Get out your old ISBN-10 number. If the book has already been printed, you can usually locate the ISBN on the back cover or on the interior copyright page.
Log on to the ISBN Agency's ISBN Online Converter website (see Resources). It is free to use online ISBN conversion tool if you do one conversion at a time (at the time of writing, fees apply if you want to do 10 or more conversions at once).
Enter in your ISBN number that you want to convert, on the ISBN conversion website, and click "Submit." The 13-digit ISBN field will auto-populate with the converted ISBN-13 number for you.