Relations between chromospheric and coronal activity diagnostics for the sample of M dwarfs from MA11 (red) with linear regression fits (red dash-dotted) and their variance (red dotted) and for comparison the fits derived by Martínez-Arnáiz et al. (2011b) for their whole sample of F-M dwarfs (black dashed): a) Ca ii K vs. Ca ii H flux; b) Hα vs. Ca ii K flux; c) X-ray vs. Hα flux; and d) Hβ vs. Hα flux. Also plotted, but not included in the fits, in all panels is DENIS 1048-3956 (green triangle), in panel c) UCDs from the literature (brown circles, orange and yellow triangles) and two UCDs for which we present new X-ray data (PC 0025+0447 and DENIS 1228-1547; green circles), and in panel d) two UCDs with both Hα and Hβ measurements from the literature (vB 10 in orange and LHS 2065 in yellow; triangles represent quiescence, and star symbols the flare state; the data points for LHS 2065 correspond to four consecutive spectra tracking the decay of a large flare and show that in the course of the flare evolution, its Hα and Hβ fluxes vary along the flux-flux relationship for earlier M dwarfs).