Paper size and weight conversions

In North America, the default paper size is "Letter" size while outside North America, the default paper size is A4.

ISO 216 - Standard Metric Paper Sizes
Standard North American Sizes
Paper weights

There are two terms commonly used to describe paper weight:
• Basis Weight
 The basis weight of a paper is based on the weight of 500 sheets,
 measured in pounds, in that paper's basic sheet size. Note that the "basic sheet size" is not the same 
for all types of paper which makes comparisons of weight between different papers difficult.
• Caliper 
Caliper refers to the thickness of a sheet of paper measured with a micrometer and expressed
 in thousandth of an inch. Paper caliper should not vary more than +/- 5% to 7% 
within a sheet. Generally, there is a relation between caliper and basis weight in that 
the greater the caliper (the thicker the paper), the greater the paper weight. But not always.

Darker colored boxes represent the
most common paper weights for that category.This Table was compiled by Micro Format, Inc., Wheeing IL. Used with permission.