It is possible, by careful radiographic technique, to demonstrate as little as 1 ml of free gas on erect chest or left lateral decubitus abdominal films. However, radiographic technique and positioning are important and a patient should be in position for 10 minutes before the film is taken, as it takes time for free gas to rise to the highest point in the abdomen.

Classic radiograph findings of pneumoperitoneum include:

• Rigler sign: visualization of the outer and inner wall of a loop of bowel
• Cupola sign: relatively large amounts of gas may accumulate beneath the diaphragm, or in the center of the abdomen over a fluid collection (the so-called "football sign")

CT is, however, the most sensitive method for the detection of peritoneal free gas, with even tiny bubbles of gas being visible.