Magnitudes of two vectors are added when finding the resultant of the two vectors. The resultant vector is the vector that represents the sum of the two vectors. To calculate the magnitude of the resultant vector, the magnitudes of the two vectors are added together.

Mathematically, if a and b are the magnitudes of the two vectors, and \theta is the angle between them, then the magnitude of the resultant vector \vec{R} is given by:

$$ \|\vec{R}\|=\sqrt{a^2+b^2+2ab\cos\theta} $$