engineering, manufacturing, and precise measurements. ### Thou (thou) One thousandth of an inch (0.0254 mm). Also called a mil in the US. Used in engineering, manufacturing, and for measuring thin materials like plastic film. ## Background 1 mm = 39.37 thou. A hair (0.075 mm) = 2.95 thou. A sheet of paper (0.1 mm) = 3.94 thou. In mixed-unit factories, converting mm to thou happens dozens of times per shift when metric and American specifications coexist. ## Good to Know The number 39.37 is the Rosetta Stone of international manufacturing. Every factory that serves both metric and American customers has this ratio posted on the wall, programmed into every calculator, and memorized by every quality inspector. It is the most important conversion factor most people have never heard of. ## FAQ ### How many thou is 1 millimeter? One millimeter equals approximately 39.37 thou (mils). More precisely: 1 mm / 0.0254 = 39.3701. ### How do I convert mm to thou? Multiply mm by 39.37. For quick estimation: 1 mm is about 40 thou. This 1.6% error is acceptable for mental math. ### What is the exact relationship? 1 thou = 0.0254 mm exactly (since 1 inch = 25.4 mm and 1 thou = 0.001 inches). So 1 mm = 1/0.0254 = 39.3701 thou exactly. ## Non-Frequently Asked Questions ### Why do Americans not just use millimeters? Some do. But 'thou' has 100 years of established use in American manufacturing. Changing would require rewriting millions of specifications, retraining millions of workers, and recalibrating millions of instruments. The cost of switching exceeds the cost of converting. So machinists memorize 39.37 and move on. ### Is 1 thou really close to 1/40 of a mm? Close. 1 thou = 25.4 um = 0.0254 mm. 1/40 mm = 0.025 mm = 25 um. The difference is 0.4 um (about 1.6%). For rough estimation, 40 thou = 1 mm works. For precision work, use 39.37. For critical aerospace work, use 39.3701. Precision depends on industry. ### Could a machinist survive with only mm? Outside America, they do. European, Japanese, and Korean machinists use only mm and um. American machinists use thou and inches. Both achieve the same precision. The units differ; the skill is identical. A 0.001 mm tolerance is a 0.039 thou tolerance. Same precision, different dialect. ## Related Articles - [Why We Measure: The Deepest Urge in Human Civilisation](https://www.unitconvertercalculator.com/blog/why-we-measure) - [The Map Is Not the Territory: Why Every Measurement Is Wrong](https://www.unitconvertercalculator.com/blog/the-map-is-not-the-territory) - [Zero: The Most Dangerous Number in Measurement](https://www.unitconvertercalculator.com/blog/zero-the-most-dangerous-number-in-measurement) - [The Body as a Ruler: Every Measurement Unit That Came From Us](https://www.unitconvertercalculator.com/blog/the-body-as-a-ruler) - [The Speed of Everything: How We Measure From Glaciers to Light](https://www.unitconvertercalculator.com/blog/the-speed-of-everything) - [15 Obscure Measurement Units You've Never Heard Of (But Still Need)](https://www.unitconvertercalculator.com/blog/obscure-measurement-units-guide) - [When Measurements Go Wrong - Disasters, Blunders and Happy Accidents](https://www.unitconvertercalculator.com/blog/when-measurements-go-wrong) - [The Surprising Stories Behind Everyday Units of Measurement](https://www.unitconvertercalculator.com/blog/stories-behind-measurement-units) - [Metric vs. Imperial - The Complete Guide to the World's Two Measurement Systems](https://www.unitconvertercalculator.com/blog/metric-vs-imperial-complete-guide) - [Length & Distance Conversion Guide - Meters, Feet, Miles & Kilometers](https://www.unitconvertercalculator.com/blog/length-and-distance-guide) ## See Also - [Thou to Millimeters](https://www.unitconvertercalculator.com/length/thou-to-millimeters/)