Angle θ between 2 planes having normal vectors \(\vec{A}\) and \(\vec{B}\) is given by the following formula

\(\theta = \cos^{-1} (\frac{\vec{A}\cdot\vec{B}}{\vert \vec{A} \vert \vert \vec{B} \vert})\)

The angle between the planes can be given as θ or π-θ. Generally the acute angle is considered.