The kappa number is determined according to ISO 302. It is a measure of the lignin content or bleachability of pulps (degree of pulping).

There are two methods: one for the kappa number range of 5 to 100 and one for the range 1 to 5. The analysis is based on the fact that potassium permanganate acts as an oxidising agent on lignin and is the amount of millilitres of a 0.02 mol/l potassium permanganate solution consumed by one gram of pulp under the specified conditions (calculated on an oven-dry basis). All compounds that are oxidised by permanganate are not just lignin, but cause consumption of permanganate and thus an increase in the kappa number.

This standard is applicable to all types of chemical and semi-chemical pulps with a kappa number range from 1 to 100. The kappa number is also a measure of the residual lignin content in pulps.

The Kappa number can be determined for archivability e.g. within the framework of ISO 9706 "Requirement for ageing resistance of paper for written and printed matter".