SI vs. imperial - a quick reference for engineering students

Almost every other engineering discipline in the world standardized on SI units decades ago. US engineering practice didn't fully follow - a lot of legacy equipment, industry codes, and reference material still run in feet, pounds, and psi, so most programs teach both and expect fluent conversion between them, not a pick-a-side approach.

Where the mixing actually shows up

• Mechanical and civil - stress and pressure specs mix psi and MPa depending on the source document; a single project can cite both.
• Fluid mechanics - flow rates show up in gallons per minute in US industry contexts and liters per minute or cubic meters per second in textbooks and international standards.
• Statics and dynamics - torque specs on real hardware are frequently in lbf-ft even when the rest of a problem set works in newton-meters.

The conversions worth actually knowing cold

A few conversions come up often enough that memorizing them saves real time on a timed exam, instead of re-deriving from a table each time:

• 1 inch = 2.54 cm exactly
• 1 pound (mass) = 0.453592 kg
• 1 psi = 6894.76 Pa
• 1 lbf-ft = 1.35582 N*m

Everything else is worth a lookup or a unit converter rather than memorization - the value is in recognizing which system a given spec is in and not silently mixing them mid-calculation, which is a far more common source of wrong answers than not knowing a conversion factor exists.

Where the real error tends to come from

Unit mistakes rarely come from a bad conversion factor - they come from carrying a value through several steps without tracking its units explicitly, then plugging a number into a formula that expects a different base unit. Writing units next to every intermediate value, not just the final answer, catches this before it compounds. That same discipline matters just as much when propagating measurement uncertainty through a calculation - a dropped or mismatched unit produces an answer that looks plausible right up until it's checked.

If you're studying for an exam that mixes both systems, treating unit conversions as their own flashcard category - not just something you'll figure out live - is worth the ten minutes it takes. See studying outside STEM for how the same spaced-repetition approach that works for MCAT vocabulary applies here too.