Mil to Inch Conversion

Understanding mils and inches

A mil (also called a thou) and an inch are both units of length used in the imperial and US customary systems. The mil is most common in engineering and manufacturing, where it describes very small thicknesses such as wire diameters, plating layers, plastic films, and printed circuit board traces. One mil is defined as exactly one-thousandth of an inch, so 1 inch contains 1000 mils.

Because the relationship is a simple power of ten, converting between mils and inches is fast and exact, with no rounding required.

Conversion formula

The two units are linked by a fixed factor:

$$1 \text{ inch} = 1000 \text{ mil}$$

To convert mils to inches, divide by 1000:

$$\text{Value in inches} = \frac{\text{Value in mils}}{1000}$$

To convert inches to mils, multiply by 1000:

$$\text{Value in mils} = \text{Value in inches} \times 1000$$

Conversion table

Below is a handy conversion table for mils and inches:

Applications and examples of use

Manufacturing and coatings

Mils are widely used to specify coating and film thickness. Suppose a protective paint layer is specified at 250 mil. To express that in inches:

$$250 \text{ mil} \div 1000 = 0.25 \text{ in}$$

So the coating is a quarter of an inch thick.

Electronics and PCB design

Printed circuit board features such as trace widths and clearances are frequently quoted in mils. If a layout calls for a trace that is 5000 mil long, the equivalent in inches is:

$$5000 \text{ mil} \div 1000 = 5 \text{ in}$$

Thus the trace is exactly 5 inches long.

Converting inches back to mils

Engineers often work in the opposite direction, taking a dimension measured in inches and expressing it in mils for fine detail. For example, a gap of 2.5 inches becomes:

$$2.5 \text{ in} \times 1000 = 2500 \text{ mil}$$

This is useful when tolerances are tighter than a single inch can conveniently express.

Frequently asked questions

How do you convert mils to inches?

To convert mils to inches, divide the mil value by 1000. For example, 1000 mil equals 1 inch. You can also simply enter the value in the calculator and read the result instantly.

How many mils are in 1 inch?

There are exactly 1000 mils in 1 inch, since a mil is defined as one-thousandth of an inch.

What is a mil the same as?

A mil is the same as a thou, both meaning one-thousandth of an inch. It should not be confused with a millimeter, which belongs to the metric system.

Convert 100 mil to inches.

Using the conversion factor:

$$100 \text{ mil} \div 1000 = 0.1 \text{ in}$$

Thus, 100 mil equals 0.1 inches.

Convert 2 inches to mils.

$$2 \text{ in} \times 1000 = 2000 \text{ mil}$$

Thus, 2 inches equal 2000 mils.

Is a mil the same as a millimeter?

No. A mil is one-thousandth of an inch (0.0254 mm), while a millimeter is one-thousandth of a meter. To convert millimeters and inches, use the mm to inches converter.