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.

Thou to Millimeters (thou to mm) Converter

1 thou = 0.0254 mm

1 Thou equals 0.0254 Millimeters (1 thou = 0.0254 mm). Convert Thou to Millimeters with formula, table, and examples.

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.

How to Convert Thou to Millimeters

mm = thou × 0.0254

Multiply the value in Thou by 0.0254

Take your value in Thou
Multiply by 0.0254
Read the result in Millimeters

Common Thou to Millimeters Conversions

Thou (thou)	Millimeters (mm)	Status
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	1 mm
50 thou	1.27 mm
100 thou	2.54 mm
200 thou	5.08 mm
500 thou	12.7 mm
1,000 thou	25.4 mm
5,000 thou	127 mm
10,000 thou	254 mm

Good to Know About Thou to Millimeters Conversion

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.

Thou to Millimeters: What You Need to Know

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.

What is a 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.

Imperial engineering plastic film thickness wire gauges

Learn more about Thou →

What is a Millimeter? mm

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

Metric engineering manufacturing rainfall

Learn more about Millimeter →

Going the other way? Use our Millimeters to Thou converter.

Thou to Millimeters FAQ

One thou equals exactly 0.0254 mm.
One millimeter contains approximately 39.37 thou.
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 About Thou to Millimeters

Questions nobody should ask - but someone did.

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.
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.
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.