In physics, the attenuation length or absorption length is the distance into a material when the probability has dropped to that a particle has not been absorbed. Alternatively, if there is a beam of particles incident on the material, the attenuation length is the distance where the intensity of the beam has dropped to , or about 63% of the particles have been stopped.

Mathematically, the probability of finding a particle at depth x into the material is calculated by Beer–Lambert law:

.

In general is material and energy dependent.