Decide the total amount in pounds that you would like to convert into U.S. dollars. Once you have decided this amount, you need to research the most updated exchange rates between British and U.S. currencies.
Multiply the current exchange rate value to the total number of pounds you are converting. If the value is one pound against 1.66 dollars and you want to convert 500 pounds, you will multiply 500 with 1.66. The result is $830.
Exchange currencies prior to your trip. You can exchange currencies at your bank or when you are going through Customs. If you prefer, you can also purchase a traveler's check in dollars.