A is 1, Z is 26
Each letter becomes its place in the alphabet: A 1, E 5, J 10, O 15, T 20, Y 25, Z 26. The numbers of one word are joined by dashes and words are kept apart by spaces, so Hi world becomes 8-9 23-15-18-12-4. Digits already in the message stay as they are, which makes them hard to tell from letters, so spell numbers out.
Numbers back to letters
Paste numbers into the Code box. Numbers joined by dashes make one word, and anything else between them, such as a space or a comma, is kept, so 8-9 23-15-18-12-4 reads hi world and 8 9 reads h i. The letters come out small, as numbers carry no capitals. A number above 26, or a 0, is marked and named.
Questions
What about A = 0, or Z = 1?
This page uses A1Z26 only. To read a code that counts from Z = 1, turn the numbers into letters here and put the letters through Atbash; to write one, do Atbash first and then this.
Why does a letter in the code box turn it red?
The code side expects only numbers, dashes and spacing. A letter there means the text is not A1Z26, or still has plain words in it.