MasterTool

Caesar Cipher Calculator

Encode and decode text with a Caesar shift cipher — including the classic ROT13 — by shifting every letter through the alphabet.

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What this tool does

Encode and decode text with a Caesar shift cipher — including the classic ROT13 — by shifting every letter through the alphabet. Only A–Z letters are shifted; case is preserved and digits, spaces and punctuation pass through unchanged. A Caesar cipher has only 25 useful keys and falls to frequency analysis in seconds — it is a teaching and puzzle tool, never real security.

How to use the Caesar Cipher Calculator

  1. Enter or select text.
  2. Enter or select shift (letters).
  3. Enter or select operation.
  4. Read the calculated result; change any measurement to compare alternatives.

Formula

Each letter is replaced by the letter k positions later in the alphabet, wrapping Z → A: cipher = (letter + k) mod 26. Decoding shifts by −k. ROT13 is the special case k = 13, which is its own inverse.
text
Text
shift
Shift (letters)
mode
Operation

Only A–Z letters are shifted; case is preserved and digits, spaces and punctuation pass through unchanged. A Caesar cipher has only 25 useful keys and falls to frequency analysis in seconds — it is a teaching and puzzle tool, never real security.

Worked example

For caesar cipher calculator, the following measurements illustrate the exact method: Text: HELLO; Shift (letters): 3; Operation: encode.

Inputs

  • TextHELLO
  • Shift (letters)3
  • OperationEncode (shift forward)

Result

  • ResultKHOOR
  • Shift used3
  • OperationEncode

Results explained

Result
Result from the formula above. Only A–Z letters are shifted; case is preserved and digits, spaces and punctuation pass through unchanged. A Caesar cipher has only 25 useful keys and falls to frequency analysis in seconds — it is a teaching and puzzle tool, never real security.
Shift used
Shift used from the formula above. Only A–Z letters are shifted; case is preserved and digits, spaces and punctuation pass through unchanged. A Caesar cipher has only 25 useful keys and falls to frequency analysis in seconds — it is a teaching and puzzle tool, never real security.
Operation
Operation from the formula above. Only A–Z letters are shifted; case is preserved and digits, spaces and punctuation pass through unchanged. A Caesar cipher has only 25 useful keys and falls to frequency analysis in seconds — it is a teaching and puzzle tool, never real security.

Frequently asked questions

A substitution cipher named after Julius Caesar, who reportedly used a shift of 3: every letter is replaced by the letter a fixed number of positions later in the alphabet, so with shift 3, A becomes D and HELLO becomes KHOOR.

ROT13 is a Caesar cipher with a shift of exactly 13. Because 13 is half the alphabet, applying ROT13 twice returns the original text, so the same operation encodes and decodes — long used online to hide spoilers and puzzle answers.

Try decoding with each shift from 1 to 25 and look for the one that produces readable words — there are only 25 possibilities. Statistical tools also work: in English, E is the most common letter, so the most common cipher letter usually maps to E.

No. Only letters A–Z and a–z are shifted, letter case is preserved, and every other character — digits, spaces, punctuation — passes through exactly as typed.

Not at all. With only 25 keys it can be broken by hand in minutes. It is included for education, puzzles and CTF challenges; use real encryption (your operating system and messaging apps provide it) for anything private.

Yes. The cipher runs entirely in your browser and your text is never uploaded or stored — though remember the cipher itself offers no secrecy once the output is shared.