Measuring frequency in hertz means counting how many times a signal completes one full cycle per second. The method you use depends on what you’re measuring: electrical signals in a circuit, sound waves in a room, or vibrations from a machine. Each requires different tools, but the underlying principle is the same.
What Hertz Actually Measures
One hertz equals one complete cycle per second. A 60 Hz electrical signal completes 60 full oscillations every second. A 440 Hz sound wave (the standard tuning note A) vibrates 440 times per second. The math behind frequency is straightforward: frequency equals 1 divided by the period, where the period is the time it takes for one complete cycle. If a wave takes 0.02 seconds to complete one cycle, its frequency is 1 / 0.02 = 50 Hz.
You can also calculate frequency from wave speed and wavelength. Divide the speed of the wave by its wavelength to get the frequency. A sound wave traveling at 343 meters per second with a wavelength of 0.78 meters has a frequency of about 440 Hz. These formulas are useful for calculating frequency from other known quantities, but for direct measurement, you’ll need an instrument.
Using a Digital Multimeter
A digital multimeter (DMM) with a frequency function is the most accessible tool for measuring electrical frequency. Many mid-range and professional multimeters include this capability, either as a primary dial position or as a secondary function accessed through a button.
If your multimeter has a dedicated Hz position on the dial, turn it there, plug the black test lead into the COM jack and the red lead into the voltage jack, then touch the probes to the circuit. The display will show the frequency in hertz. If your meter uses a frequency button instead, start by setting the dial to AC voltage. For unknown circuits, set the voltage range to the highest setting, though most modern meters will autorange. Then press the Hz button to switch the display from voltage to frequency.
A few things can trip you up. Circuits with heavy distortion, like those near variable frequency drives (VFDs), can produce noise that throws off the reading. If your meter has a low-pass filter setting for AC voltage, use it in those situations. You can also try switching to the DC voltage setting and pressing the Hz button again, or changing the voltage range to compensate for electrical noise. Every multimeter has a specific frequency measurement range listed in its manual. If the signal falls outside that range, the reading won’t be accurate.
When you’re finished, remove the red lead first, then the black. When measuring anything connected to mains power, check for dangerous voltage between all conductor pairs, including each conductor to ground, before handling the circuit. Use a meter rated for the voltage category of the environment you’re working in.
Using an Oscilloscope
An oscilloscope gives you the most detailed frequency measurement because it shows the actual shape of the waveform on screen. You measure frequency by capturing one or more complete cycles and reading the time between them.
Connect your probe to the signal and set the time-per-division (the horizontal scale) to a value appropriate for the frequency you expect. The goal is to see at least one or two full cycles clearly on screen without the waveform being stretched too thin or compressed into a blur. Set the trigger mode to “Auto” or “Normal” and adjust the trigger level until the waveform stabilizes. A stable display means the oscilloscope is consistently capturing the waveform at the same point in each cycle.
To calculate frequency manually, measure the time for one complete cycle using the horizontal grid divisions, then divide 1 by that time. If one cycle spans 4 divisions and each division represents 5 milliseconds, the period is 20 milliseconds, giving a frequency of 1 / 0.020 = 50 Hz. Most digital oscilloscopes also have built-in measurement tools that calculate the frequency automatically once the waveform is captured cleanly.
Oscilloscopes are particularly useful when you need to see what the signal looks like, not just its frequency. A distorted sine wave and a clean one can have the same fundamental frequency, but the oscilloscope reveals the difference instantly.
Measuring Sound Frequency With a Phone
For audio frequencies, your smartphone can work surprisingly well. Apps like Phyphox (free, available on Android and iOS) use your phone’s built-in microphone to capture sound and run a Fast Fourier Transform, which is an algorithm that breaks a complex audio signal into its individual frequency components. The audio spectrum feature in Phyphox displays the peak frequency and even identifies the corresponding musical note. Physics Toolbox by Vieyra Software is another free option that offers similar functionality.
Phone microphones have limitations. They’re typically optimized for voice frequencies (roughly 300 Hz to 3,400 Hz) and lose accuracy at very low or very high frequencies. Background noise also muddles results. For best accuracy, measure in a quiet environment with the sound source close to the phone. These apps are excellent for tuning instruments, identifying hums from appliances, or classroom demonstrations, but they aren’t precise enough for professional acoustic analysis.
The Sampling Rate Limit
Any digital measurement tool, whether it’s a multimeter, oscilloscope, or phone app, has a fundamental limit on the highest frequency it can accurately capture. This limit comes from a principle called the Nyquist theorem: the tool must sample the signal at least twice per cycle to represent it correctly. A device sampling at 16,000 times per second can only measure frequencies up to 8,000 Hz accurately.
When a signal exceeds this limit, something called aliasing occurs. The frequency doesn’t just disappear. Instead, it folds back and appears as a false, lower frequency in the measurement. If that 16,000 Hz sampler encounters a 10,000 Hz signal, the signal sits 2,000 Hz above the 8,000 Hz limit and appears as a ghost signal at 6,000 Hz, with the same amplitude as the original. It looks completely real but is entirely wrong.
In practice, this means you should always check your instrument’s maximum frequency specification before trusting a reading. For a phone app, the sampling rate is usually 44,100 Hz or 48,000 Hz (depending on the device), which limits accurate measurement to about 20,000 Hz. For oscilloscopes and dedicated frequency counters, the bandwidth specification in the manual tells you the true upper limit, and it’s the bandwidth, not the sample rate alone, that determines how high you can go.
Choosing the Right Tool
- Multimeter with Hz function: Best for quick checks of electrical signal frequency in circuits, especially AC mains or motor drives. Simple, portable, and affordable.
- Oscilloscope: Best when you need to see the waveform shape, measure signals with complex patterns, or capture very fast signals in the megahertz range and above.
- Smartphone app: Best for audio frequencies in non-critical applications like music, acoustics experiments, or identifying an annoying hum.
- Dedicated frequency counter: Best for high-precision measurements in calibration labs or RF work, where you need accuracy down to fractions of a hertz over a wide range.
The right choice depends on what you’re measuring, how precise you need to be, and whether you care about the shape of the signal or just its repetition rate. For most home and workshop tasks, a multimeter with a frequency function covers the basics. For anything involving waveform analysis or high-speed signals, an oscilloscope is worth the investment.