Wavelength Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
The wavelength of an electromagnetic wave in a vacuum is calculated with λ = c ÷ f, where c is the speed of light (299,792,458 m/s). For a frequency of 100 MHz (FM radio), the wavelength is about 3 meters.
Explanation
Every electromagnetic wave (radio waves, microwaves, visible light, X-rays…) propagates in a vacuum at the speed of light, a universal constant denoted c and fixed exactly at 299,792,458 meters per second since the redefinition of the meter in 1983. The wavelength (the distance the wave travels in one full oscillation period) and the frequency (the number of oscillations per second) are therefore directly linked by this constant speed: λ = c ÷ f. The higher the frequency, the shorter the wavelength, and vice versa — this relationship explains why radio waves (frequencies from a few kHz to a few GHz) have wavelengths from several meters down to a few centimeters, while visible light, whose frequency is on the order of several hundred trillion hertz (terahertz), has a wavelength of only a few hundred nanometers. This same speed-frequency-wavelength relationship underlies the physics of black-body radiation covered in our Planck's law calculator. This calculator assumes propagation in a vacuum (or, in practice, in air, where the speed of light is extremely close to c): in another medium (water, optical fiber, glass), the wave travels more slowly, and this formula no longer applies directly. For the resonant frequency of an electronic circuit rather than a freely propagating electromagnetic wave, see our Ohm's law calculator for the underlying electrical relationships.
Example: FM radio at 100 MHz
Inputs
Frequency: 100,000,000 Hz (100 MHz).
Calculation
Wavelength = 299,792,458 ÷ 100,000,000 ≈ 2.9979 m, i.e. about 299.79 cm.
Result
An FM radio wave at 100 MHz has a wavelength of about 3 meters.
Frequently asked questions
Why is the speed of light an exact value, with no uncertainty?
Since 1983, the meter has been defined from the speed of light (the distance light travels in a vacuum in 1/299,792,458 of a second), rather than the other way around. The speed of light in a vacuum has therefore become an exact constant by definition, with no experimental margin of uncertainty, unlike most other measured physical constants.
Does this formula also apply to sound?
No, sound is not an electromagnetic wave but a mechanical wave, which propagates at a much lower speed that varies significantly with the medium (about 343 m/s in air, much faster in water or solids). For a wave whose speed depends on the medium, like sound, the wavelength must be calculated with that medium's actual propagation speed instead of c.
Why does Wi-Fi use such a high frequency compared to FM radio?
A higher frequency (and therefore a shorter wavelength) allows more data to be transmitted per second, an essential requirement for Wi-Fi. In exchange, short-wavelength waves pass through obstacles (walls, furniture) less easily than the long wavelengths of FM radio, which explains the more limited range of Wi-Fi inside a building.