  The main purpose of this paper is to provide formulas for the Hilbert-Kunz
multiplicity of fiber products and idealization rings. We calculate the
Hilbert-Kunz multiplicity of a fiber product $R \times_T S$, with $R$, $S$ and
$T$ being Noetherian local rings with common characteristic and residue field.
Inspired by a work of Strazzanti and Armengou we calculate the Hilbert-Kunz
multiplicity of an idealization ring $R \ltimes M$, where $M$ is a finite
$R$-module. We explore certain examples and structural consequences and obtain
some bounds for the Hilbert-Kunz multiplicity.
