There are a number of standardized tests that are used to determine the strength and durability of containerboard components.
The below video shows how to measure tensile strength, which tells you how resistant a paper is to web breaks. The tensile strength measurement is used for calculating tensile stiffness. The measurement unit is kN/m, kilonewton per metre.
It's only since recently that lightweight papers have been able to manifest sufficient strength to be comparable for the intended end uses. Please be aware that many test methods are developed for 100 grams per square metre and up, and may need adaptions to avoid limitations. Therefore, CEPI Containerboard association defines their test results for papers <100 grams per square metre only as indicative.