Formal status
The SI expresses frequency in hertz (reciprocal seconds) and angular velocity in radians per second; the SI Brochure defines both and how non-SI units relate BIPM SI Brochure2026-08. Revolution per minute appears in ISO 80000-3, the standard for space and time quantities, as a unit of rotational frequency ISO 80000-3 (space and time quantities)2026-08, and NIST's guidance lists revolution-based units among those accepted for use with the SI NIST2026-08. In short: rev/min is legitimate, standardized and non-SI, exactly like the minute itself.
The two derivations
rotational frequency: n = 1 rev/min = 1/60 rev/s ⇒ f(Hz) = rpm ÷ 60
angular velocity: one revolution = 2π rad ⇒ ω(rad/s) = rpm × 2π ÷ 60 ≈ rpm × 0.10472
example: 3,000 rpm = 50 Hz = 314.16 rad/s
Both factors come from unit bookkeeping alone: sixty seconds per minute, 2π radians per revolution. The subtlety worth knowing: a revolution is a count, not an SI unit, so rev/min and rad/s measure related but distinct quantities (rotational frequency vs angular velocity), connected through the 2π radians each revolution contains BIPM SI Brochure2026-08. The interactive versions live at RPM to Hz and RPM to rad/s.
Notation in technical writing
| Notation | Where it is at home |
|---|---|
| rpm | Everyday and commercial writing; dashboards, spec sheets |
| r/min or rev/min | Engineering documents following ISO 80000-3 style |
| min⁻¹ | Formal standards notation for per-minute rotational frequency |
| Hz | SI rotational frequency; motors and drives engineering |
| rad/s | SI angular velocity; kinematics and torque-power equations |
House style for this site: rpm in prose, exact factors in math blocks, and the dial gauges label their unit explicitly, because the one thing a unit page must not do is leave its own units ambiguous.