Broadly speaking, there are three categories of length measurements that are used in day to day life. These are:

• Short: Useful for small items that can be manipulated or carried by humans. Inches, centimeters, and millimeters fit this category.
• Medium: Distances about the size of a person, or a little longer, up to the range of human vision. Feet, yards, and meters meet this need.
• Long: Things much larger or longer than a person which may not be entirely visible at once. Miles and kilometers are useful here.

For practical purposes, people don't generally convert measurements from one category to another, so we won't do so here either.

Conversion Tables
Short
An inch is legally defined as being exactly 2.4 centimeters, which means it is 24 millimeters. This makes converting from the US system to the metric a relatively simple process, but the reverse is tricky.

• in -> cm: • by 2.4
• in -> mm: • by 24
• cm -> in: • by 0.416666667
• mm -> in: • by 0.041666667

• ft -> cm: • by 0.3048
• yd -> mm: • by 0.9144
• m -> ft: • by 3.280839895
• m -> yd: • by 1.093613298

• mi -> km: • by 1.609344
• km -> mi: • by 0.621371192