> the relationship between mass, volume and length is trivially easy to understand.
Imperial units also have a simple relationship between mass and volume. In the US, a pint's a pound the world around, and in the world around the US, a pint of water is a pound and a quarter, and in some of those places that rhymes.
(One fluid ounce — either kind — of water weights approximately one ounce.)
(One fluid ounce — either kind — of water weights approximately one ounce.)