Every day in the endoscopy unit we use instruments and accessories with measures in “French” (Fr). In Germany we refer to the “French measurement” as “Ch, Charr or Charrière”. Who was Charrière? Joseph-Frédéric-Benoît Charrière (1803-1876) was a Swiss-French artisan medical instrument designer who created a panoply of surgical products, mostly used in urology, but he also developed, scissors, syringes, forceps, and non-rebreathing systems for administering anesthesia (1, 2).

To be useful, the products had to be reproducible and follow standards for keeping the same size. Charrière devised a famous system of gauging, known as the Charrière or “French” gauge system, which is still used today. It has uniform increments between gauge sizes; a third of a millimeter equates one Ch, Charr or French: i.e. 1 Charr [French] = 0.333 mm.

In essence, one increment on the French scale is equal to 1/3 millimeter, e.g. 10 Fr catheter is 10 x 0.33 mm = 3,33 mm in caliber. 

Good to know! If you are using a gastroscope with a 2.8 mm working channel, it’s impossible to pass a delivery device, stent or pushing catheter larger than 8 Fr. Most delivery systems for self-expanding metal stents (SEMS) used to palliate gastric outlet, small bowel and colonic obstruction are 10 Fr. Therefore, you can deploy these SEMS only using scopes with a large enough working channel, for example colonoscope or duodenoscope (see figure below).

However, if you don’t have a scope with a large enough channel, there are always tricks to pass thicker catheters or stent to the target lesion or position using overtubes or guidewires like shown in this example: 

https://www.thieme-connect.com/products/ejournals/pdf/10.1055/s-0043-119686.pdf

Just multiple 1 Fr x 0,33 = 0,33 mm

1 Fr is 0.33 mm

2 Fr is 0,66 mm

3 Fr is 1 mm

4 Fr is 1.33 mm

5 Fr is 1.67 mm

6 Fr is 2 mm

7 Fr is 2.33 mm

8 Fr is 2.67 mm

9 Fr is 3 mm

10 Fr is 3.33 mm

12 Fr is 4 mm

14 Fr is 4.67 mm

16 Fr is 5.33 mm

20 Fr is 6.66 mm

Fig. legend: Balloon-assisted endoscopic retrograde cholangiopancreatography. a Magnetic resonance cholangiopancreatography revealed a tight distal bile duct stricture (yellow arrow). b Tight opening of the hepaticojejunostomy. c Balloon dilation of the hepaticojejunostomy. d The balloon-assisted enteroscopy overtube worked as a giant working channel. The wire was left inside, connecting the bile ducts to the operator. The operator then advanced a fully covered metal stent, over-the-wire and through-the-overtube, using radiological guidance, and then successfully inserted the stent inside the bile duct (3).

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Ref:

1. Bowen DK, Wan J, Engel R, Lyon RP, Dielubanza E, Bloom DA. Sounds and Charrière: the rest of the story. J Pediatr Urol. 2014;10(6):1106-10. doi: 10.1016/j.jpurol.2014.04.010.

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

3. Hewitt BA, Jovanovic I, Martínez-Alcalá A, Kröner PT, D'Assunçao M, Mönkemüller K. Insertion of fully covered self-expandable metal stent during balloon-assisted ERCP in patient with surgically altered upper gastrointestinal anatomy. Endoscopy. 2018;50(1):E20-E21. doi: 10.1055/s-0043-1196