Convert frequency

About the units

• Hertz (Hz) – The hertz is the SI coherent unit of frequency, one cycle per second, and the name was adopted by the General Conference on Weights and Measures in 1960. It is a legal unit throughout the EU and is used for the mains, radio, sound, clock speeds and screen refresh rates. The definition of the second is expressed in hertz: the caesium frequency is exactly 9,192,631,770 hertz. The trap is what may be counted. The hertz is only for periodic phenomena; random events are measured in becquerels.
• Kilohertz (kHz) – A kilohertz is 1,000 hertz, since the prefix kilo means 10³, and as a decimal multiple of the hertz a legal unit throughout the EU. It belongs to sound and radio: sampling rates in digital audio are given in kilohertz, as is the medium wave band. The spelling is a trap in itself – a lower-case k and a capital H, so kHz, never “KHz”. Another is that kilohertz and kilobits per second sound alike but measure different things: oscillations against data.
• Megahertz (MHz) – A megahertz is a million hertz. The unit measures FM radio, mobile networks, older processors and memory buses, and it is legal throughout the EU as a decimal multiple of the hertz. The trap is to confuse frequency with data: megahertz counts oscillations per second, megabits per second counts bits. A router transmitting on 2.4 gigahertz does not send 2.4 thousand million bits per second – the figure describes the carrier wave, not the speed. The spelling is MHz, with a capital M and a capital H.
• Gigahertz (GHz) – A gigahertz is a thousand million hertz. The unit gives processor clock speeds, wifi on 2.4 and 5 gigahertz, 5G, radar and satellite links. One cycle of a gigahertz signal lasts just long enough for light to travel about 30 centimetres, since the speed of light is exactly 299,792,458 metres per second – which is why track lengths start to matter on circuit boards at these frequencies. The trap is reading clock speed as performance: two processors at the same figure can differ widely.
• Revolutions per minute (rpm) – Revolutions per minute is a rotational frequency: one revolution per minute is exactly one sixtieth of a hertz, because the minute is exactly 60 seconds. It is used for engine speeds, hard drives, spin cycles and machine tools. There is no standard symbol “rpm” – the American NIST writes r/min and EU type-approval papers min⁻¹. The anchor is that 3,000 revolutions per minute is exactly 50 hertz. The trap is angular velocity: in radians per second it is 2π times the rotational frequency.