Wavelength to Frequency Calculator
Converts the wavelength of an electromagnetic wave to its frequency using the speed of light (c ÷ wavelength). Enter a wavelength in metres and get the corresponding frequency in hertz.
Last updated: May 2026
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About this calculator
Every electromagnetic wave — radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma — travels through a vacuum at the speed of light, c = 299,792,458 metres per second. Its wavelength (λ, the distance between successive crests) and frequency (f, the number of cycles per second) are tied together by c = f × λ, so frequency = c ÷ wavelength. Long wavelengths mean low frequencies and short wavelengths mean high frequencies; the product is always the speed of light. For a 3-metre wavelength, the frequency is 299,792,458 ÷ 3 ≈ 99,930,819 Hz, or about 99.93 MHz — right in the FM radio band. This relationship lets you move freely between the two descriptions of a wave. Radio engineers think in frequencies (MHz, GHz), while optics and spectroscopy often use wavelengths (nanometres for visible light). To use this calculator, enter the wavelength in metres; for other units convert first — 1 nanometre is 1e-9 m, 1 micrometre is 1e-6 m, and 1 centimetre is 0.01 m. So green light at 550 nm is 5.5e-7 m, giving a frequency around 5.45e14 Hz. One important caveat: the speed of light used here is the vacuum value. In a medium such as glass, water, or a cable dielectric, light travels slower (the medium’s refractive index reduces the speed), so the frequency-wavelength relationship uses the slower speed; frequency stays the same as in vacuum while the wavelength shortens. For everyday radio and free-space optics the vacuum value is an excellent approximation.
How to use
Example 1 — FM radio. A station broadcasts at a 3-metre wavelength. Enter 3. Result: about 99,930,819 Hz, i.e. 99.93 MHz. Verify: 299,792,458 ÷ 3 ≈ 99,930,819. ✓ That is squarely within the 88–108 MHz FM band. Example 2 — Visible green light. Green light has a wavelength of 550 nm = 0.00000055 m. Enter 0.00000055. Result: about 5.451e14 Hz (545 THz). Verify: 299,792,458 ÷ 5.5e-7 ≈ 5.451e14. ✓ Remember to convert nanometres to metres before entering.
Frequently asked questions
What value for the speed of light does this use?
It uses the exact vacuum speed of light, c = 299,792,458 metres per second, which is a defined constant in the SI system. This is the correct value for electromagnetic waves travelling through a vacuum or, to an excellent approximation, through air. In denser media such as glass, water, or a coaxial cable’s dielectric, light slows down by a factor equal to the medium’s refractive index, so the calculation would need the reduced speed. For most radio, free-space optics, and general physics problems, the vacuum value is the right choice and any error from using it in air is negligible.
How do I enter wavelengths given in nanometres or centimetres?
Convert to metres first, because the calculator expects metres. Multiply by the appropriate power of ten: nanometres to metres is ×1e-9 (so 550 nm = 0.00000055 m), micrometres is ×1e-6, millimetres is ×0.001, and centimetres is ×0.01. For very small wavelengths it is easiest to enter scientific notation, e.g. 5.5e-7 for 550 nm. Forgetting this conversion is the most common mistake and produces frequencies that are off by many orders of magnitude. Once you enter the wavelength in metres, the frequency comes out correctly in hertz.
Why does shorter wavelength mean higher frequency?
Because the speed of light is fixed, the product of wavelength and frequency must stay constant (c = f × λ). If the waves are packed closer together (shorter wavelength), more of them must pass a point each second to maintain the same speed, which means a higher frequency. Conversely, long, stretched-out waves pass less often, giving a lower frequency. This inverse relationship is why gamma rays (tiny wavelengths) have enormous frequencies and energies, while long radio waves have low frequencies. It is a direct consequence of all electromagnetic radiation sharing the same travel speed in a vacuum.
What mistakes do people make converting wavelength and frequency?
The dominant mistake is unit confusion — entering a wavelength in nanometres or centimetres as if it were metres, which skews the answer by factors of millions or hundreds. Another is using the speed of sound (about 343 m/s) instead of the speed of light; this calculator is for electromagnetic waves, not sound, where the wave speed is completely different. People also forget that in a material medium the effective speed is lower, so a fibre-optic or in-glass calculation needs the refractive-index-adjusted speed. Finally, mixing up which quantity is which — treating a frequency as a wavelength — produces nonsensical results; wavelength goes in, frequency comes out.
When should I not use this calculator?
Do not use it for sound waves: sound is a mechanical wave whose speed (around 343 m/s in air, and very different in water or solids) is not the speed of light, so you would need the medium-specific sound speed instead. It is also inaccurate for electromagnetic waves travelling through a dense medium unless you substitute the slower, refractive-index-adjusted speed — relevant in fibre optics and inside glass or water. For matter waves in quantum mechanics (the de Broglie wavelength of particles), an entirely different relationship applies. This tool is specifically for electromagnetic radiation in vacuum or air, where the speed of light is the correct constant.