The archives of the Assyrian merchants, found at Kaneš in Central Anatolia, highlight the use of a weight system, well adapted to quantify the metals they traded from East to West and back. An atypical progressive metrological list of weights shows the importance of fractions in Old Assyrian numeracy and computing practices. Accounts documents reveal two different cultures for the use of weighing units according to the commodities quantified and where they were quantified. In Upper Mesopotamia, tin and silver were measured in gu, mana and gin, while in Anatolia, locally traded copper was often counted only in the mana unit. In addition, merchants had to deal with different weight standards: The Anatolian mana weighed ten per cent less than the Mesopotamian mana. Texts also mention the verification of weights and reveal methods for correcting discrepancies. A hundred and sixty stone weights found at Kaneš may be compared with the data provided by the metrological list. These weights, which show significant variations, witness the existence of many individual weight standards, and confirm the recurrent necessity to correct discrepancies.