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RPM to Hz Converter

Use this free RPM to Hz converter to turn rotational speed in revolutions per minute into shaft frequency in hertz (Hz = RPM / 60) and angular velocity (rad/s = RPM × 2π / 60), and back again. A live 1X–10X harmonic-order table shows the multiples of running speed you would look for in a vibration spectrum, each annotated with its typical diagnostic meaning.

ℹ The conversions here are exact arithmetic — Hz = RPM/60 is simply a unit change. The order→fault notes (1X imbalance, 2X misalignment, ½X oil whirl, etc.) are common diagnostic conventions, not guarantees or a diagnosis: the same order can have several causes, and confirming one needs a calibrated accelerometer, phase data and machine context. This tool tells you which frequencies to look for; it does not measure vibration or judge severity. Verify your running speed (a slipping induction motor turns slightly below its synchronous speed).

Enter a speed or frequency

Type in any one box — the others update instantly. Use the presets for common motor and mains-tied speeds.

Presets:

Harmonic orders (1X–10X of running speed)

Each order is a whole-number (or simple-fraction) multiple of the shaft frequency above. The “typically” column lists the most commonly cited meaning of energy at that order — a starting point for diagnosis, never a verdict.

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How It Works

A rotating shaft completes some number of revolutions per minute (RPM). Frequency in hertz (Hz) counts cycles per second, and one revolution is one cycle, so the conversion is just a change of time base: divide by the 60 seconds in a minute. Hz = RPM / 60, and inversely the Hz to RPM formula is RPM = Hz × 60. At 1800 RPM a shaft turns 30 times a second, i.e. 30 Hz. This shaft frequency is the running speed or 1X order of the machine — the same frequency at which a mass imbalance produces its characteristic spectral peak. If you need to check whether imbalance exceeds the ISO 21940-11 grade limit for your rotor, the Rotational Imbalance Detector takes the same RPM value and computes the permissible residual unbalance and a single-plane trial-weight correction.

Angular velocity measures the same rotation in radians per second. One full turn is 2π radians, so ω = RPM × 2π / 60 = 2π × Hz. Engineers use ω (rad/s) in dynamics, control and resonance equations; technicians use RPM on the nameplate; vibration analysts use Hz and orders on the spectrum. As a rotational speed converter, this tool keeps all three in lock-step so you can move between the worlds in one place.

In vibration analysis the running speed sets a comb of orders — integer multiples 1X, 2X, 3X… and a few sub-synchronous fractions like ½X. Because faults excite characteristic orders, knowing the exact 1X–10X frequencies in Hz tells you where on the spectrum to look. The mapping in the table (1X often imbalance, 2X often misalignment, harmonics with looseness, sub-1X with oil whirl) reflects long-standing field conventions, but these are diagnostic clues, not certainties: one order can arise from several causes, and a real diagnosis needs amplitude from a calibrated accelerometer, phase relationships, the spectrum shape, and knowledge of the machine. Once you have your 1X frequency, many derived fault frequencies scale directly from it: bearing fault frequencies (BPFO, BPFI, BSF, FTF) are fixed multiples of shaft Hz determined by the bearing geometry — use the Bearing Fault Frequency Calculator to compute them. Similarly, blade-pass frequency (BPF = number of blades × shaft Hz) and gear mesh frequency (GMF = tooth count × shaft Hz) both require the exact shaft frequency in Hz as their starting point. For variable-speed drives (VSDs or VFDs), the output frequency changes continuously, so recalculate the order table at each operating speed — fault frequencies shift with the drive, which is why order-tracked spectra (tracking the shaft frequency rather than a fixed Hz axis) are standard practice on variable-speed machinery. Use these frequencies as targets, then confirm with measurement.

How to Convert RPM to Hz: Worked Example

Formula: Hz = RPM / 60  •  rad/s = RPM × 2π / 60

Example — a 2-pole, 50 Hz induction motor running at 3000 RPM:

  1. Shaft frequency: 3000 ÷ 60 = 50.00 Hz
  2. Angular velocity: 3000 × 2π / 60 = 314.16 rad/s
  3. 2X order (misalignment check): 2 × 50.00 = 100.00 Hz

Common reference speeds (synchronous, unloaded AC induction motors): 1500 RPM = 25 Hz (4-pole, 50 Hz mains); 1800 RPM = 30 Hz (4-pole, 60 Hz mains); 3000 RPM = 50 Hz (2-pole, 50 Hz mains); 3600 RPM = 60 Hz (2-pole, 60 Hz mains). Once you have the shaft frequency in Hz, downstream calculations follow directly: bearing fault frequencies (BPFO, BPFI) and gear mesh frequency are both fixed multiples of this value, and the Angular Frequency Calculator extends the same rad/s conversion to period and wavelength.

