What is the normal size of the appendix in adults and children?

Normal Appendix Size

The normal appendix measures up to 6 mm in maximal outer diameter on imaging, though diameters between 6-8 mm represent a diagnostic gray zone where normal and inflamed appendices overlap significantly. 1

Diameter Measurements

Standard Diagnostic Thresholds

• Appendices <6 mm in diameter are nearly always normal on both ultrasound and CT imaging 1, 2
• Appendices >8 mm are nearly always abnormal, with optimal CT cutoff values ranging from 8.2 mm (sensitivity 88.8%, specificity 93.4%) to 11 mm (sensitivity 62.7%) depending on the study 1
• The frequently referenced 6 mm cutoff yields high sensitivity (97.5%) but poor specificity (59.6%), meaning it captures most appendicitis cases but generates many false positives 1

The Borderline Zone Problem

• Diameters of 6-8 mm create diagnostic uncertainty because significant overlap exists between normal and inflamed appendices in this range 2
• In patients without appendicitis, >20% have appendiceal diameters >7 mm, and nearly half (48.12%) exceed 6 mm on CT 3, 4
• The mean diameter of normal appendices ranges from 6.3-8.2 mm across multiple studies, with some normal appendices measuring up to 12.8 mm 4, 5
• Diameter alone should not be relied upon to diagnose appendicitis in the borderline range; secondary findings (wall thickening >3 mm, periappendiceal fat stranding, appendicoliths, intraluminal gas) become critical 1, 3

Wall Thickness

• Normal appendiceal wall thickness averages 2.2-2.4 mm (range 1.2-4.2 mm) 4, 5
• Wall thickness >3 mm is significantly associated with perforation (OR 3.2, P=0.02) 1

Length Measurements

• Mean appendiceal length is 66.7-81.1 mm (approximately 6.7-8.1 cm) in adults 4, 5, 6
• Length ranges widely from 6.3 mm to 158.8 mm across individuals 4, 5
• Men have significantly longer appendices than women (mean 8.4 cm vs 7.4 cm, P=0.02) 5, 6
• Intermediate lengths (4-10 cm) are more frequently complicated by appendicitis (90% of appendicitis cases), while very long (>10 cm) or very short (<4 cm) appendices are more common in unaffected adults 6

Age-Specific Considerations

Pediatric Population

• The same 6 mm diameter threshold applies to children on ultrasound, with diagnostic criteria including noncompressibility and appendiceal tenderness 1
• CT demonstrates high accuracy in children with sensitivities of 88-100% and specificities of 87-100% when using appropriate contrast protocols 1

Clinical Pitfalls

Common Diagnostic Errors

• Relying solely on diameter in the 6-8 mm range leads to overdiagnosis and unnecessary appendectomies 4
• The appendix is visualized in only 74-76% of abdominal CT scans, with non-visualization more common in patients with higher BMI 3, 5
• Compressed diameter (after accounting for compressible luminal contents) performs better than maximal outer diameter, with optimal cutoff of 6.6 mm yielding 93.8% sensitivity and 94.9% specificity 1

Secondary Findings to Assess

When appendiceal diameter is borderline (6-8 mm), evaluate for:

• Periappendiceal fat stranding (OR 5.63 for perforation, P<0.001) 1
• Appendicoliths (OR 2.47-2.67 for perforation) 1
• Intraluminal gas (OR 2.64 for perforation; sensitivity 36.9%, specificity 81.9%) 1
• Focal wall defects (OR 23.40 for perforation; specificity 98.8%) 1
• Hyperemia on color Doppler ultrasound in borderline cases 2