The Kappa number is an indication of the lignin content or bleachability of wood pulp. It estimates the amount of chemicals required during bleaching of wood pulp to obtain a pulp with a given degree of whiteness. Since the amount of bleach needed is related to the lignin content of the pulp, the Kappa number can be used to monitor the effectiveness of the lignin-extraction phase of the pulping process. The number is in the range of 1-100 (10-70 for most types of wood) and is established by measuring the amount of a standard potassium permanganate solution that is consumed by the pulp being considered. Details on how the Kappa number should be determined is published in ISO 302:2004.

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The Kappa number measurement is inflated by the presence of hexenuronic acids in the pulp. These compounds are formed during the chemical pulping process, from the hemicelluloses. Thus the Kappa number measurement does not only represent the residual lignin in the pulp.