# Kilogram is SI unit but not gram at the same time meter is SI unit and not kilometer. What decides whether a unit should be considered as SI or not?

centi, milli,kilo,micro etc all are prefixes used in SI system of measurement. So in centimeter both the prefix-centi and meter belong to SI system but centimeter is not SI unit. If it is not SI unit then should we call it non-SI unit.

OR Should we say both meter and centimeter are SI units but meter is the coherent SI unit?

The SI is defined in the BIPM’s “The International System of Units” brochure, currently in its 9th edition.

https://www.bipm.org/en/publications/si-brochure

centimeter is not SI unit

This is not correct. Section 2.3.4 says that “The complete set of SI units includes both the coherent set and the multiples and sub-multiples formed by using the SI prefixes.”

Should we say both meter and centimeter are SI units but meter is the coherent SI unit?

That is correct and is described also in section 2.3.4. The meter is a coherent SI unit. The meter is also a base unit. Although the distinction is less important now than it was in previous editions.

What decides whether a unit should be considered as SI or not

The BIPM (Bureau of Weights and Measures) decides. It has a very lengthy process for deciding. Once it has decided the definition is published in their brochure, referenced above.