Look up the most current exchange rates on a currency conversion website (see References). To calculate your conversion, you will need to find out how much 1 GBP is worth in USD.
Multiply the number of GBP you have by the exchange rate. This will determine how many dollars you can get for your pounds.
Use a currency calculator, available on most currency conversion websites, for a quick and convenient conversion. Some currency calculators, such as Oanda's (see Resources), will also allow you to perform conversions based on past exchange rates if you wish to compare.