Frequently Asked Questions

How do I convert RPM to Hz?
Divide the RPM by 60, because there are 60 seconds in a minute and one revolution equals one cycle. So Hz = RPM / 60: 1800 RPM is 30 Hz, 3600 RPM is 60 Hz. To go the other way, multiply hertz by 60 (RPM = Hz × 60). This is exact unit arithmetic, not an estimate.
What is the rad/s (angular velocity) result?
Angular velocity ω expresses the same rotation in radians per second. One revolution is 2π radians, so ω = RPM × 2π / 60, which is also 2π × frequency in Hz. It is the form used in dynamics, control systems and resonance formulas; 1800 RPM equals about 188.50 rad/s.
What does the 1X / 2X / 3X order table mean?
An "order" is a multiple of the running speed (1X = shaft frequency, 2X = twice it, and so on), the natural grid a vibration spectrum is read against. The notes are common conventions: 1X commonly relates to imbalance, 2X to misalignment, multiple harmonics to mechanical looseness, and sub-synchronous components below 1X to oil whirl or rub. They point you to the frequency to inspect — they are not a diagnosis on their own.
Does a peak at 1X always mean imbalance?
No. 1X is the single most common signature of mass imbalance, but a bent shaft, a soft foot, certain misalignments and even resonance near running speed can also raise 1X. Confirming imbalance needs phase measurements (a steady phase that follows the rotor) and amplitude from a calibrated sensor — the order alone is a clue, not proof.
Why might my motor not run at exactly 1800 or 3600 RPM?
Those are the synchronous speeds of AC induction motors (120 × line frequency ÷ poles). Under load an induction motor slips a little below synchronous speed — a "1800 RPM" motor often runs at roughly 1750 RPM. Use the actual measured speed for accurate order frequencies; the presets give the synchronous reference, not the loaded running speed.
Can this tool measure my machine's vibration?
No. It is a converter and a frequency map, not a measurement. It tells you which frequencies correspond to each order so you can search for them in a spectrum, but it does not capture vibration, give amplitude in mm/s or g, or assess severity. For real measurement use a calibrated accelerometer and analyzer; for severity see ISO 10816, which requires a calibrated instrument.
How do I convert RPM to Hz for bearing fault frequency calculations?
Convert RPM to shaft frequency in Hz first (Hz = RPM / 60), then multiply by the bearing’s geometry-specific multipliers. For example, the ball-pass frequency outer race (BPFO) = (number of balls / 2) × (1 − ball diameter/pitch diameter × cosα) × shaft Hz. The exact multipliers depend on your bearing’s dimensions, so use a dedicated bearing fault calculator with those values. The RPM→Hz step here is always the first step, regardless of which fault frequency you are targeting.
How does RPM to Hz conversion work for variable-speed drives (VFDs)?
The conversion formula Hz = RPM / 60 still applies at any instantaneous speed, but on a VFD-driven machine the running speed changes continuously, so all fault frequencies shift with it. The standard practice is order tracking: synchronize the spectrum to the shaft speed so the X-axis is in orders rather than fixed Hz, making faults appear at consistent order positions regardless of speed. This tool gives you the Hz values at any specific RPM you type in, which is useful for steady-state analysis or confirming a specific operating point.
What is the sub-synchronous 0.5X (half-order) component and when does it appear?
The 0.5X component appears at half the shaft running speed in Hz (RPM / 120). In fluid-film (sleeve) bearings this is the classic signature of oil whirl — the oil wedge circulates at roughly 43–48% of shaft speed, which rounds to the 0.5X region in the spectrum. In journal bearings above a critical speed, oil whirl can become oil whip and lock onto the system’s natural frequency rather than tracking shaft speed, which is far more destructive. A 0.5X component can also appear from a rub or from sub-harmonic resonance; phase and amplitude trends over time help distinguish the causes.

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