Introduction: Hydration of an organism is important for its functioning. Osmolality is one of the most appropriate parameters for the assessment of hydration in an organism. The relative volume mass of urine is routinely measured in laboratories with a urine test strip or an urometer. Measuring relative volume mass with an urometer is more sensitive, but also more demanding method. Aim of the research: to assess the state of hydration in the organism based on osmolality measurements and relative volume mass in samples of disposable urine, to compare values of relative volume mass and osmolality in randomized urine samples, to compare values of relative volume mass measured by urine test strip and urometer. Materials and methods: one hundred samples of disposable urine are used in this research from the ambulance and hospital patients from the Clinical Institute for laboratory diagnostics with the request for routine analysis of urine. The relative volume mass of disposable urine is measured by the Roche’s urine test strip Combur 10 and standard urometer method. Osmolality of the same samples are measured by Arkray’s automatic osmometer OM-6060. Results: Results show a good correlation between osmolality and the relative volume mass of urine. Also, there is a good correlation between osmolality measured by an osmometer and osmolality calculated with formulas. Comparison of test strip urine method and urometer measurements of relative volume mass show a slight, but statistically significant constant difference (Passing-Bablog regression) and no significant proportional difference. Conclusion: osmolality is the best indicator of hydration of an organism. Relative volume mass shows a good correlation with osmolality. Determination of relative volume mass of urine with test strip gives reliable results when measuring hydration in organisms. Laboratories that do not have an osmometer can calculate osmolality with formulas.