The wavelength of a wave is a length, so it is measured in units of distance.

  • The SI unit of wavelength is the meter (m).

Depending on how large or small the wavelength is, you often see convenient multiples or submultiples of the meter used:

  • Kilometer (km) – for very long radio waves.
  • Meter (m) – general-purpose unit for all kinds of waves.
  • Centimeter (cm), millimeter (mm) – for longer sound waves, microwaves, some radio waves.
  • Micrometer (µm) – common in infrared.
  • Nanometer (nm) – standard in optics for visible and ultraviolet light.
  • Angstrom (Å) – used for X‑rays and atomic scales (1 Å = 10−1010^{-10}10−10 m).

In short: wavelength is always a length, so any length unit can be used, but meter (m) is the official SI unit, and nm, µm, Å, cm, mm, km are widely used in different contexts for convenience.