I found this thickness guide.
Chipboard thickness is measured by a point system. Each point is 1/1000th of an inch.
Light - 20 pt or 1/42nd of an inch, about the thickness of a cereal box.
XL - 32 pt. or 1/32nd of an inch, about the thickness of a credit card
Extra Heavy - 50-52 pt or 1/20th of an inch, about the thickness of a penny.
2X - 85 pt. or 3/32nd of an inch, about the thickness of 2 dimes.