Stellar R_X, defined as L_X/L_bol, as a function of Rossby number for the sample of main sequence stars collected by Wright et al. (2011). The Rossby numbers were calculated using the convective turnover times given by Spada et al. (2013). The dashed red line shows our best fit relation, given by Eq. (17). The green line shows the range of values for the non-flaring Sun, with the green circles showing the 10th and 90th percentiles of the Sun’s X-ray luminosity.