How to Convert from MM to Mils: a Guide for Beginners

The Metric System vs. the Imperial System

To convert mm to mils, divide the measurement in millimeters by 0.0254, or multiply by approximately 39.37. For example, 1 mm is approximately 39.37 mils. Before using the formula, it helps to distinguish these metric and inch-based units.

• The metric system is used worldwide and employs units like milliliters, centimeters, and millimeters. 
•  The imperial system is more common in the United States, employing units like fluid ounces and inches. 

Our focus is on mm, a metric unit, and its conversion to mils, an imperial unit.

The Basics: Millimeters and Mils

• Millimeters, denoted as "mm," are a metric unit that is one-thousandth of a meter, or 0.001 meters.

What units are mils?

• Mils are one-thousandth of an inch, meaning 1 mil is equivalent to 0.001 inches.

However, mils are sometimes used in a different context as milliradians (mils), which are units of angular measurement commonly used in ballistic calculations and optical instruments.

mm to mils Conversion Formula

To convert from millimeters to mils, you can use the following straightforward conversion formula:

Mils = Millimeters ÷ 0.0254 ≈ Millimeters × 39.37

We can derive this formula the following way:

• 1 inch equals 25.4 millimeters (2.54 centimeters)
• 1 mil equals 0.001 inch
• So, 1 mil equals 0.0254 mm.
• Therefore, 1 mm equals 1 ÷ 0.0254 ≈ 39.37 mils.
• Therefore, multiplying millimeters by 39.37 gives us the equivalent measurement in mils.

Let's illustrate this with an example: Suppose you have a measurement of 0.5 mm and want to convert it to mils:

Mils = 0.5 mm * 39.37 ≈ 19.685 mils

So, 0.5 millimeters is approximately 19.685 mils. Keep the original value in view when converting PCB dimensions: 50 mm would be approximately 1,968.5 mils, not 19.685 mils.

Mils to mm Conversion Formula

This conversion is essentially the reverse of our previous equation. You can use the following straightforward conversion formula:

Let's walk through an example of converting from mils to millimeters using the conversion formula:

Suppose you have a measurement of 50 mils, and you want to convert it to millimeters. You can use the formula as follows:

Millimeters = 50 mils × 0.0254 = 1.27 mm

So, 50 mils equals 1.27 millimeters. Multiplying by 0.0254 uses the exact conversion factor and avoids rounding from dividing by 39.37.

How thick is 1 mil in mm?

1 mil = 0.0254 Millimeter

MM to Mils and Mils to MM Conversion Chart

For those who prefer visual aids, having a conversion chart on hand is common. Here's a short conversion chart we made to give you an idea:

This chart lists selected conversions from 10 to 120 mils. For other values, use the formulas above and round only the final result to the precision needed for your design.

Why Convert to Mils in Electrical Engineering?

Mils are frequently employed in precision applications in the US, where small measurements are crucial. Below are specific scenarios where mils come into play for electrical engineers:

1. Printed Circuit Boards (PCBs): Mils are used to specify dimensions such as trace widths, clearances, and drill sizes. For example, 6 mils equals 0.1524 mm. This is a unit conversion, not a universal design rule: choose dimensions based on your fabricator’s capabilities and the electrical requirements of your board.
2. Component Sizing: SMT (Surface Mount Technology) packages and PCB land patterns may be dimensioned in millimeters or mils. For example, 20 mils equals 0.508 mm and 40 mils equals 1.016 mm. Use the component datasheet’s recommended land pattern; the package body size is not the same as its pad size.
3. Wire Gauge Measurements: American Wire Gauge (AWG) is a wire-size numbering system, not a diameter in mils. An AWG number such as 12 or 14 must first be looked up in a wire-size table before its diameter can be converted between millimeters and mils.

SMT package codes identify nominal body dimensions, not an exact mil-to-mm conversion or a recommended PCB footprint. The table below shows common chip-package sizes; check the specific component datasheet for dimensions, tolerances, and land patterns.

Don't be Confused! Understanding the Milliradian "Mil"

We've mentioned milliradians earlier, so let's clarify what they are. A milliradian, often denoted as "mrad" or "mil," is a unit of angular measurement. It is used in various fields, including ballistics and engineering. When you hear talk about a mil-dot scope or mil reticle, it's referring to measurements in milliradians. In this context, one radian is divided into 1,000 milliradians, and the reticle markings are spaced at one milliradian intervals.