The French gauge (Fr) (also known as the French scale or system) is used to size catheters, and other instruments, in interventional radiology and surgery. In some parts of the world, the Charrière (Ch) is used as the name of the unit, in honour of its inventor.
The French system is simple, one increment on the French scale is equal to 1/3 millimetre, e.g. 8 Fr catheter is 8 x 0.33 mm = 2.67 mm in calibre.
Unlike the needle gauge system, the French system has no set lower or upper limit, and users generally find it a lot less confusing, as the French size is proportional to the diameter.
Some common French sizes with equivalent metric diameter and circumferences:
- 3 Fr is 1 mm (diameter) and 3.14 mm (circumference)
- 4 Fr is 1.33 mm and 4.19 mm
- 5 Fr is 1.67 mm and 5.24 mm
- 6 Fr is 2 mm and 6.28 mm
- 7 Fr is 2.33 mm and 7.33 mm
- 8 Fr is 2.67 mm and 8.34 mm
- 9 Fr is 3 mm and 9.42 mm
- 10 Fr is 3.33 mm and 10.47 mm
- 12 Fr is 4 mm and 12.57 mm
- 14 Fr is 4.67 mm and 14.66 mm
- 16 Fr is 5.33 mm and 16.76 mm
- 20 Fr is 6.66 mm and 20.94 mm
History and etymology
Joseph-Frédéric-Benoît Charrière (1803-1876) 1,2, a renowned Swiss-French surgical instrument manufacturer invented the French scale in 1842.
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- 2. Bowen DK, Wan J, Engel R, Lyon RP, Dielubanza E, Bloom DA. Sounds and Charrière: the rest of the story. (2014) Journal of pediatric urology. 10 (6): 1106-10. doi:10.1016/j.jpurol.2014.04.010 - Pubmed
- 3. Osborn NK, Baron TH. The history of the "French" gauge. (2006) Gastrointestinal endoscopy. 63 (3): 461-2. doi:10.1016/j.gie.2005.11.019 - Pubmed