Mil to MM Converter

Convert mil (thousandths of an inch) to millimetres and vice versa. This conversion is essential for engineering, manufacturing, coating thickness measurements, and PCB design where precision matters.

Mil (thou)

Millimetres (mm)

Quick Conversions

Mil to MM Conversion Table

Here are common mil to millimetre conversions you’ll encounter in manufacturing and engineering projects.

Mil (thou)	Millimetres (mm)	Common Application
1 mil	0.0254 mm	Thin film coatings
5 mil	0.127 mm	PCB copper traces
10 mil	0.254 mm	Standard PCB spacing
20 mil	0.508 mm	Thick film resistors
50 mil	1.27 mm	Wire insulation
100 mil	2.54 mm	PCB edge spacing
250 mil	6.35 mm	Sheet metal thickness
500 mil	12.7 mm	Protective coatings
1000 mil	25.4 mm	Industrial barriers

Conversion Formula

Mil to Millimetres:

mm = mil × 0.0254

Millimetres to Mil:

mil = mm × 39.37007874

Step-by-Step Conversion

Converting 250 mil to millimetres:

Start with your measurement: 250 mil
Multiply by the conversion factor: 250 × 0.0254
Result: 6.35 mm

Converting 5 mm to mil:

Start with your measurement: 5 mm
Multiply by the conversion factor: 5 × 39.37007874
Result: 196.85 mil (approximately)

What’s the Difference?

Mil (Thou)

System: Imperial

Definition: One thousandth of an inch (0.001″)

Used in: United States, UK manufacturing specs

Symbol: mil or thou

Millimetre

System: Metric

Definition: One thousandth of a metre (0.001 m)

Used in: Worldwide standard

Symbol: mm

Where You’ll Use This

The mil to millimetre conversion appears frequently in several industries and everyday situations across the UK.

Engineering and Manufacturing

Manufacturing tolerances often specify dimensions in mil, especially when working with American suppliers or legacy British standards. A typical engineering drawing might specify a tolerance of ±2 mil (0.0508 mm) for precision machined parts. Sheet metal thickness is commonly measured in mil in aerospace and automotive sectors.

Electronics and PCB Design

Printed circuit board designers work with mil measurements daily. Trace widths, spacing between components, and via hole diameters are typically specified in mil. For instance, a standard PCB might have 10 mil traces with 10 mil spacing (0.254 mm each), whilst high-density boards might use 4 mil traces.

Coating and Paint Industries

Paint and coating thickness measurements rely heavily on mil. A typical automotive paint job might be 4-6 mil thick (0.102-0.152 mm). Industrial coatings for corrosion protection can range from 10 mil to several hundred mil depending on the environment.

Packaging and Films

Plastic films, shrink wrap, and packaging materials have their thickness specified in mil. Standard cling film is about 0.5 mil (0.0127 mm), whilst heavy-duty industrial wrapping might be 6 mil (0.152 mm) or thicker.

Understanding Imperial to Metric

The mil originates from the imperial system, representing exactly one thousandth of an inch. It’s also called “thou” in British English, short for “thousandth”. This unit remains prevalent in certain industries despite the UK’s adoption of the metric system.

The conversion factor (0.0254) comes from the definition of an inch. Since 1 inch equals exactly 25.4 mm, one mil equals 0.0254 mm. This relationship is precise and standardised internationally, making conversions straightforward.

British engineers often encounter both units because UK manufacturing maintains links with American suppliers who specify dimensions in mil. European suppliers use millimetres. Being fluent in both systems prevents costly errors and miscommunication.

Precision Matters

When working with mil and millimetre conversions, decimal precision is crucial. In manufacturing, a difference of even 0.01 mm can determine whether a part meets specifications or gets rejected.

For most engineering purposes, maintaining four decimal places when converting from mil to millimetres provides sufficient accuracy. For example, 15 mil converts to 0.3810 mm. Going beyond four decimal places rarely offers practical benefit in real-world manufacturing.

However, when converting from millimetres to mil, you’ll often work with more decimal places because the mil is a smaller unit. Converting 1 mm gives you 39.37007874 mil, though most applications round this to 39.37 mil.

Related Length Conversions

From	To	Multiply by
Mil	Micrometres (µm)	25.4
Mil	Inches	0.001
Mil	Centimetres	0.00254
Millimetres	Micrometres (µm)	1000
Millimetres	Inches	0.0393701
Millimetres	Centimetres	0.1
Inches	Millimetres	25.4
Micrometres	Millimetres	0.001

FAQs

How many millimetres are in one mil?

One mil equals exactly 0.0254 millimetres. This conversion is precise and standardised internationally. The mil, also called thou, represents one thousandth of an inch.

Is mil the same as millimetre?

No, they’re completely different units from different measurement systems. A mil (from the imperial system) is much smaller than a millimetre (metric system). It takes approximately 39.37 mil to equal one millimetre.

Why do engineers still use mil instead of millimetres?

Many industries continue using mil because of historical standards, compatibility with American specifications, and legacy equipment. PCB manufacturing, coating thickness measurement, and aerospace engineering commonly specify dimensions in mil. British engineers often work with both units to collaborate with international suppliers.

What’s the difference between mil and thou?

There’s no difference—they’re two names for the same unit. “Mil” is more common in American English, whilst “thou” (short for thousandth) is traditional British terminology. Both refer to one thousandth of an inch (0.001 inches).

How thick is 10 mil in everyday terms?

10 mil (0.254 mm) is roughly the thickness of three sheets of standard printer paper stacked together. It’s a common thickness for heavy-duty bin liners, laminating pouches, and protective films used in manufacturing.

Can I use this conversion for coating thickness?

Yes, mil to millimetre conversion is essential for coating thickness specifications. Paint manufacturers, powder coating services, and galvanising operations all specify thickness in mil. Most coating thickness gauges can display readings in both mil and millimetres.

What measurement is smaller than a mil?

Microinches (µin) represent millionths of an inch and are smaller than mil. In metric terms, micrometres (µm) are commonly used for very small measurements. One mil equals 1,000 microinches or 25.4 micrometres.

How accurate does my mil to mm conversion need to be?

This depends on your application. For most manufacturing and engineering work, four decimal places (e.g., 0.2540 mm) provides sufficient precision. Ultra-precision machining or scientific applications might require more decimal places, but this is uncommon in typical industrial settings.