Many currency brokerage websites have currency converters, like XE.co, Oanda.com, and USForex.com. In addition, financial news outlets like Finance.Yahoo.com and Bloomberg.com will often have a currency converter.
Currency converters make use of market-driven currency exchange rates to calculate how much of one currency can be purchased with a given amount of another currency. For example, calculating how many British pounds can be bought for $50 is a matter of setting the purchasing currency to US dollars and the amount to 50, then setting the target currency to British pounds.
The volatility of the foreign exchange market is famous, so any given price is subject to change. Likewise, currency converters do not usually give any indication of fees or premiums charged by the institution facilitating the exchange.