Millimeters Conversion

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mil (informal)

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Millimeters are widely used as a unit of measurement across the globe due to their versatility and precision. This metric unit is commonly used in various fields, including engineering, manufacturing, construction, and science. One of the key reasons for the worldwide use of millimeters is their compatibility with the International System of Units (SI), which is the most widely adopted system of measurement.

In engineering and manufacturing, millimeters are crucial for precise measurements and dimensional accuracy. They are used to determine the size and specifications of components, ensuring proper fit and functionality. In construction, millimeters are used to measure distances, thicknesses, and dimensions of various building materials, enabling precise planning and execution of projects. Additionally, millimeters are extensively used in scientific research, particularly in fields such as physics, chemistry, and biology, where accurate measurements are essential for conducting experiments and analyzing data.

The global adoption of millimeters as a standard unit of measurement promotes consistency and facilitates international collaboration. It eliminates the confusion and inconsistencies that can arise from using different measurement systems, allowing for seamless communication and exchange of information between professionals from different countries. Moreover, the use of millimeters simplifies calculations and conversions, as it is based on the decimal system, making it easier to work with and understand. Overall, the worldwide use of millimeters highlights their importance in various industries and underscores their significance in achieving accurate and standardized measurements.


The millimetre is a unit of length in the metric system, equivalent to one thousandth of a metre (the SI base unit of length).

One meter was defined in 1983 by the 17th conference of weights and measures as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second” and the millimetre by definition is derived as being 1/1000th of that value.

Millimeters are a unit of measurement commonly used in the metric system to quantify lengths and distances. The term "millimeter" is derived from the Latin words "milli," meaning one thousandth, and "metrum," meaning measure. As the name suggests, a millimeter is equal to one thousandth of a meter, making it a very small unit of measurement.

To put the size of a millimeter into perspective, it is roughly the thickness of a credit card or the diameter of a standard paperclip. This tiny unit of measurement allows for precise and detailed measurements, making it an essential tool in various industries.


John Wilkins first proposed the length of a “seconds pendulum” as a universal measurement – ie a measurement that could be carried out, to avoid a definition with respect to a stored reference unit. He thought that a pendulum that made one half oscillation per second would have a fixed length. He was almost correct, but it was later found there is a slight difference in this length around the world. Because of this variability, the Academy of Sciences devised a new measure, equal to 1/10000000 of the distance from the Equator to the North Pole, measured along the meridian through Paris. This measurement was used to create the international prototype meter – a metal bar stored at the BIPM’s headquarters. In time this standard was dropped in favour of more accurate measures – firstly in 1960 the 11th conference of weights and measures defined the meter as “1,650763.73 wavelengths of the orange-red emission line of a krypton-86 atom (in a vacuum)”. Finally in 1983 the 17th conference defined the meter as “the distance travelled by light in a vacuum in 1/299792458 of a second” which therefore leads to a definition of a millimetre as the distance travelled by light in a vacuum in 1/299792458000 of a second.

The origin of millimeters can be traced back to the metric system, which was developed during the French Revolution in the late 18th century. The metric system was created to establish a universal and decimal-based system of measurement that would be easy to use and understand. The system was intended to replace the various inconsistent and complex systems of measurement that were in use at the time.

The metric system was based on the meter as the fundamental unit of length. The meter was defined as one ten-millionth of the distance from the North Pole to the equator, passing through Paris. This definition provided a consistent and measurable standard for length. To further divide the meter into smaller units, the metric system introduced the concept of prefixes. The prefix "milli-" was derived from the Latin word "mille," meaning one thousand. Therefore, a millimeter is one thousandth of a meter. This subdivision allowed for more precise measurements, particularly in scientific and engineering fields.

Common references:

There are 25.4 millimetres in one inch.

The head of a pin is approximately 2mm diameter.

A CD is approximately 1.2mm thick.

00 gauge model railways measure 16.5mm between rails.

Grade 1 hair clippers will cut hair to approximately 3 mm in length (grade 2 cuts to 6 mm, grade 3 to 9 mm etc)

1 metre, 1000 mm

A piece of normal legal paper is approximately .1mm thick

A fine human hair is approximately .04mm wide

An average matchstick is approximately 2mm wide

An AA battery can be between 49.2 and 50.5mm from its flat end to the end of the battery button

Usage context:

The millimetre, as part of the metric system, is used as a measure of length across the globe. The most notable exception is the United States, where the imperial system is still used for most purposes.