How to convert French size to mm
- Enter the French size, such as 14 Fr, 16 Fr, or 18 Fr.
- The converter divides by 3 to estimate outside diameter in millimeters.
- Use the reverse modes if you have a measured diameter in mm or inches.
Catheter French Size to mm Converter
Convert catheter French sizes, also written Fr or CH, to approximate outside diameter in millimeters and inches. You can also enter a measured diameter to find the nearest French size.
Convert Fr, CH, mm, or inches
Use the standard relationship: 1 Fr = 0.333 mm outside diameter.
Approximate outside diameter
—
—
French size—
Inches—
Nearby common sizes—
Enter a French size or diameter to see the conversion.
Common catheter French size chart
This quick chart shows outside diameter. Always check the device package, product instructions, and clinical guidance for actual use.
| French / CH | OD mm | OD inch | Common note |
|---|
Fr vs CH: are they the same?
Yes for practical sizing. Fr means French scale and CH refers to Charrière. Both describe the same catheter size scale where the French number is three times the outside diameter in millimeters.
Catheter French size FAQ
What is 18 Fr in millimeters?
18 Fr is approximately 6 mm outside diameter because 18 divided by 3 equals 6.
What is 16 Fr in mm?
16 Fr is approximately 5.33 mm outside diameter, or about 0.210 inches.
Does a larger French number mean a larger catheter?
Yes. Unlike needle gauge, larger French numbers mean a larger outside diameter.
Can this choose the right catheter size for a patient?
No. This is a measurement converter only. Device selection should follow a clinician’s instructions, product labeling, and local clinical policy.