Urinary catheter French (Fr) gauge to mm and inch conversion with typical clinical use.
A French (Fr or CH) catheter size chart converts French gauge to outer diameter in millimetres. The French scale is used worldwide for urinary catheters, vascular catheters, and surgical drains. Unlike needle gauge (inverse scale), French gauge is direct: higher Fr = larger diameter.
The relationship is simple: 1 Fr = 1/3 mm exactly. So a 12 Fr catheter has an outer diameter of 4 mm; a 24 Fr catheter is 8 mm. The most common urinary catheter sizes are 12 Fr (small/female), 14–16 Fr (adult standard), and 20+ Fr (drainage of large clots or pus).
OD_{mm} = \frac{Fr}{3}
Inputs: Selecting a Foley urinary catheter for routine adult use
Latex catheters: 7–14 days. Silicone or PTFE-coated: 28 days. Hydrogel-coated long-term: up to 12 weeks. CDC/NHS guidelines recommend removing or changing the catheter as soon as clinically indicated to reduce CAUTI (catheter-associated urinary tract infection) — the most common healthcare-associated infection globally.
A standard Foley has two lumens — one for urine drainage and one to inflate the retention balloon. A triple-lumen (3-way) catheter adds a third channel for continuous bladder irrigation with saline — used after prostate surgery or in haematuria with clot retention. Triple-lumen catheters are larger (22–24 Fr) because they accommodate three internal channels.
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