Introduction: A healthy fluid balance is necessary to maintain normal vital functions. Detecting the amount of body water is important both in clinical practice and in sports in assessing dehydration and/or fluid overload. Among the methods used to determine the amount and distribution of body water, devices operating on the principle of bioelectrical impedance are becoming more common. Their use is simple, fast and the results obtained are reliable. Objective: The aim of our study was to determine the relative body water content changing dependent on age in men and women of normal weight, overweight and obese, using a device that works on the principle of bioelectrical im¬pedance. Method: The data of a total of 2,354 persons between the ages of 7 and 81 are included in the study conducted with the InBody 720 type device. Normal body weight, overweight and obesity were determined using the body mass index. Results: There were significant differences between gender in body weight, body mass index between normal weight and overweight/obese participants in all age groups. The differences between relative body water content of men and women in normal body weight are significant in all age groups. The averages have not changed until the 3rd age group (21-50 years), then decreased in the 4th age group (above 51 years) in the normal body weight men. In thecase of normal body weight women, the averages decreased linearly dependent on age. The relative body water con¬tents in overweight/obese subjects were significantly lower by gender and age group than the averages of those with normal body weight. The relative water contents in overweight/obese men and women have not changed dependent on age. Discussion: Although bioelectrical impedance method is less commonly used, it is suitable for measuring total body water content. The reliability of the data obtained by the method is confirmed by more and more studies in the case of people of normal body weight, overweight and obese people as well. Conclusion: The bioelectrical impedance procedure can help prognosis and diagnosis in many areas. After presenting our data, we hope that our study will contribute to the spread of the bioelectrical impedance method.