The French size (Fr) of a catheter is calculated by multiplying its diameter in millimeters by 3.
Here's a more detailed explanation:
The French scale, also known as Charrière scale, is a unit of measure for the outer diameter of medical catheters. It is a straightforward system for determining the correct size.
The Formula:
The core principle is:
*French size (Fr) = Diameter in millimeters (mm) 3**
Example:
If a catheter has an outer diameter of 4.7 mm, its French size is calculated as follows:
- 1 Fr = 4.7 mm * 3
- 2 Fr = 14.1 Fr
This is commonly rounded to 14 Fr. Therefore, a catheter with a 4.7 mm diameter is a 14 Fr catheter.
Using the Formula in Reverse:
You can also determine the diameter of a catheter if you know its French size. Simply divide the French size by 3:
Diameter in millimeters (mm) = French size (Fr) / 3
For instance, if you have a 16 Fr catheter:
Diameter = 16 Fr / 3 ≈ 5.3 mm
Importance of Correct Sizing:
Choosing the correct catheter size is crucial for patient comfort and to minimize the risk of trauma to the urethra or other body parts. Too large a catheter can cause pain and damage, while too small a catheter may not drain effectively.
