The present paper aims to present a new methodology to measure the thermoplasticity of coals by the permeation distance method. This development is a reproduction of a methodology created at JFE Steel and adapted to the reality of the coals used for the production of metallurgical coke in Brazil. This method consists of evaluating the thermoplastic behaviour of coals in a complementary way to Plastometry and Dilatometry analyses. Experiments were carried out for coals of different ranks and petroleum coke. Furthermore, the hypotheses of additivity and tests with different particle sizes for the same coal were validated. It was concluded that there are significant differences between permeation distances for coals of the same fluidity. It is necessary to bring emphasis for the anomalous behaviour of the petroleum coke (CVP). The granulometry is a relevant factor for the permeation distance with a direct impact on the quality of the semi-coke, with a direct influence on the mechanical strength of metallurgical coke.