MM to Mils Converter
Convert millimetres (mm) to mils (thou) with precision. A mil, also known as a thou, represents one-thousandth of an inch and is commonly used in engineering, manufacturing, and material specifications to measure thickness, wire diameter, and machined dimensions.
Common MM to Mils Conversions
| Millimetres (mm) | Mils (thou) | Typical Application |
|---|---|---|
| 0.01 mm | 0.3937 mils | Ultra-thin coatings |
| 0.1 mm | 3.937 mils | Thin films |
| 0.5 mm | 19.685 mils | Paper thickness |
| 1 mm | 39.37 mils | Standard measurement |
| 2 mm | 78.74 mils | Plastic sheets |
| 5 mm | 196.85 mils | Sheet metal |
| 10 mm | 393.7 mils | Plywood |
| 25.4 mm | 1000 mils | One inch equivalent |
| 50 mm | 1968.5 mils | Structural materials |
| 100 mm | 3937 mils | Construction materials |
Conversion Formula
Millimetres to Mils:
Mils = Millimetres × 39.3700787402
Mils to Millimetres:
Millimetres = Mils × 0.0254
Conversion Steps
To convert millimetres to mils:
- Take your measurement in millimetres
- Multiply by the conversion factor 39.37
- The result is your measurement in mils (thou)
- Example: 3 mm × 39.37 = 118.11 mils
To convert mils to millimetres:
- Take your measurement in mils
- Multiply by the conversion factor 0.0254
- The result is your measurement in millimetres
- Example: 100 mils × 0.0254 = 2.54 mm
Visual Comparison
1 Millimetre
Approximately the thickness of a credit card
0.1 Millimetre
Thickness of standard printer paper
25.4 Millimetres
Exactly one inch
What Is a Mil?
A mil (also called thou) is a unit of length equal to one-thousandth of an inch (0.001 inch). This term derives from the Latin word “millesimus” meaning thousandth. In the UK and Commonwealth countries, the term “thou” is more commonly used in engineering and manufacturing contexts, whilst “mil” is the preferred term in North America.
The mil is primarily used for measuring small thicknesses and diameters where metric measurements would result in decimal values. Industries such as engineering, electronics, aerospace, and manufacturing frequently specify tolerances and dimensions in mils because it provides convenient whole numbers for precision work.
Note: Do not confuse “mil” with “millimetre”. Despite the similar abbreviations, a mil is 0.0254 mm, making it approximately 40 times smaller than a millimetre.
When to Convert MM to Mils
Engineering and Manufacturing
Engineers working with imperial-based technical drawings often need to convert metric measurements to mils. Machined parts, bearing clearances, and surface finishes are frequently specified in mils, particularly when working with international suppliers or legacy designs.
Material Thickness Specifications
Industries such as plastics manufacturing, label production, and protective coatings specify material thickness in mils. Converting from millimetres helps when sourcing materials from metric-based suppliers or comparing product specifications across different measurement systems.
Wire and Cable Manufacturing
Wire diameter, insulation thickness, and cable jacketing are often measured in mils in the electrical industry. Technicians converting from metric wire gauge specifications need accurate mm to mils conversion for proper material selection.
Printed Circuit Board (PCB) Design
PCB manufacturers specify copper trace thickness, laminate thickness, and clearances in mils. Designers working with metric component datasheets must convert these measurements to meet manufacturing specifications.
Mils vs Millimetres
| Aspect | Mil (Thou) | Millimetre |
|---|---|---|
| Definition | 1/1000 of an inch | 1/1000 of a metre |
| Measurement System | Imperial | Metric |
| Common Usage | USA, UK engineering | Worldwide standard |
| Symbol | mil or thou | mm |
| Precision Context | Thin materials, coatings | General measurements |
| Equivalent | 1 mil = 0.0254 mm | 1 mm = 39.37 mils |
Frequently Asked Questions
How many mils are in 1 millimetre?
There are exactly 39.3700787402 mils in 1 millimetre. For practical purposes, this is typically rounded to 39.37 mils. This conversion factor is derived from the relationship between inches and millimetres: since 1 inch equals 25.4 mm and 1 mil equals 0.001 inches, 1 mm equals 1÷0.0254 = 39.37 mils.
Is a mil the same as a millimetre?
No, a mil is not the same as a millimetre. A mil (or thou) is 1/1000 of an inch, whilst a millimetre is 1/1000 of a metre. One mil equals 0.0254 millimetres, making a millimetre approximately 39.37 times larger than a mil.
Why are mils used instead of millimetres?
Mils are primarily used in industries and countries that work with imperial measurements, particularly in the United States and in UK engineering contexts. Mils provide convenient whole numbers for small measurements that would otherwise require decimals in millimetres. For instance, 3 mils is easier to work with than 0.0762 mm in manufacturing specifications.
What is the difference between mil and thou?
There is no difference; “mil” and “thou” are two names for the same unit of measurement. Both terms refer to one-thousandth of an inch. “Mil” is derived from the Latin “millesimus” (thousandth), whilst “thou” is short for “thousandth”. In the UK, “thou” is more commonly used in engineering, whereas “mil” is preferred in North America.
How do I convert 0.5 mm to mils?
To convert 0.5 mm to mils, multiply by 39.37: 0.5 × 39.37 = 19.685 mils. This thickness is typical for standard office paper or thin cardstock materials.
What thickness is 10 mils in millimetres?
10 mils equals 0.254 millimetres (10 × 0.0254 = 0.254 mm). This thickness is commonly used for plastic bags, laminating pouches, and thin protective films.
Are mils used in the UK?
Yes, mils (more commonly referred to as “thou” in the UK) are widely used in British engineering, manufacturing, and machining industries. Whilst the UK officially uses the metric system, imperial units like thou remain prevalent in certain technical and industrial contexts, particularly for legacy specifications and when interfacing with American designs.
How precise should my conversion be?
The precision required depends on your application. For general purposes, rounding to two decimal places (39.37) is sufficient. For high-precision engineering and manufacturing, use the full conversion factor (39.3700787402) or maintain at least four decimal places to minimise rounding errors.
Related Length Conversions
| From | To | Multiply By |
|---|---|---|
| Millimetres | Inches | 0.03937 |
| Millimetres | Microns (μm) | 1000 |
| Mils | Inches | 0.001 |
| Mils | Microns (μm) | 25.4 |
| Inches | Millimetres | 25.4 |
| Inches | Mils | 1000 |