iSTARMOD quantifies chromospheric activity by using the spectral subtraction technique. It measures excess chromospheric emission in late-type stars across several activity indicators, such as Hα and the other Balmer lines in the visible, He I D3 and Na I D1, D2, Ca II H and K, and Ca II infrared triplet, as well as the Paschen series and He I λ10830 lines in the near-infrared. The iSTARMOD code is accompanied with a series of calibrations of χ-functions, to transform the excess emission equivalent widths measured through iSTARMOD into absolute surface fluxes.