The subject of this thesis is determining C20 strength class for six timber products. The C20 strength class is rare in Finland. Sawmills in Europe usually use initial settings in strength grading machines. It is possible to start using different rare strength classes but companies usually have to conduct some research for that. This thesis was made with Koskisen Oy Sawn Timber Industry. Koskisen Oy Sawn Timber Industry would like to use the C20 strength class with a few products because one of their export customer specifically asked for them. The theory part of the thesis explains strength grading in industry. Many different standards are used in strength grading and it must be done with license. Standard EN 338 explains the so-called C classes, which are the most commonly used strength classes in Europe. The theory part also discusses different strength grading machines. In Europe are used different machines to strength grading and the machines have to follow the standard EN 14081. The practical part of the thesis consists of describing how the C20 products are tested in industry. In this study, an output control method was used in determining the C20 class, which is one way to determining timber’s wanted strength class for strength grading machine. Research and calculations on the strength properties are needed when determining the strength class. All instructions are found in the standards concerning the strength grading of timber. The result of the thesis was got determining to six products C20 streght class parameter values.