random medical sizing: the French catheter system & needle gauges

30 Apr


where you’ll see it:  chest tubes, foley catheters, etc.

bigger number =:  bigger tube size

quick reference:  external diameter is (French #)/3

example:  18 French catheter has a 6mm external diameter (18/3 = 6)




where you’ll see it:  hypodermic needles. duh.

bigger number =: smaller needle size

quick reference:  not simple. see the chart, if you want.


  • 14 gauge needle ~= 2mm outer diameter
  • 20 gauge needle ~= 1mm outer diameter
  • 30 gauge needle ~= 0.3mm outer diameter


Thanks, medicine, for keeping it easy and consistent.  But there you go.


References: dr. wikipedia french; dr. wikipedia gauges; picture

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