Enter a frequency in Hz or a wavelength in metres. The tool applies λ = c/f both ways and lists kHz, MHz, GHz, cm and mm values.
λ = c ÷ f, where c is 299,792,458 m/s, the exact speed of light in vacuum.
100 MHz gives λ = c/f ≈ 2.9979 m, a typical FM-radio wavelength.
Yes — it uses the vacuum speed of light; in other media, wavelength shrinks by the refractive index.