3,600 rpm ÷ 60 s/min = 60.00 Hz
The worked pair every engineer memorizes
3,600 rpm ÷ 60 = 60 Hz (a two-pole motor synchronized to a 60 Hz grid)
7,200 rpm ÷ 60 = 120 Hz (the performance hard-drive spindle class)
reverse: 50 Hz × 60 = 3,000 rpm (two-pole motor on a 50 Hz grid)
The motor examples are why this conversion appears constantly in electrical engineering: AC motor speeds are tied to grid frequency, so 50 Hz countries see 3,000 rpm two-pole synchronous speeds where 60 Hz countries see 3,600. The hard-drive example is worked further on the HDD page.
Why the factor is exactly 60
Rotational frequency is revolutions per unit time; only the time unit changes between rev/min and rev/s, and hertz is the SI name for per-second frequency BIPM SI Brochure2026-08. NIST's units guidance covers the accepted non-SI status of minute-based units NIST2026-08: the conversion is exact by definition, with no empirical constant anywhere in it.
When engineers quote which
- Hz: electrical machines, vibration analysis, anything sharing math with signal frequency.
- RPM: mechanical shafts, engines, spindles: anywhere a human reads a dial or a spec sheet.
- rad/s: dynamics equations; the third leg of the triangle, converted on its own page.