The biomass density in biotechnological processes is often determined by indirect and manual methods. Electrical impedance spectroscopy can provide online viable biomass density estimators. In this work, we present two linear estimators obtained with this technique. Four different microorganisms were measured. The detection threshold was approximately 1 g/L (dry weight) for bacteria and 0.5 g/L for yeast. Liposome suspensions were also used to validate the methods. The monitoring of the continuous growth of a yeast culture is also presented.