  Atmospheric escape is an important factor shaping the exoplanet population
and hence drives our understanding of planet formation. Atmospheric escape from
giant planets is driven primarily by the stellar X-ray and extreme-ultraviolet
(EUV) radiation. Furthermore, EUV and longer wavelength UV radiation power
disequilibrium chemistry in the middle and upper atmosphere. Our understanding
of atmospheric escape and chemistry, therefore, depends on our knowledge of the
stellar UV fluxes. While the far-ultraviolet fluxes can be observed for some
stars, most of the EUV range is unobservable due to the lack of a space
telescope with EUV capabilities and, for the more distant stars, to
interstellar medium absorption. Thus, it becomes essential to have indirect
means for inferring EUV fluxes from features observable at other wavelengths.
We present here analytic functions for predicting the EUV emission of F-, G-,
K-, and M-type stars from the log $R'_{HK}$ activity parameter that is commonly
obtained from ground-based optical observations of the Ca II H&K lines. The
scaling relations are based on a collection of about 100 nearby stars with
published log $R'_{HK}$ and EUV flux values, where the latter are either direct
measurements or inferences from high-quality far-ultraviolet (FUV) spectra. The
scaling relations presented here return EUV flux values with an accuracy of
about three, which is slightly lower than that of other similar methods based
on FUV or X-ray measurements.
