How many in in 400 mm?

A unit of length equal to 1/1000 of a meter and is equivalent to 0.03937 inch. Abbreviation: mm

How big is a millimeter?

Here are examples of things that measure around 1 mm:

• The thickness of a credit card
• A typical mechanical pencil tip measures, for example, 0.5, 0.7, or 0.9 mm
• A medium pen tip size delivers a line that’s about 1.0 mm in width for a ballpoint pen.

Here are more examples of things that measure about one millimeter (order of magnitude):

The width of the lead in a mechanical pencil
The thickness of a credit card
The length of a flea
The width of a grain of salt
The width of a staple
The diameter of the tip of a ballpoint pen
The thickness of a single strand of human hair
The height of the lettering on a penny

An inch is a unit of length or distance in a number of systems of measurement, including in the US Customary Units and British Imperial Units. One inch is defined as 1⁄12 of a foot and is therefore 1⁄36 of a yard. According to the modern definition, one inch is equal to 25.4 mm exactly.

The inch is abbreviated as "in" or sometimes as double quotes ("), especially when referring to the size of a screen or display.

Inches are commonly used to measure the length, width, or height of various objects, such as:

The diagonal size of computer or TV screens is often expressed in inches.
The size of furniture, such as tables, desks, or shelves, may be expressed in inches.
Clothing sizes, such as the length of pants or the circumference of a waist, may be expressed in inches.
The size of paper, such as letter or legal size, is often expressed in inches.
The dimensions of construction materials, such as lumber or drywall, are often expressed in inches.

Overall, the inch is a widely used unit of measurement for expressing length or distance in many different contexts and applications in the United States and other countries that use the customary or imperial systems of measurement.