ROT13 Cipher Encoder / Decoder
Free ROT13 encoder and decoder. Type or paste text and instantly apply the ROT13 substitution cipher. Supports A–Z and keeps punctuation, numbers, and spacing unchanged.
Full original guide (expanded)
ROT13 Cipher Encoder / Decoder
ROT13 is a special case of the Caesar cipher that shifts each letter by 13 positions. Because the alphabet has 26 letters, applying ROT13 twice gives you back the original text. Use this tool to encode or decode instantly.
Letters A–Z and a–z are rotated. Numbers, punctuation, emojis and other characters are left unchanged.
How ROT13 works
For letters A–Z:
\( \text{ROT13}(x) = (x - 'A' + 13) \bmod 26 + 'A' \)
For letters a–z:
\( \text{ROT13}(x) = (x - 'a' + 13) \bmod 26 + 'a' \)
Because applying ROT13 twice returns the original message, you don't need to know whether the text is “encoded” or “decoded”: just run ROT13 and it flips it.
Common uses
- Hide spoilers or puzzle answers.
- Obfuscate email addresses or quick strings.
- Simple classroom demos of substitution ciphers.
Formula (LaTeX) + variables + units
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For letters A–Z: \( \text{ROT13}(x) = (x - 'A' + 13) \bmod 26 + 'A' \) For letters a–z: \( \text{ROT13}(x) = (x - 'a' + 13) \bmod 26 + 'a' \)
- No variables provided in audit spec.
- NIST — Weights and measures — nist.gov · Accessed 2026-01-19
https://www.nist.gov/pml/weights-and-measures - FTC — Consumer advice — consumer.ftc.gov · Accessed 2026-01-19
https://consumer.ftc.gov/
Last code update: 2026-01-19
- Initial audit spec draft generated from HTML extraction (review required).
- Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
- Confirm sources are authoritative and relevant to the calculator methodology.