Fr? CH? OD?
Understanding the Correlation Between Fr, CH and OD in Urinary Catheters
When selecting urinary catheters, we often refer to the catheter size using the Fr or CH for Charrière scale, which directly correlates to the catheter’s outer diameter (OD). One Fr equals 1/3 of a millimeter in OD (e.g., 18 Fr = 6 mm OD). This measurement is important because the size of the catheter must match the patient’s anatomy to ensure proper drainage, minimize trauma, and avoid complications like urethral injury or discomfort.
We focus on OD for urinary catheters because the outer size determines how well the catheter fits into the urethra. The internal diameter (ID) is less relevant in this context, as the urethral passage is defined by the external width of the device.
In contrast, for endotracheal (ET) tubes, we typically refer to the internal diameter instead. This is because airflow is the priority in airway management, so the internal space (ID) of the tube determines how much air can pass through — not how tightly it fits in the trachea. The trachea can accommodate some variability in outer diameter, making OD less critical for ET tubes.