Numbering the letters so A=1, B=2, etc is the simplest way of converting them to numbers. This is called the A1Z26 cipher or simply letters to numbers. However, the numbering can also start somewhere else than at 1. This translation tool and tables will help you easily convert between letters and numbers.
Convert between letters and numbers
Note that you can type in either the letters or numbers area.
Letter numbers are used frequently in geocaching mystery caches (puzzle caches), CTFs and logic puzzles.