# Thou to Millimeters (thou to mm)

Source: https://www.unitconvertercalculator.com/length/thou-to-millimeters/

**1 thou = 0.0254 mm**

One thou equals exactly 0.0254 millimeters. This is the most commercially important cross-system precision conversion in the world. Every international engineering drawing, every mixed-unit factory, every cross-border supply chain depends on 1 thou = 0.0254 mm.

## Formula

Convert Thou to Millimeters

## Conversion Table

| Thou (thou) | Millimeters (mm) |
|---|---|
| 1 thou | 0.0254 mm |
| 2 thou | 0.0508 mm |
| 5 thou | 0.127 mm |
| 10 thou | 0.254 mm |
| 20 thou | 0.508 mm |
| 39.37 thou | 0.999998 mm |
| 50 thou | 1.27 mm |
| 100 thou | 2.54 mm |
| 200 thou | 5.08 mm |
| 500 thou | 12.7 mm |
| 1000 thou | 25.4 mm |
| 5000 thou | 127 mm |
| 10000 thou | 254 mm |

## Units

### 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.

### Millimeter (mm)

A metric unit of length equal to one thousandth of a meter. Used in engineering, manufacturing, and precise measurements.

## Background

1 thou = 0.0254 mm. 1 mm = 39.37 thou. A 5-thou tolerance = 0.127 mm. A 0.1 mm tolerance = 3.94 thou. Engineers worldwide convert between these units hundreds of times per day.

## Good to Know

0.0254 mm per thou and 39.37 thou per mm. These two numbers keep global manufacturing interoperable. They appear on every international engineering drawing, every quality report, every tolerance analysis. They are the most important numbers in industrial commerce that most people have never heard of.

## FAQ

### How many millimeters is 1 thou?

One thou equals exactly 0.0254 mm.

### How many thou are in 1 millimeter?

One millimeter contains approximately 39.37 thou.

### Why is this the most important conversion?

Because American engineering specs use thou and metric specs use mm. Every product crossing the Atlantic requires this conversion. It is the most performed unit conversion in manufacturing.

## Non-Frequently Asked Questions

### Why not just round to 0.025 mm per thou?

That would introduce a 1.6% error. Over a 1,000-thou (1-inch) dimension, the error would be 0.4 mm. In precision machining, 0.4 mm is enormous - about 16 thou of error. Rounding kills precision. So we keep the ugly 0.0254 and maintain accuracy.

### How many thou-to-mm conversions happen per day globally?

Probably billions. Every CNC machine cutting a metric part from an American drawing, every quality inspector comparing thou specs to mm measurements, every CAD system converting between systems. 0.0254 is processed more times per day than most web pages are loaded.

### If 1 thou = 0.0254 mm, what is 1 mm in thou?

39.3701 thou. For quick mental math: 1 mm is about 40 thou (1.6% error). This approximation is taught in every machine shop on day one. '1 mm is about 40 thou' is the machinist's Rule of Thumb. It is wrong, but usefully wrong.

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## See Also

- [Millimeters to Thou](https://www.unitconvertercalculator.com/length/millimeters-to-thou/)