I was born and raised in Europe. I live in Canada that has (mostly) gone metric in the 1970's. Yet, I use inches. They are nicer! I recently built a shelf and hanger rack in a weirdly-shaped closet. I used Sketchup to design it, and naturally made everything in inches. Inches are "semi-metric" too, thanks to mils: thousandths of an inch. Mils are very nice units for fine work, like circuit boards. Millimeters are way…
I just assumed that everyone was using such standards when I was little and then came things like miles, pounds, yards, foot and I was like wtf, it's so complicated.
I know how long a cm is by pinching my index and thumb finger and measuring the distance between the nails. I know roughly how long is a meter based on my height. I know roughly how long is a km based on my running distance.