How many micrometers is 1 thou?

One thou equals exactly 25.4 um. This is the exact ratio since 1 inch = 25.4 mm = 25,400 um and 1 thou = 0.001 inches.

How do I convert thou to um?

Multiply thou by 25.4. For example, 5 thou = 127 um. For rough estimation: 1 thou is about 25 um.

Why is 25.4 so important?

Because 1 inch = exactly 25.4 mm (by international agreement since 1959). This cascades to 1 thou = 25.4 um. The number 25.4 is the bridge between every imperial and metric precision conversion.

Thou to Micrometers (thou to μm) Converter

1 thou = 25.4 μm

1 Thou equals 25.4 Micrometers (1 thou = 25.4 μm). Convert Thou to Micrometers with formula, table, and examples.

One thou equals exactly 25.4 micrometers. This is THE critical cross-system precision conversion. Every machinist working internationally memorizes 1 thou = 25.4 um. It is the handshake between American and metric precision.

How to Convert Thou to Micrometers

μm = thou × 25.4

Multiply the value in Thou by 25.4

Take your value in Thou
Multiply by 25.4
Read the result in Micrometers

Common Thou to Micrometers Conversions

Thou (thou)	Micrometers (μm)	Status
0.1 thou	2.54 μm
0.25 thou	6.35 μm
0.5 thou	12.7 μm
1 thou	25.4 μm
2 thou	50.8 μm
5 thou	127 μm
10 thou	254 μm
25 thou	635 μm
50 thou	1,270 μm
100 thou	2,540 μm
500 thou	12,700 μm
1,000 thou	25,400 μm
5,000 thou	127,000 μm
10,000 thou	254,000 μm

Good to Know About Thou to Micrometers Conversion

25.4 is the number that keeps global manufacturing interoperable. Every international engineering project, every cross-border supply chain, every mixed-unit factory depends on 1 thou = 25.4 um. It is the most commercially important conversion factor in human industry.

Thou to Micrometers: What You Need to Know

1 thou = 25.4 um. A 3-thou hair = 76.2 um. A 0.8 um Ra surface finish = 0.0315 thou. Surface roughness, coating thickness, and tolerance bands all cross this boundary daily in mixed-unit factories worldwide.

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 Micrometer? μm

One millionth of a meter, also called a micron. Used in biology for cell sizes, in engineering for surface finishes, and in manufacturing tolerances.

Metric biology manufacturing engineering tolerances

Learn more about Micrometer →

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

Thou to Micrometers FAQ

One thou equals exactly 25.4 um. This is the exact ratio since 1 inch = 25.4 mm = 25,400 um and 1 thou = 0.001 inches.
Multiply thou by 25.4. For example, 5 thou = 127 um. For rough estimation: 1 thou is about 25 um.
Because 1 inch = exactly 25.4 mm (by international agreement since 1959). This cascades to 1 thou = 25.4 um. The number 25.4 is the bridge between every imperial and metric precision conversion.

Non-Frequently Asked Questions About Thou to Micrometers

Questions nobody should ask - but someone did.

It was negotiated. The 1959 International Yard and Pound Agreement defined 1 yard = 0.9144 m exactly, making 1 inch = 25.4 mm exactly. Before 1959, the ratio varied slightly between countries. International diplomacy produced 25.4. It is a treaty number, not a mathematical necessity.
All of them. 25.4 appears on the first page of every machining textbook, the first slide of every manufacturing lecture, and the first line of every CNC programming guide. It is the most taught number in industrial education after pi. Machinists dream in 25.4s.
Tremendously. 25 is divisible by 5. 25.4 is not divisible by anything useful. But changing to 25 would make every existing part, drawing, and specification wrong by 1.6%. The cost of fixing 1.6% across all manufacturing exceeds the cost of memorizing 25.4. So 25.4 remains forever.