Background A challenging task in the intensive care unit is weaning intubated patients from mechanical ventilation. The most commonly used weaning parameter, the rapid shallow breathing index (RSBI), gives thorough guidance on extubation timing with spontaneous breathing trials. Diaphragm plays vital role in tidal volume generation. The main objective of the study was to compare ultrasound-based diaphragmatic excursion (DE) with RSBI as weaning predictors. Methods We conducted an observational prospective cohort study on patients on mechanical ventilation. During a spontaneous breathing trial (SBT) we simultaneously evaluated right hemidiaphragm excursion by using M-mode ultrasonography as well as the RSBI. To be included, patients must have been on mechanical ventilation for longer than 48 hours, have no excessive tracheobronchial secretions, and their underlying critical illness (for which they were intubated) must be resolved. Patients younger than 14 years, patients with neuromuscular disorder, patients with pneumothorax, and patients with cervical spine injury were excluded from the study. We analyzed the data to determine the significance of DE and RSBI. Results A total of 90 patients were included in our study; 54 (60%) were men, and 36 (40%) were women. The average age of all the participants was 55 ± 16 years (range, 19 to 83 years). Sixty-two patients (68.9%) were successfully weaned. The mean DE was 1.44 ± 0.26 cm, and the mean RSBI was 56.88 ± 8.30 in all patients. Successful weaning patients had a mean DE of 1.51 ± 0.26 cm and a mean RSBI of 54.05 ± 7.00. The greater the DE value, the greater the weaning success rate, and the lesser the RSBI value, the greater the weaning success rate. The area under the receiver operator curve for DE and RSBI was 0.795 and 0.815, respectively (p < 0.0001). Conclusion RSBI is an optimized clinical predictor in classifying weaning outcomes for intubated patients, but DE is also helpful in extubation assurance and reintubation prevention.