Thermal conductivity measurements of a wellbore fluid may be used to derive the sag of the wellbore fluid (i.e., the inhomogeneity or gradation in particle distribution in the fluid as a result of the particles settling). For example, a method may include measuring a thermal conductivity of a fluid at two or more locations along a height of a vessel containing the fluid that comprises particles dispersed in a base fluid; and calculating a sag of the fluid based on the thermal conductivity at the two or more locations. In some instances, the temperature and pressure of the wellbore fluid may be changed and/or the wellbore fluid may be sheared to investigate their effects on sag.