Psi is used a lot in engineering. But honestly, pressure units are a bit of a mess. The metric unit is a Pascal, which is fundamentally defined as a Newton per square meter – unsurprisingly, that is an incredibly small quantity of pressure. It’s roughly 101,500 Pascals for standard atmospheric pressure. You’ll typically see pressure written in either kPa, MPa, or bars (1E5 Pascals) within a metric framework. For perspective, it’s 14.7 psi (lbs per square inch) for an atmosphere.

And personally, I think all of these are pretty silly when we could be using 1 atm instead, which is literally defined as standard atmospheric pressure. It’s a much easier way to visualize and intuitively grasp pressures.

BTU is another fun one. It’s the energy needed to raise 1 lb of water by 1 degF. Calorie is the energy to raise 1 g of water by 1 degC. Both are very pragmatic definitions and have a degree of intuition. Then they’re the metric unit, the Joule, which suffers from the same issue as Pascal. It’s the work done by a 1 Newton force pushing an object 1 meter. Once again, pretty small.