The hertz (symbol Hz) is the SI unit of frequency. It is named in honour of the German physicist Heinrich Rudolf Hertz who made some important contributions to science in the field of electromagnetism.

One hertz simply means "one per second"; 100 Hz means "one hundred per second", and so on. The unit may be applied to any periodic event – for example, a clock might be said to tick at 1 Hz.

The old name for the unit was cycles per second (cps), and this term is sometimes seen, along with its related multiples (kilocycles, megacycles, and so forth) in writing from before the second world war.

SI Multiples

1 kilohertz	kHz	103 Hz	1 000 Hz
1 megahertz	MHz	106 Hz	1 000 000 Hz
1 gigahertz	GHz	109 Hz	1 000 000 000 Hz
1 terahertz	THz	1012 Hz	1 000 000 000 000 Hz
1 petahertz	PHz	1015 Hz	1 000 000 000 000 000 Hz
1 exahertz	EHz	1018 Hz	1 000 000 000 000 000 000 Hz

Examples

• 50 Hz or 60 Hz, electromagnetic — standard AC mains power
• 20 Hz to ~16 kHz, acoustic — normal range of adult human hearing (most children and some animals perceive sounds outside this range)
• 261.626 Hz, acoustic — the musical note middle C
• 440 Hz, acoustic — concert pitch (A above middle C), used for tuning musical instruments
• 740 kHz, transitions — the clock speed of the world's first commercial microprocessor, the Intel 4004 (1971)
• 1 MHz to 8 MHz, transitions — clock speeds of early home/personal computers (mid-1970s to mid-1980s)
• 30 MHz to 300 MHz, electromagnetic — VHF terrestrial TV broadcasts
• 88 MHz to 108 MHz, electromagnetic — FM radio broadcasts
• 1420 MHz, the spectral frequency of hydrogen, the most common element in the universe.
• 3.4 GHz, transitions — clock speed of the Pentium 4 "Prescott" microprocessor (2004)
• 428 THz to 750 THz, electromagnetic — visible light, from red to violet
• 30 PHz, electromagnetic — x-rays

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Hertz is also the name of a car rental company. See The Hertz Corporation.