Banks, currency traders, tourists, and others use exchange rate calculators to determine how much of one currency they can buy with another currency based on current market conditions. Exchange rate calculators are often referred to as currency calculators. X-rates.com maintains a currency calculator on their site.
The foreign exchange market uses symbols like EURJPY to identify currency pairs, in this case the Euro and the Japanese Yen. A number is assigned to this symbol by the market, such as 139.89, the actual value at 6:20 pm EST on June 22, 2015. This means one Euro buys 139.89 Yen, exclusive of exchange fees or other premiums. Exchange rate calculators work with any number in the purchasing currency slot, so 100 Euro in the example would buy 13,989 Yen.