Answer
Feb 04, 2026 - 07:14 AM
Catheter sizes are measured using the French (Fr) scale rather than standard U.S. inch sizing. The French scale is based on the outer diameter of the catheter, where 1 French (Fr) equals approximately 0.33 millimeters. Because this involves very small measurements, Fr sizes do not convert cleanly to common inch sizes.
For reference, a 16 Fr catheter has an outer diameter of about 5.3 mm (approximately 0.21 inches). Most catheter selection is done using the French size recommended by a healthcare provider rather than inch measurements.
