Why So Many Pressure Units Exist
Pressure units proliferated because different fields historically standardized on different reference points and measurement traditions, and none ever fully displaced the others. Pascals (the SI unit) define pressure directly as force per unit area — clean and metric, but pascals are a tiny unit, so kilopascals are more commonly seen in practice. Bar (close to, but not exactly, standard atmospheric pressure) remains common in European engineering and meteorology. PSI (pounds per square inch) dominates in the US, especially for tire pressure and plumbing. Atmospheres define pressure relative to sea-level air pressure specifically, useful for anything comparing against ambient conditions. Torr (roughly one millimeter of mercury) persists in vacuum and scientific contexts, tracing back to mercury barometer measurements.
None of these units are simple round-number multiples of each other, which is exactly why converting by memory or rough estimation is risky for anything where the actual pressure value matters practically — an incorrectly converted tire pressure or industrial system spec isn't just an academic rounding error, it's a real safety-relevant mistake.
Standard atmospheric pressure at sea level — 101,325 pascals, or 1 atmosphere, or roughly 14.7 PSI — is the one reference value worth memorizing, since it's the anchor most other pressure comparisons implicitly relate back to.