FORMULATION OF AN EQUATION TO PREDICT FAT MASS USING BIOELECTRICAL IMPEDANCE IN ADULTS  in a wide range of ages and body mass index.   Background:  Bioelectrical impedance (BIA) has  a good correlation and agreement with reference techniques, such as dual energy X-ray absorptiometry (DEXA), to assess body composition. Aim: To develop and assess the concordance of an equation to predict body fat mass derived from anthropometric data, gender, age and resistance obtained from bioelectrical impedance in adults, using  DEXA as the reference method. Patients and Methods: Cross-sectional study of  62 women and 59 men aged 18 to 64 years with a body mass index ranging from 18.5 to 34.8 kg/m2. The equation was constructed using a predictive statistical model, considering sex, age, weight, resistance index (height2(cm)/ resistance (ohms)), as independent variables, and fat mass as the dependent variable. Results: The R2 of the regression model was 0.96, and the standard error of estimation was 2.58 kg (p 0.05) and - 4.5 ± 8,9% (p