Background To evaluate the diagnostic utility of gas under the diaphragm on abdominal radiograph for patients with a perforated appendix. Methods A prospective, observational study at General Surgery Department, Jinnah Postgraduate Medical Centre, Karachi, Pakistan was conducted between 2018 to 2020. All patients with suspected appendicitis, irrespective of gender were included in the study. Patients with age younger than 12 years or older than 60 years were excluded from the study. Findings from the imagings for each patient were noted. The plain radiograph was looking for “Gas under diaphragm”. All imaging was performed by a radiologist with experience of five years or more. All data were entered into a statistical package for social sciences (SPSS version 24). Results The appendix diameter was highly significantly associated with the risk of perforation in patients with appendicitis (p<0.001). Similarly, the patients with a perforated appendix have a significantly higher white blood cell count compared to those without perforation (15.6 ± 5.1 versus 13.2 ± 4.4; p-value = 0.002). It was found that out of the 76 patients with perforated appendicitis pneumoperitoneum was positive in 6 (7.8%) patients while out of those with a non-perforated appendix, 8 (10.5%) had gas under the diaphragm. Conclusion Gas under the diaphragm on a plain radiograph was detected in only a few cases with perforated appendicitis. Nevertheless, if positive, it warrants immediate abdominal surgery and requires comprehensive investigation.