ROT18 is the combination of two simple ciphers applied together: ROT13 rotates each letter by 13 positions, and ROT5 rotates each digit by 5 positions. The name comes loosely from 13 + 5.

Why is ROT18 self-inverse?

Both halves are self-inverse. ROT13 rotates by half of 26 and ROT5 by half of 10, so applying each twice returns the original. That makes the whole ROT18 operation reversible with the exact same step — no separate decode mode is needed.

What characters does ROT18 change?

Only A–Z, a–z and 0–9 are transformed. Letters use ROT13 (A↔N, B↔O, …) and digits use ROT5 (0↔5, 1↔6, …). Spaces, punctuation and other symbols pass through untouched.

How does it handle case?

Case is preserved. Uppercase letters rotate to uppercase letters and lowercase to lowercase, so a capital G becomes a capital T and a lowercase g becomes a lowercase t.

Is ROT18 secure encryption?

No. It has no key and a tiny, fixed mapping, so anyone who recognises it can reverse it instantly. Use ROT18 for puzzles, light obfuscation, or to hide spoilers, never for real secrets.

ROT18 Encoder/Decoder

ROT18 rotates an entire alphanumeric string in one pass by combining two famous toy ciphers: ROT13 for letters and ROT5 for digits. Where ROT13 alone leaves numbers readable, ROT18 scrambles letters and digits together, making it a tidy choice for puzzle hunts, hiding spoilers, and lightly obfuscating mixed text like serial numbers. This tool runs the transformation instantly in your browser.

How it works

Each character is classified and rotated independently:

Uppercase A–Z: shifted by 13 — (code − 65 + 13) mod 26 + 65
Lowercase a–z: shifted by 13 — (code − 97 + 13) mod 26 + 97
Digits 0–9: shifted by 5 — (code − 48 + 5) mod 10 + 48
Everything else: left unchanged

Because 13 is half of 26 and 5 is half of 10, both rotations are self-inverse. Applying ROT18 a second time undoes it exactly, which is why a single button serves as both encoder and decoder.

Example

The string Gera Tools 2026 becomes Tren Gbbyf 7571: the letters rotate via ROT13 (G→T, e→r, r→e, a→n) while the digits rotate via ROT5 (2→7, 0→5, 2→7, 6→1). The space is preserved.

Notes

ROT18 is a classic obfuscation device, not encryption — there is no key and the mapping is public. Everything runs locally in your browser, so your text is never sent to a server.