Both Parker and Swagelok provide stainless steel tube dimensions in their tubing catalogs. The Swagelok catalog provides tube dimensions in metric, imperial and fractional sizes, while the Parker catalog provides dimensions only in metric and imperial sizes.
The inch is the unit of measure for both fractional and imperial dimensions, and the millimeter is the unit of measure for metric dimensions. Imperial outer diameters increase in increments of 1/16 inch and fractional outer diameters increase in increments of 1/8 inch. Tubing is sold by the outer diameter and wall thickness.
The tubing wall thickness, or gauge, determines the safe working pressure for the tubing. Both Parker and Swagelok provide the acceptable working pressures for their various diameters and gauges of tubing.