Background and objectives The main aim of this study was to validate a nutrient-estimation method applied using OPEN, a Slovenian platform for clinical nutrition. As the most desirable validation is a direct comparison of the calculated values with the values obtained from a chemical analysis of the same food, we performed a chemical analysis and a calculation for a representative set of daily meals ( ) and compared the data. The secondary aim was to evaluate the usability of the OPEN for dietary assessment. Methods We used a Student's t test to observe any differences between the analyzed and calculated nutrient-content data, considering the 0.01 significance level. Results No statistically significant differences were observed in the mean contents of energy, total dietary fibre, water, macro-nutrients, and selected essential minerals: Ca, Fe, Mg, Zn, Na, P, Cu, I and Se. Conclusions We noticed remarkable, but not statistically significant, differences in the mean values of iodine (-11%) and selenium (-19%). The present study also indicates that the OPEN is a useful tool for both dieticians and patients.