The differentiation between premenstrual syndrome (PMS) and premenstrual dysphoric disorder (PMDD) has been widely discussed. PMDD is listed as a mental disorder in the DSM-5, whereas PMS is not considered as a mental disorder in any diagnostic manual. Consequently, PMS is operationalized in different ways. Keeping a symptom diary is required to diagnose PMDD but is also recommended for PMS. The aim of our study was, therefore, to operationalize PMS and PMDD within a DSM-5-based symptom diary. We developed a symptom-intensity-score (SI-score) and an interference-score (INT-score) to evaluate the symptom diary. Ninety-eight women (aged 20-45 years) completed a symptom diary over two menstrual cycles, a retrospective screening for premenstrual symptoms, and answered additional impairment questionnaires from August 2013 to August 2015. The scores revealed moderate to good reliability (Cronbach's α = 0.83-0.96). Convergent validity was shown by significant correlations with a retrospective screening, the Pain Disability Index, and the German PMS-Impact Questionnaire. Discriminant validity was indicated by low correlations with the Big Five Inventory-10. These scores may facilitate the evaluation of prospective symptom ratings in research and clinical practice. Future research should focus on continuing to validate the scores (e.g., in an ambulatory setting).