ROT13
ROT13 rotates every letter halfway round the alphabet. Run it again and the original text returns, so one tool does both jobs.
Opens Unscramble Words 4U, our free word-puzzle site, in a new tab. Free, with no sign-up.
What it does
ROT13 stands for 'rotate by 13'. A turns into N, B into O, and once you pass Z you loop back to A. With 26 letters in the English alphabet, two rotations of 13 make a full circle, which is why encoding and decoding are the same action. Type or paste text and the result shows immediately; letter case is kept, and digits, spaces and punctuation stay unchanged.
Usenet newsgroups took it up in the early 1980s for tucking away spoilers, joke punchlines and puzzle answers, so readers chose when to look. It is still handy for answer keys, quiz sheets and riddles, and because the rule never changes, children can learn to read it by eye with a little practice. In fact it is just Caesar with the shift fixed at 13, and it gives no real privacy because there is no key.
How to use it
- Type or paste text into the box.
- Read the rotated version, which appears as you type.
- To reverse it, paste the ROT13 text back in; it returns to normal.
- Press Copy result to use it elsewhere.
More tips
- For any other shift, or to crack an unknown one, use the Caesar cipher tool.
- Try ROT13 on whole sentences and ask learners which coded words look most like real words; it makes a fun spotting game.
Ideas for the classroom and home learning
- KS2 (7–11)Write jokes with the punchline in ROT13 and let partners decode the ending.
- KS2 (7–11)Maths: explain why 13 + 13 = 26 makes the code undo itself, and ask which other shift would do the same (none, for 26 letters).
- KS3 (11–14)Computing: discuss the difference between hiding text from a glance and actually securing it.
- KS3 (11–14)Have learners write a short algorithm in pseudocode for ROT13, handling capitals and leaving other characters alone.
Questions
How can I reverse ROT13 text?
Put it back into the box. Rotating by 13 a second time completes the circle and restores the original.
Are digits affected?
No, just letters. A separate scheme called ROT47 also moves digits and symbols, but you will meet it far less often.
Does ROT13 count as encryption?
Not really. It stops an accidental glance, nothing more.
What is CAT in ROT13?
PNG. C moves to P, A to N and T to G. Put PNG back in and you get CAT again, which is a quick way to show learners the code reversing itself.
Opens Unscramble Words 4U, our free word-puzzle site, in a new tab. Free, with no sign-up.
ROT13 has no key and no real security. It is for spoilers, puzzles and learning.