The aim of the paper was to estimate how the value of 1 GJ of energy in coal with a calorific value of 5500 kcal/kg varies on the international coal market compared to 1 GJ of energy in coal with a calorific value of 6000 kcal/kg. The analysis of data from different ports was intended to answer the question of whether the pricing of coals of different producers according to their calorific value is convergent. The best-known price standard for thermal coal is 25.1 MJ/kg coal (6000 kcal/kg) and, until recently, coals with such quality parameters dominated international trade. Currently, coals with parameters other than considered to be standard parameters are traded on the coal market, hence it is necessary to price a unit of energy (e.g. 1 GJ) contained in these coals. The indices have been selected of the largest exporters of thermal coal for which data was available and referred to the same coal types (grades) determined on the same basis (FOB). Theoretically, the price differential between 6000 kcal/kg and 5000 kcal/kg coal (in USD/ton) should be (at least) as much as the difference in calorific value, i.e. about 9% per USD/ton. In reality, the price differential between these types of coal is greater, though. The overall conclusion of the analysis is that the price calculated per 1 GJ of energy fluctuated on average by 5.9% over the entire period considered. The analytical results obtained for coal from four countries are quite convergent, so it can be assumed that the calculated relationship between the prices of coal with different calorific values (6000 and 5000 kcal/kg) is a good approximation of the observed relationships in the international trade. The calculation results provide a simple formula allowing to estimate the price of coal with a calorific value other than the standard 25.1 MJ/kg (6000 kcal/kg) using the relationships from the international market.