It’s only arbitrarily easy since water has a density of 1 kg/l in metric, as it was designed to do so. If you happened to know the density of water is 62.2 lb/ft^3 then the equation is roughly 123*60 which is 360 lb. 372 if you can actually paid attention to what common core was trying to teach. If the material was anything other then water the math would be just as difficult to do in imperial or metric.

Metric is still far superior as the harmonized units make density in particular much easier to convert between. About the only thing imperial is better at is thread pitch of screws. I will also maintain that when describing human temperatures for weather Fahrenheit is a superior scale, but that’s just more personal preference and experience then any rational basis.

Gauge is historically number of passes through gauging machine. With the machine and material in question being different for every single one. We took that and put it to a standard, so it’s super messy and makes no sense.