A foot is a foot. Fantastic. Glad to know everyone has the same sized feet.
First off, it’s an estimate. Your feet don’t need to be exactly 12 inches/1 foot. If your feet are only 10 inches long, it’s still useful information because you know your margin of error.
That being said, there’s no reason why you can’t also do this trick with the metric system. You would just need to divide the amount-of-human-foot-length by around 4 to get your human -foot-measurement into meters.
Integrals. I can have an area function, integrate it, and then have a volume.
And if you look at it from the Rieman sum angle, you are pretty much adding up an infinite amount of tiny volumes (the area * width of slice) to get the full volume.