We describe a cluster algebra algorithm for calculating $q$-characters of Kirillov–Reshetikhin modules for any untwisted quantum affine algebra $U_q\left(\widehat{𝔤}\right)$. This yields a geometric $q$-character formula for tensor products of Kirillov–Reshetikhin modules. When $𝔤$ is of type $A,D,E$, this formula extends Nakajima’s formula for $q$-characters of standard modules in terms of homology of graded quiver varieties.