Among Old Assyrian school texts from Aššur and Kaneš, a group of lenticular tablets presents conversion exercises. Their statements, as well as the numerical data employed, are very similar to those given by merchants in their letters. For their everyday activities, the Assyrians used the Mesopotamian weight system to quantify the metalgold, silver, tin and copperobjects of their commercial exchanges. Weighed silver was often used as a means of payment. In order to compute the value of a given metal in silver, it was necessary to multiply or to divide the quantity of metal by the specified conversion ratio. As we do not have tablets showing the intermediary steps, we do not know, up to now, how the merchants carried out the conversions. A systematic analysis of the data given by the Assyrians when purchasing metals and the results they obtained gives clues about the computation methods they might have been using. These appear to be very different from those utilized by contemporaneous Babylonian administrators