ELi5: Where did all the different units of measurement come from? And how did we get a uniformed decision on what is what?

Question

I’m thinking distance or weight. Where did a fathom or metre come from and how do I know that my kilo is the same as yours?

In: Mathematics

Answer 1

For the most part, somebody just creates a unit, and if it catches on, we use is.

One thing that you might find interesting is to read up on the ‘Le Grand K’.

Basically, the kilogram was (until last year) defined as ‘exactly the weight of ‘Le Grand K, which is stored in France. This causes some issues. 1. I can’t really go there and measure it, and 2, the weight is not stable. Since that object ALWAYS (pre 2019) weights 1 kilogram, as it gains or loses weight, the definition of a kilogram should change. For many practical reasons, it doesn’t, which is why there was an effort to replace the standard. Here is an interesting video about it (pre 2019), so this is an attempt to replace the standard.