What this calculator does
A Caesar cipher decoder shifts every letter of a piece of text a fixed number of places through the alphabet, wrapping from Z back to A when it runs past the end. It is one of the oldest ciphers on record, reportedly used by Julius Caesar himself for military messages, and it survives today mainly as an introduction to cryptography.
This calculator handles all three directions: encoding text with a chosen shift, decoding text when the shift is already known, and brute-forcing all 25 possible shifts when it is not, so the correct one can be spotted by eye from the resulting table.
The formula
Each letter is converted to its position in the alphabet, the shift amount is added (for encoding) or subtracted (for decoding), and the result wraps back into the 26-letter range before being converted back to a letter. Anything that is not a letter, such as spaces, numbers and punctuation, is left unchanged.
| Term | Meaning |
|---|---|
| Shift | The fixed number of alphabet positions each letter moves, also called the key. |
| Encode | Shifting plain text forward by the chosen amount to produce ciphertext. |
| Decode | Shifting ciphertext back by the same amount to recover the original plain text. |
| Brute force | Trying every one of the 25 possible non-zero shifts and displaying each result, for when the shift used to encode a message is unknown. |
The inputs explained
| Field | What to enter |
|---|---|
| Text | The text to encode or decode. Letters are shifted; everything else passes through unchanged. |
| Shift amount | The number of alphabet positions to shift by. Ignored in brute-force mode, where every shift from 1 to 25 is tried. |
| Mode | Encode to create ciphertext, decode if the shift is already known, or brute force to try every shift at once. |
When to use it
Solving a puzzle or classroom cryptography exercise
Caesar ciphers are a standard teaching example precisely because they are simple enough to break by hand; entering ciphertext in brute-force mode shows every possible decoding at once.
Checking a message encoded with a known shift
If the shift used is already known, decode mode recovers the original text directly without needing to try every option.
Demonstrating why the cipher is weak
With only 25 possible shifts, a brute-force table makes it obvious how quickly a Caesar cipher can be broken without knowing the key in advance, which is the usual point of introducing it.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the encoded text changes with the shift amount
The same message encoded with a range of shift values.
Questions
What shift did Julius Caesar reportedly use?
Historical accounts credit Caesar with a shift of 3, encoding A as D, B as E and so on, which is why 3 is the default shift on this calculator.
Is a Caesar cipher secure?
No. With only 25 possible non-zero shifts, it can be broken almost instantly by trying every one, which the brute-force mode above demonstrates directly. It has no practical security value beyond puzzles and teaching.
What is ROT13?
ROT13 is a Caesar cipher with a fixed shift of exactly 13. Because the alphabet has 26 letters, applying the same ROT13 operation twice returns the original text, which made it a popular simple way to hide spoilers or punchlines in early online forums.
Does capitalisation and punctuation survive encoding?
Yes. Letters keep their original case through the shift, and anything that is not a letter, such as spaces, numbers and punctuation, is left exactly as typed.
For Unicode text styling rather than letter substitution, see the bold text generator, which also offers an italic style using the same "type text, get transformed text" pattern.