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

The Metric System vs. the Imperial System

Before we discuss how to convert MM to mils, it's essential to mention the metric system and the imperial system. 

• 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 * 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.
• And 1 mil divided by 0.0254 mm is 39.37 mils per mm
• 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, 50 millimeters is approximately 19.685 mils. That's about the thickness of two business cards!

Mils to mm 

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

Millimeters (mm) = Mils / 39.37

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 (mm) = 50 mils / 39.37 ≈ 1.27 mm 

So, 50 mils are approximately equivalent to 1.27 millimeters. That's about the thickness of a U.S. Dime!

How thick is 1 mil in mm?

1 mil = 0.0254 Millimeter

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:

In this chart, we have provided conversions for mils ranging from 1 to 120. This should cover most conversion needs. However, if you require conversions for values outside this range, simply use the conversion formulas mentioned earlier.

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): Electrical engineers often use mils to specify the spacing between components and the traces on a PCB. For example, a common requirement for PCB trace spacing to prevent electrical interference is 6 mils (0.1524 mm). This level of precision ensures that signals do not cross over and disrupt the proper functioning of the circuit.
2. Component Sizing: SMT (Surface Mount Technology) components often have specific pad sizes and pitches specified in mils. Engineers ensure that components fit snugly and securely on the PCB, reducing the risk of manufacturing errors or circuit malfunctions. Standard component pad sizes for SMT components can range from 20 mils (0.508 mm) to 40 mils (1.106 mm), depending on the component and its intended use.
3. Wire Gauge Measurements: The American Wire Gauge (AWG), a U.S. standard for wire conductor size, relates to the diameter of the wire in mils. For example, typical household copper wiring is AWG number 12 or 14, translating to 12 or 14 mils in diameter.
   
   

For SMT components in PCB design, there are part size references that are commonly used and are crucial for specifying dimensions on the PCB. This table provides their size values in both mils and millimeters for easy reference during PCB design and assembly:

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.