How many inches is 1 thou?

One thou equals exactly 0.001 inches (one thousandth). This is the defining relationship.

Why was the thou invented?

To avoid fractions. Saying '3 thou' is cleaner than '3/1000 of an inch' or '0.003 inches.' The thou gives machinists a whole-number language for sub-inch precision.

Is a thou the same as a mil?

Yes. 'Thou' (British) and 'mil' (American) both mean 0.001 inches = 25.4 um. Same unit, two names across the Atlantic.

Thou to Inches (thou to in) Converter

1 thou = 0.001 in

1 Thou equals 0.001 Inches (1 thou = 0.001 in). Convert Thou to Inches with formula, table, and examples.

One thou equals exactly 0.001 inches - this IS the definition of a thou. The thou (also called 'mil') was invented to give the inch a decimal subdivision. 1,000 thou = 1 inch, bringing base-10 math to the imperial system's precision layer.

How to Convert Thou to Inches

in = thou ÷ 1,000

Divide the value in Thou by 1,000

Take your value in Thou
Divide by 1,000
Read the result in Inches

Common Thou to Inches Conversions

Thou (thou)	Inches (in)	Status
0.5 thou	0.0005 in
1 thou	0.001 in
2 thou	0.002 in
5 thou	0.005 in
10 thou	0.01 in
25 thou	0.025 in
50 thou	0.05 in
100 thou	0.1 in
250 thou	0.25 in
500 thou	0.5 in
1,000 thou	1 in
2,000 thou	2 in
5,000 thou	5 in
10,000 thou	10 in
12,000 thou	12 in

Good to Know About Thou to Inches Conversion

The thou is the imperial system's secret decimal unit. While inches divide into halves, quarters, and eighths (base-2), the thou divides the inch into thousandths (base-10). Below the inch, America is secretly decimal. The thou is where imperial and metric philosophy converge.

Thou to Inches: What You Need to Know

1 inch = 1,000 thou. A human hair is about 3 thou (0.003 inches). A sheet of paper is about 4 thou (0.004 inches). A credit card is about 30 thou (0.030 inches). The thou makes sub-inch measurement clean and decimal.

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 Inch? in

An imperial unit of length equal to exactly 2.54 centimeters. Commonly used in the US and UK for screen sizes, body measurements, and construction.

Imperial Us-customary screen sizes construction (US) body measurements

Learn more about Inch →

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

Thou to Inches FAQ

One thou equals exactly 0.001 inches (one thousandth). This is the defining relationship.
To avoid fractions. Saying '3 thou' is cleaner than '3/1000 of an inch' or '0.003 inches.' The thou gives machinists a whole-number language for sub-inch precision.
Yes. 'Thou' (British) and 'mil' (American) both mean 0.001 inches = 25.4 um. Same unit, two names across the Atlantic.

Non-Frequently Asked Questions About Thou to Inches

Questions nobody should ask - but someone did.

Because fractions of fractions are miserable. Below 1/16 inch, imperial fractions become 1/32, 1/64, 1/128... nobody wants to work with 1/128-inch tolerances. The thou (0.001 inch) is base-10 salvation for the imperial system. Below the inch, America secretly goes metric in spirit.
The naked eye resolves about 4 thou (0.004 inches / 100 um). A magnifying glass extends this to about 1 thou. A microscope goes below 1 thou. Most machining tolerances (1-5 thou) are at or below naked-eye visibility. Precision lives just beyond human sight.
The gap on a spark plug: typically 25-45 thou (0.025-0.045 inches). Every car owner who has gapped a spark plug has worked in thou without knowing it. The feeler gauge used is literally a thou-measuring instrument. Millions of people use thou annually. Most call it 'that little gap thing.'