Millimeters are length measurements used in the metric system. They are the smallest measurement most people will use in their everyday lives, though there are still smaller length measurements used in the metric system.

'Milli' comes from the Latin 'mille,' meaning 1,000. Because all metric system measurements are based around the meter, everything is defined in relation to the exact length of the meter. In this case, a millimeter is one-thousandth of a meter.

Although millimeters are very small measurements, they are still commonly used to make precise and accurate readings.

We are going to take a look at eight things commonly measured in millimeters.

The following things are commonly measured in millimeters:

  • Gun calibers
  • Tumors
  • Pen, pencil, and crayon tips and nibs
  • Construction materials
  • Paintbrushes
  • Jewellery
  • Smoking filters
  • Glasses

So let's jump in an take a closer look at each of these.


The caliber of a gun refers to the diameter of the gun's barrel. In the US, the caliber of a gun is measured in inches. For example, a 'US 45 caliber' has a barrel diameter of 0.45 inches.

However, in countries that use the metric system, the caliber of a gun is measured in millimeters. Revolvers and rifles are usually measured at 9 mm.

The measurement of the caliber is important as it allows the user to purchase the correctly sized bullets.

Tumor size

In metric-based countries, tumor sizes are usually measured in millimeters. The tumor's length, width, and height are measured to provide a comprehensive overview of the tumor's size, which can indicate its threat and potency depending on whether it is malignant or benign.

Measuring tumors in millimeters offers precision and accuracy to the measurement that inches and centimeters do not. However, you may also see tumor measurements in both centimeters and millimeters.

Pencils, pens, crayons

The tips and nibs of writing and drawing materials are often measured in millimeters. While most people can get by without knowing the measurement of their writing or drawing stylus, there are artists, illustrators, and even writers who are particular about the exact size of the pen, pencil, or crayon they are working with.

Construction materials

Certain construction materials are measured in millimeters. For example, screw sizes are shown in millimeters on size charts, as are the drill diameters to match the screws.

Millimeters are also used to measure the frames of doors and windows. Again, this is to ensure precise measurements are taken so that the structures fit perfectly.


As with pencils, pens, and crayons, paintbrushes are measured in millimeters to meet the accuracy requirements of the painters. This includes both artists' paintbrushes and brushes for painters and decorators.

Paintbrushes usually come in 10 mm, 20 mm, 40 mm, 50 mm, 60 mm, 70 mm, 80 mm, 90 mm, and 100 mm sizes. The measurement refers to the width of the brush head.


Jewellery is also often measured in millimeters. For example, ring measurements measure the ring's inner circumference to ensure you can find a good fit for your finger.

Similarly, if you have a chain with beads on it, the holes in the beads are usually measured in millimeters. This is because the hole size is generally very small, so you need a precise measurement to make sure the chain will fit through and that the beads won't be too loose when they are on.

Smoking filters

Smoking filters come in different sizes depending on the size of the cigarette the roller intends to roll. Standard filters are 8 mm in diameter and 15 mm long.


The frame size of glasses is measured in millimeters. The frame size has nothing to do with the prescription. Instead, it is about the size of your eyes and your head.

Glasses frames come in the following measurements:

Small: 40 mm to 48 mm

Medium: 49 mm to 54 mm

Large: 55 mm to 58 mm

Extra large: 58 mm+

A millimeter is exactly one-tenth of a centimeter (0.1 cm) and one-thousandth of a meter (0.001 m).

One inch is defined as being exactly 25.4 millimeters. Therefore, a millimeter is equal to exactly 5/127ths of an inch. This is also equal to roughly 0.0393701 inches.

All measurements in the metric system are based on the length of a meter. A meter is defined as the length of the path traveled by light in a vacuum during 1/299792458th of a second.

In terms of real-life comparisons, a millimeter is roughly the thickness of a plastic id card or about 10 stacked sheets of paper. 

There are two metric length measurements smaller than a millimeter. The micrometer and the nanometer.

One millimeter is equal to 1,000 micrometers. And one micrometer is equal to 1,000 nanometers. Therefore, one millimeter is also equal to 1,000,000 nanometers. This means that a nanometer is one billionth of a meter.

Given the microscopic sizes of these measurements, most people are unlikely to ever use them. For example, scientists use nanometers to measure the width of particles.

On a metric ruler or tape measure, there are usually two or three units of measurement; centimeters, meters, and millimeters. The thick numbered lines denote the centimeter units, while the thin, unmarked lines correspond to millimeters, the smallest unit.

The marked line between each centimeter represents half a centimeter, five millimeters. The same unit annotation is used on meter sticks and metric tape measures.

Millimeters are the smallest common measurements in the metric system that people use in their everyday lives. While scientists use smaller measurements, millimeters are the smallest metric length measurement that is visible to the naked eye.

If you need to take a very small measurement with a ruler or measuring tape, you can use millimeter measurements by following the small, un-numbered lines on a metric ruler. You can also easily convert millimeter measurements by using our conversion values outlined above.