Clinical practice is frequently challenged by limited funding and resources, which finally limit both clinical effectiveness and safety of some therapies. Electrolyte disorders represent serious problems in the clinical management. Nonetheless the osmometer, that is the reference instrument for routine assessment of osmolality, it is only available in a limited number of healthcare facilities. The diagnosis of the leading electrolyte disorders relies therefore on indirect criteria, frequently inaccurate, especially when inappropriately used. According to recent evidences emerged on prevalence, severity and therapeutic approach of patients with electrolyte disturbances such as hyponatremia, the diagnostic appropriateness is now regarded as an essential aspect of the clinical decision making. Recent multidisciplinary guidelines indicate that urinary osmolality is a mainstay in the differential diagnosis of hyponatremic states. Since hyponatremia is commonplace across a broad range of clinical conditions, it is noteworthy that accurate knowledge of the different equations that may be used for its calculation in serum or urine is not widespread among general and hospital physicians. To couple with these clinical issues, this article is aimed to briefly describe the epidemiology and clinics of osmolality disturbances and to suggest some equations that may be useful for its routine assessment in serum or urine, and which can be applied to different categories of patients. The usefulness and reliability of additional indirect methods used in the diagnostic approach of electrolyte disturbances, such as the assessment of urine specific gravity, will also be briefly discussed. The equations that will be proposed have been validated in small sample population studies, but are commonly used as a surrogate or replacement of direct osmolality assessment. A larger multicentric study is hence necessary to validate the clinical use of the equations used for the calculation of serum and urine osmolality.