  Strong spectral lines are useful indicators of stellar chromospheric
activity. They are physically linked to the convection efficiency, differential
rotation, and angular momentum evolution and are a potential indicator of age.
However, for ages > 2 Gyr, the age-activity relationship remains poorly
constrained thus hampering its full application. The Ca II infrared triplet
(IRT lines) has been poorly studied compared to classical chromospheric
indicators. We report in this paper absolute chromospheric fluxes in the three
Ca II IRT lines, based on a new calibration tied to up-to-date model
atmospheres. We obtain the Ca II IRT absolute fluxes for 113 FGK stars from
high signal-to-noise ratio and high-resolution spectra covering an extensive
domain of chromospheric activity levels. We perform an absolute continuum flux
calibration for the Ca II IRT lines anchored in atmospheric models calculated
as an explicit function of effective temperatures, metallicity, and gravities
avoiding the degeneracy present in photometric continuum calibrations based
solely on color indices. The internal uncertainties achieved for continuum
absolute flux calculations are 2\% of the solar chromospheric flux, one order
of magnitude lower than photometric calibrations. We gauge the impact of
observational errors on the final chromospheric fluxes due to the absolute
continuum flux calibration and find that $T_{\rm eff}$ uncertainties are
properly mitigated by the photospheric correction leaving [Fe/H] as the
dominating factor in the chromospheric flux uncertainty. Across the FGK
spectral types, the Ca II IRT lines are sensitive to chromospheric activity.
The reduced internal uncertainties reported here enable us to build a new
chromospheric absolute flux scale and explore the age-activity relation from
the active regime down to very low activity levels and a wide range of $T_{\rm
eff}$, mass, [Fe/H], and age.

Solar and Stellar Astrophysics