Frequency Converter – Hz, kHz, MHz, GHz and RPM

Hertz and its multiples

Frequency counts how many times something repeats per second. The SI unit is the hertz (Hz): one cycle per second. Larger frequencies use metric prefixes, each 1,000 times the previous one:

• 1 kilohertz (kHz) = 1,000 Hz
• 1 megahertz (MHz) = 1,000 kHz = 1,000,000 Hz
• 1 gigahertz (GHz) = 1,000 MHz
• 1 terahertz (THz) = 1,000 GHz

Revolutions per minute

Engines, motors, fans and hard drives give their speed in revolutions per minute (RPM). Since a minute has 60 seconds, 60 RPM equals 1 Hz. A car engine idling at 900 RPM turns 15 times per second, and a 7,200 RPM hard drive spins at 120 Hz. Mains electricity at 50 Hz drives a two-pole motor at up to 3,000 RPM.

Where frequencies are used

• Audio: human hearing spans roughly 20 Hz to 20 kHz; concert pitch A is 440 Hz.
• Electricity: mains power is 50 Hz in Europe, Asia and Africa and 60 Hz in the Americas.
• Radio: AM radio uses kilohertz (around 530–1,700 kHz) and FM radio uses megahertz (87.5–108 MHz).
• Wi-Fi and mobile networks: Wi-Fi uses the 2.4, 5 and 6 GHz bands; 5G uses bands from below 1 GHz to above 24 GHz.
• Computers: processor clock speeds are measured in gigahertz, for example 3.5 GHz.
• Displays: monitor refresh rates such as 60, 120 or 144 Hz show how many images appear per second.

Frequency and period

The period is the time of one cycle and is the inverse of frequency: period = 1 ÷ frequency. A 50 Hz signal repeats every 20 milliseconds, and a 1 kHz tone every millisecond. For radio waves, the wavelength in metres is about 300 divided by the frequency in MHz, so 100 MHz corresponds to a wavelength of 3 metres.

Sources

Conversion factors follow the SI definitions in the BIPM SI Brochure and the exact factors for US customary and imperial units in NIST Special Publication 811.