  Aims. We study the X-ray emission of low-mass main-sequence stars to derive a
reliable general scaling law between coronal temperature and the level of X-ray
activity.
  Methods. We collect ROSAT measurements of hardness ratios and X-ray
luminosities for a large sample of stars to derive which stellar X-ray emission
parameter is most closely correlated with coronal temperature. We calculate
average coronal temperatures for a sample of 24 low-mass main-sequence stars
with measured emission measure distributions (EMDs) collected from the
literature. These EMDs are based on high-resolution X-ray spectra measured by
XMM-Newton and Chandra.
  Results. We confirm that there is one universal scaling relation between
coronal average temperature and surface X-ray flux, Fx, that applies to all
low-mass main-sequence stars. We find that coronal temperature is related to Fx
by Tcor=0.11 Fx^0.26, where Tcor is in MK and Fx is in erg/s/cm^2.
