French gauge

Last revised by Dr Derek Smith on 26 Jun 2021

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 honor of its inventor. 

The French system is simple, one increment on the French scale is equal to 1/3 millimeter, e.g. 8 Fr catheter is 8 x 0.33 mm = 2.67 mm in caliber. 

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 

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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