Background Intermediate care functional recovery (ICFR) units in Catalonia, Spain, provide post acute care and geriatric rehabilitation. Providers report a Minimum Data Set (MDS), initially developed to classify patients into resource utilization groups (RUG), to adjust public reimbursement. The MDS, completed by health professionals, is time consuming and diverges from routine clinical assessment. Objectives We developed new intermediate care case-mix indexes (ICMI), based on admission diagnosis and function, and tested whether ICMIs better predict length of stay (LOS) and discharge destination, compared to RUG. Methods We developed the ICMIs using 122,754 ICFRs admissions (years 2017–18–19–22): we categorized individuals by diagnosis and four levels of activities of daily living (ADL). To obtain ICMIs, each category was assigned a weight based on associations with ICFR LOS and “unsuccessful discharge” (death or other service versus home). Then, in the 2023 cohort, we compared the performance of four models (base model (age, sex, income), base+RUG, base+ADL, base+ICMIs) to predict LOS >57 days and unsuccessful discharge. Results The 2023 cohort included 31,640 patients (median age [IQR]=82.0 [75.0;88.0], 57% women). When predicting dichotomous LOS, the area under the ROC curve (AUC) increased from 0.56 [95% CI=0.544–0.57] to 0.56 [95% CI=0.55–0.56] (base+RUG), 0.59 [95% CI=0.58–0.61] (base+ADL), and 0.63 [95% CI=0.62–0.65] (base+ICMI). AUC to predict discharge destination showed similar improvements (0.55 [95% CI=0.54 0.55], 0.59 [95% CI=0.58–0.59], 0.64 [95% CI=0.63–0.64], 0.66 [95% CI=0.65–0.66], respectively). Conclusions In Catalan ICFRs, new, simplified, case-mix indicators improved prediction of LOS and discharge destination, compared to existing tools. This study can inform new policies for intermediate care, adding a validated instrument to improve evaluation and reimbursing systems.