French Gauge diameter conversion in millimeters

Q: What is the French Gauge diameter conversion in millimeters (mm)?

Answer: The French gauge system is commonly used to measure the size of a catheter. It is often abbreviated in the USA as F.

A catheter of 1 French has a diameter of ⅓ mm 

For example Size 9 French converts to a diameter of 3 mm.

An increasing French size corresponds to a larger external diameter.

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Reference: 

Iserson KV. J.-F.-B. Charrière: the man behind the "French" gauge. J Emerg Med. 1987 Nov-Dec;5(6):545-8. doi: 10.1016/0736-4679(87)90218-6. PMID: 3323304.