Hertz (Hz) is a unit that counts how many times something repeats in one second. One hertz equals one cycle per second. If a guitar string vibrates back and forth 440 times in a single second, it’s oscillating at 440 Hz. That’s really all there is to the core concept, but hertz shows up everywhere: in sound, light, electricity, brain activity, and the screen you’re reading this on.
What Counts as a “Cycle”
A cycle is one complete repetition of a pattern. Picture a wave rising up, dipping down, and returning to where it started. That’s one cycle. Frequency is simply how many of those cycles happen per second, and the hertz is the SI unit for measuring it. A slow ocean wave might complete one cycle every 10 seconds, giving it a frequency of 0.1 Hz. A hummingbird’s wings beat about 50 times per second, or 50 Hz.
The relationship between frequency and time is straightforward. If something cycles at 100 Hz, each individual cycle takes 1/100th of a second (0.01 seconds) to complete. That duration of a single cycle is called the period. Frequency and period are inverses of each other: double the frequency, and each cycle takes half as long.
Where the Name Comes From
The unit is named after Heinrich Hertz, a German physicist who in 1887 became the first person to produce and detect radio waves in a lab. His experiments confirmed James Clerk Maxwell’s earlier theoretical prediction that electromagnetic waves existed. Hertz showed these invisible waves traveled in straight lines and could be focused, reflected, and polarized, just like light. The waves were initially called “Hertzian waves” before being renamed radio waves. In 1930, the international scientific community honored his work by naming the frequency unit after him.
Hertz in Sound
Sound is vibrations traveling through air (or water, or solid material), and pitch is determined by frequency. A deep bass note might sit around 80 Hz, meaning the air pressure fluctuates 80 times per second. A high-pitched whistle could be 4,000 Hz or more. Human hearing spans roughly 20 Hz to 20,000 Hz (20 kHz), though that upper limit drops with age. Most adults top out around 15,000 to 17,000 Hz. Infants can hear slightly above 20 kHz, but that sensitivity fades during childhood.
Anything below 20 Hz is called infrasound. You can’t hear it, but you can sometimes feel it as a rumble or pressure. Anything above 20 kHz is ultrasound. Some animals operate well outside our range. Certain bat species detect frequencies up to 200 kHz, with a lower limit right around where human hearing drops off.
Hertz in Light and Radio Waves
Light, radio signals, microwaves, and X-rays are all electromagnetic waves, and they’re all measured in hertz. The only difference between them is frequency. Radio waves sit at the low end, roughly 300 kilohertz (kHz) to 30 gigahertz (GHz). Visible light is much higher, in the hundreds of terahertz range (a terahertz is a trillion hertz). X-rays and gamma rays go higher still.
When you tune a radio to 101.5 FM, you’re selecting a signal oscillating at 101.5 million cycles per second (101.5 MHz). Your Wi-Fi router operates at either 2.4 GHz or 5 GHz, meaning the electromagnetic wave it produces cycles billions of times every second. The principle is always the same: hertz tells you how many cycles happen per second, whether the wave is a sound vibration or a beam of light.
Hertz in Your Power Outlets
The electricity in your wall outlets uses alternating current (AC), meaning the flow of electrons reverses direction at a set frequency. In the United States, that frequency is 60 Hz, so the current switches direction 60 times per second. In Europe, the UK, most of Asia, Africa, and Australia, the standard is 50 Hz.
These numbers trace back to early industrial decisions. Westinghouse Electric in America settled on 60 Hz, while General Electric’s European operations adopted 50 Hz. The choices stuck and became national standards. The difference matters for electrical equipment: devices designed for 60 Hz systems may not run correctly on a 50 Hz grid without adapters or converters, which is why international travelers sometimes need more than a simple plug adapter.
Hertz on Screens and Monitors
When a monitor advertises a 60 Hz, 144 Hz, or 240 Hz refresh rate, it’s telling you how many times per second the display redraws the image. A 60 Hz screen updates 60 times per second. A 144 Hz screen updates 144 times, producing noticeably smoother motion, especially in fast-paced video games or high-speed video.
There’s a catch, though. Your screen’s refresh rate only matters if your computer’s processor and graphics card can actually produce frames fast enough to match. If your hardware outputs 30 frames per second, a 144 Hz monitor won’t look any better than a 60 Hz one for that content. The monitor can refresh as fast as it wants, but it’s just redrawing the same frame if new ones aren’t ready. For everyday tasks like reading or browsing, most people won’t notice much difference above 60 Hz. For competitive gaming, higher refresh rates can make actions feel more responsive and animations appear much smoother.
Hertz in Brain Activity
Your brain produces electrical signals that can be measured with sensors on the scalp, and these signals oscillate at specific frequencies tied to different mental states. Neuroscientists group them into bands:
- Delta waves (2 to 4 Hz) are the slowest and are linked to deep sleep, basic motivation, and mental concentration. They also play a role in processing speech.
- Theta waves (4 to 7 Hz) increase during tasks that load working memory, like mental math or holding a phone number in your head.
- Alpha waves (8 to 12 Hz) are prominent when you close your eyes and relax. The lower end of this band (8 to 10 Hz) behaves differently from the upper end (10 to 12 Hz), with increases in alpha power reflecting the brain actively suppressing certain regions.
- Beta waves (16 to 25 Hz) are associated with active movement and engaged thinking.
- Gamma waves (30 to 50 Hz) are the fastest commonly measured waves and tend to increase during complex cognitive tasks.
These aren’t separate signals your brain switches between. Multiple frequency bands are active simultaneously, with different regions of the brain producing different patterns depending on what you’re doing.
Hertz in Medical Ultrasound
Medical ultrasound devices use sound waves at frequencies far above human hearing. Diagnostic imaging typically operates between 1 and 3 MHz (1 million to 3 million cycles per second). At these frequencies, sound waves penetrate tissue, bounce off internal structures, and return to a sensor that assembles them into an image. Lower frequencies penetrate deeper into the body but produce less detailed images. Higher frequencies give sharper detail but don’t reach as far.
Therapeutic ultrasound, used in physical therapy to promote tissue healing, operates in similar frequency ranges. Some newer “longwave” devices use frequencies around 40,000 to 50,000 Hz, which is still well above audible sound but much lower than traditional medical ultrasound.
How Hertz Scales Up
Because hertz can range from single digits to trillions, scientists use standard metric prefixes to keep numbers manageable. One kilohertz (kHz) is 1,000 Hz. One megahertz (MHz) is 1 million Hz. One gigahertz (GHz) is 1 billion Hz. One terahertz (THz) is 1 trillion Hz. Your computer’s processor speed, listed in GHz, tells you how many billions of basic operations it can perform per second. A 3.5 GHz chip cycles 3.5 billion times every second.
At the foundation of all these measurements is the scientific definition of a second itself. The international standard defines one second as exactly 9,192,631,770 oscillations of a cesium-133 atom. That atomic vibration is the reference clock against which all frequency measurements are ultimately calibrated.