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.

Millimeters to Thou (mm to thou) Converter

1 mm = 39.3701 thou

1 Millimeter equals 39.3701 Thou (1 mm = 39.3701 thou). Convert Millimeters to Thou with formula, table, and examples.

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.

How to Convert Millimeters to Thou

thou = mm × 39.3700787402

Multiply the value in Millimeters by 39.3700787402

Take your value in Millimeters
Multiply by 39.3700787402
Read the result in Thou

Common Millimeters to Thou Conversions

Millimeters (mm)	Thou (thou)	Status
0.01 mm	0.3937 thou
0.025 mm	0.9843 thou
0.05 mm	1.9685 thou
0.1 mm	3.937 thou
0.25 mm	9.8425 thou
0.5 mm	19.685 thou
1 mm	39.3701 thou
2 mm	78.7402 thou
5 mm	196.8504 thou
10 mm	393.7008 thou
25 mm	984.252 thou
50 mm	1,968.5039 thou
100 mm	3,937.0079 thou
500 mm	19,685.0394 thou
1,000 mm	39,370.0787 thou

Good to Know About Millimeters to Thou Conversion

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.

Millimeters to Thou: What You Need to Know

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.

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 →

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 →

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

Millimeters to Thou FAQ

One millimeter equals approximately 39.37 thou (mils). More precisely: 1 mm / 0.0254 = 39.3701.
Multiply mm by 39.37. For quick estimation: 1 mm is about 40 thou. This 1.6% error is acceptable for mental math.
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 About Millimeters to Thou

Questions nobody should ask - but someone did.

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.
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.
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.