Caesar Cipher Decoder and Encoder

The Caesar cipher decoder is a free, online tool that shifts each letter of a message through the alphabet to decode or encode a Caesar cipher. Paste or type your text, choose Decode or Encode, and set the shift from 1 to 25. The result updates as you type.

The tool also works as a Caesar cipher translator, a shift cipher decoder, a Caesar cipher encoder, and a ROT13 converter. If you do not know the shift, it can test all 25 and rank the most likely readings. Everything runs in your browser, no sign-up is needed, and nothing you type is sent anywhere.

How to use the Caesar Cipher Decoder

1. Paste the coded text into the input box.
2. Select Decode to move letters back by the shift, or Encode to move them forward.
3. Set the shift with the number box or the slider. The default is 3. The result box updates live and shows a heading such as “Decoded text (shift 3)”.
4. If you do not know the shift, press I Don’t Know the Shift. The tool lists all 25 shifts, best first. Each row shows a “Use shift k” button and a count such as “16 of 16 words recognised”. The best shift loads into the result box automatically. Click any row’s button to try another.
5. Check that the result reads as real words.
6. Press Copy Result to copy the text, or Use Result as Input to send it back to the input and flip Encode and Decode. This is useful for chaining steps.

You can press Ctrl+Enter (or Cmd+Enter on a Mac) in the text box to run the unknown-shift search. The ROT13 button sets the shift to 13 in one click. Encoding a message with one shift and then decoding it with the same shift always gives back the original text. Only letters A-Z are changed, so spaces, digits and punctuation stay where they are.

What is a Caesar cipher? (definition)

A Caesar cipher is a substitution cipher. Each letter in the message is replaced by the letter a fixed number of places further along the alphabet, called the shift. When the shift runs past Z, it wraps back to A. The cipher is named after Julius Caesar, who, according to the Roman historian Suetonius, used a shift of 3 for private letters.

Caesar cipher meaning: what does the shift key do?

The shift is the key. For a Caesar cipher it can be any whole number from 1 to 25. A shift of 0 or 26 changes nothing, because the alphabet has 26 letters and a full loop returns each letter to itself. That leaves only 25 useful keys.

How to use a Caesar cipher to encode and decode a message

To encode, move each letter forward by the shift. With a shift of 3, A becomes D, B becomes E, and so on. X becomes A, Y becomes B, and Z becomes C, so XYZ with a shift of 3 gives ABC. HELLO becomes KHOOR.

Letter case is kept, so “Hello, World!” with a shift of 3 becomes “Khoor, Zruog!”. Spaces, digits and punctuation are not changed, so word breaks stay visible. To decode, move each letter back by the same shift: KHOOR with a shift of 3 becomes HELLO.

How to solve a Caesar cipher by hand

You can solve it in two ways. The first is to try each of the 25 shifts and check which one gives real words. The second is to find the most common letter in the coded text, assume it stands for E (the most common letter in English), and work out the shift from the gap between the two letters.

If every shift gives nonsense, the message probably does not use a single shift. A general letter-swap puzzle needs a cryptogram solver, not a Caesar tool. A Vigenère cipher, where the shift changes for each letter, cannot be broken with this tool.

How to read the Caesar cipher alphabets

The Alphabet at this shift strip shows two rows. The Plain row is A to Z. The Cipher row, below it, shows which letter each Plain letter becomes at the current shift. To encode a letter, find it in the Plain row and read the letter below it. To decode, find the letter in the Cipher row and read the one above it.

Caesar cipher example: what does a real solve look like?

Take this coded sentence: “Wkh txlfn eurzq ira mxpsv ryhu wkh odcb grj. Phhw ph dw wkh oleudub dw qrrq!” With a shift of 3 it is ranked first and reads “The quick brown fox jumps over the lazy dog. Meet me at the library at noon!” It recognises 16 of 16 words, while the next-best shifts recognise only 3.

Caesar cipher explained: how the tool picks the best shift

For each shift, the tool gives a score. The first part is 100 multiplied by the share of letters that sit inside recognised words. A word is recognised if it is “a”, “I”, or in a list of 17,156 English words with two or more letters. The second part is 25 multiplied by how closely the letter pattern matches English letter frequencies, such as E at 12.7% and T at 9.1%. If the word list fails to load, the ranking uses letter frequency alone.

This ranking can go wrong with very short messages, names and non-English text. For example, “Pb” is “Me” with a shift of 3, but the tool does not rank it first, so check the other lines too.

What is ROT13, and how is it related to the Caesar shift?

ROT13 is a Caesar cipher with a shift of 13. Because the alphabet has 26 letters, applying ROT13 twice returns the original text. So encoding and decoding are the same operation. For example, Hello becomes Uryyb, and Uryyb becomes Hello again.

Frequently asked questions

Is the caesar cipher decoder free to use?

Yes. The tool is free and runs in your browser. You do not need to create an account.

Does the shift cipher decoder need sign-up or installation?

No sign-up is needed. Open the page and start using the tool. The word list loads the first time you use the unknown-shift search.

Is my text sent to a server?

No. Everything runs in your browser, and nothing you type is sent anywhere.

Can I use a Caesar cipher to keep messages secret?

No. With only 25 possible shifts, anyone can try them all in seconds. Caesar ciphers are used for puzzles, teaching, and hiding spoilers, not for protecting private data.

Does the tool work with accented letters or other alphabets?

No. Only the letters A to Z are shifted. Accented letters, other alphabets, and non-English text are not supported, and the ranking may not pick the right shift for them.

Is there a printable caesar cipher wheel?

No. The page does not have a printable wheel or a print button. The Alphabet at this shift strip shows the same pairing of letters on screen.

Can the tool break a Vigenère cipher?

No. A Vigenère cipher changes the shift for each letter, and this tool only applies one fixed shift to the whole message.

Related tools

• Cryptogram Solver: Use this when every Caesar shift gives nonsense, because the message uses a general letter-swap code.
• Cryptogram Maker: Use this to create your own substitution puzzle for friends to solve.
• Cryptograms: Use this to practise solving coded quotes for fun.
• Codeword Solver: Use this to fill in crossword-style codeword grids.
• Word Unscrambler: Use this to rearrange a set of letters into real words.
• Anagram Solver: Use this to find words made from the same letters as a given word.
• Hangman: Use this to guess words one letter at a time.
• Word Ladder Solver: Use this to change one word into another one letter at a time.