It has been suggested in the literature than volume deficit in hypertonic dehydration can be calculated from actual osmolarity. But for an accurate estimation of volume deficit a number of factors must be given. The initial concentration and the loss of substance during the development of dehydration must be known, the distribution of the substance within the compartment must be homogeneous, and no shift between dissolved and undissolved compartments is allowed. For the osmotically active substances the second and the forth condition are not satisfied. Therefore, the calculation of volume deficit, based on actual osmolarity, can induce an error from 40 ml to as much as 100 ml per kg body weight.