The ASTM D5663-15 (2020) standard guide provides a calculation and verification method for recycled content in Encasement paper and Cardboard products. The guide recommends the use of mass equilibrate to determine the recycled fiber content, as there are currently no physical or chemical test methods that can accurately determine the amount of recycled finished paper. The calculation method is based on a time-weighted average Flow rate and takes into account the Flow rate and consistency of the ingredient composition. It is suitable for a variety of Cardboard products such as folding cardboard, corrugated fibreboard, tubing, etc. The guide also covers how to handle situations containing recycled non-fiber materials and reminds users to comply with relevant safety and regulatory requirements in practical applications.