Turn Roman alphabet letters A–Z into a sequence of numbers between 10 and 35. Keeps other numbers as numbers. Below you can turn numbers back into letters, with the same settings. There's another tool on this site that turns letters into numbers 1-26.
Options:
Reverse transformation: turn numbers back into letters. This uses the settings set above. If "No spaces between numbers in the output" is not used, then this can do a pretty good job of getting the original letters back, even if there are some numbers in between. But if there are no spaces between the numbers, then this will try its best to find number pairs that look like letters, but it can get confused if there are other numbers mixed in.