[header] How to calculate arbitrage in forex [title] Understand the foreign exchange market. [step] The foreign exchange market, commonly referred to as forex, is an international exchange for the trading of currencies. It allows investors (from large banks to individuals and everyone in between) to trade one national currency for another. 
Each trade is both a purchase and a sale, as one currency is sold in order to buy another one. This duality means that each currency is priced only in relation to another currency.