The standard French, or Charriere, Scale (abbreviated Fr.) is generally used in the size calibration of cathers and other tubular instruments.  It is based on the metric system, with each unit being approximately 0.33 mm in diameter between consecutve sizes.

A simple guide

1mm = .039 inches = 3Fr

 2mm = .079 inches = 6 Fr

3mm = .118 inches = 9 Fr

4mm = .158 inches = 12 Fr

5 mm = .179 inches = 15 Fr

6 mm = .236 inches = 18 Fr

mm = millimeter

Fr = French