Introduction
Frequency is 'cycles per second' — the rate at which something repeats. Audio engineers talk in Hertz (20 Hz-20 kHz is the human hearing range). Radio engineers talk in MHz and GHz. Car mechanics talk in RPM (revolutions per minute). All the same underlying quantity, different scale.
Why frequency units exist and how they diverged
The Hertz (Hz) was officially adopted in 1930, replacing 'cycles per second' — Heinrich Hertz proved electromagnetic waves exist in 1887, and the unit honors him. RPM (revolutions per minute) is just Hz × 60, and persists because engines and motors are timed in minutes, not seconds. 3,600 RPM = 60 Hz (the frequency of US AC power, useful to match for motor speed).
Wireless frequencies have been subdivided by international treaty into bands: HF (3-30 MHz, shortwave), VHF (30-300 MHz, FM radio and TV), UHF (300 MHz-3 GHz, cell phones and TV), SHF (3-30 GHz, satellite and radar), EHF (30-300 GHz, 5G mmWave). Each band has propagation characteristics dictating its use.
How to convert frequency
Hz to kHz: divide by 1,000. Hz to MHz: divide by 1,000,000. Hz to RPM: multiply by 60. The relationship to wavelength: wavelength (m) = speed of light / frequency (Hz) = 300,000,000 / Hz. A 100 MHz FM station has 3-meter wavelength; a 5 GHz WiFi signal has 6 cm wavelength.
Units supported by this frequency calculator
- Hertz
- Kilohertz
- Megahertz
- Gigahertz
- RPM
Common frequency conversion mistakes
- Hz vs Hz/s. Hz is already per-second. 'Cycles per second per second' would be angular acceleration (unusual).
- Bits vs Hz. A '2.4 GHz processor' runs at 2.4 billion clock cycles per second. But one clock cycle is not one instruction — modern CPUs execute multiple instructions per cycle. Clock rate is not computing power.
- RPM vs rad/s. Motor specs sometimes use radians per second. 1 rev = 2π rad, so 3,000 RPM = 50 rev/s = 314 rad/s.
- Confusing sampling rate with frequency content. A 44.1 kHz audio sample rate captures frequencies up to 22.05 kHz (Nyquist limit) — exactly the top of human hearing. 96 kHz or higher captures nothing audible for humans.
- FM vs AM station numbers. FM is in MHz (87.5-108.0). AM is in kHz (535-1,705). '93.5' on the FM dial is 93.5 MHz; '1040' on AM is 1,040 kHz (not MHz).
Real-world frequency examples
- Middle C (music): 261.63 Hz.
- Concert A (tuning reference): 440 Hz.
- Human hearing range: 20 Hz-20 kHz (declines with age, especially above 12 kHz).
- FM radio: 87.5-108 MHz.
- WiFi 5 GHz band: 5.15-5.85 GHz.
- Cell phone 5G mmWave: 24-47 GHz.
- US AC power: 60 Hz. Europe: 50 Hz.
- Idle car engine: 600-900 RPM = 10-15 Hz.
- Highway engine: 2,500-3,000 RPM = 42-50 Hz.
- Red light: ~460 THz (teraHertz, visible light).
Tips for accurate frequency conversion
- For audio, 44.1 kHz is the CD standard. 48 kHz is video-industry standard. Higher is only marginally useful.
- For radio, always match band class to use — HF for long-distance, VHF for line-of-sight, UHF/SHF for high-bandwidth short-range.
- Check motor RPM specs against the power frequency (60 Hz in US, 50 Hz in EU). A European motor at 60 Hz runs 20% faster than rated.
Related: Time Converter · Angle Converter · SI Prefix Converter.