Three different ways of describing the size of tube-shaped medical devices such as endotracheal tubes, intravenous catheters, hypodermic needles, etc. are commonly used.
They are metric, i.e. millimeters, 'French' or Fr, and 'gauge' or G.
Conversion between 'French' and millimeters
'French' is a measurement primarily of circumference rather than just 'size'. As far as airway equipment is concerned, the 'French' measurement is only rarely applied, for example to the Bronchocath double-lumen endobronchial tube.
The convenrsio to metric diameter is straightforward.
Diameter (mm)= French / 3
A "three French" catheter therefore has an external diameter of 1mm.
Conversion between 'gauge' and millimeters
'Gauge' measurements are seldom used in airway equipment but common to describe intravascular catheter and hypodermic needle sizes.
Easy conversion between 'gauge' and millimeters is not possible as no straightforward mathematical relationship exists between the two. Steps between sizes in the lower gauge-ranges are smaller than in the higher gauge-ranges. You have to resort to look-up tables.