A1Z26 Cipher Decoder

A1Z26 has no key. A is 1, Z is 26, and that's the entire specification, which means it hides nothing from anybody who can count. Calling it a cipher is generous.

What it does is more useful than that. It's the step that turns letters into numbers, and until you've done that you can't do arithmetic on a message at all. Any shift cipher is doing this arithmetic whether or not it writes the numbers down, though most implementations count from zero rather than one.

Key Takeaways

• A is 1 through Z is 26. No key, so nothing to guess.
• It's an encoding, not a cipher. It changes representation, not readability.
• A Caesar shift is A1Z26 plus one sum: add the shift, wrap at 26.
• Suetonius records Caesar's shift as three, which in these numbers is just "+3".
• Wrapping at the end of the alphabet is modular arithmetic, which is the notation real cryptography is written in.

What does A1Z26 actually do?

Numbers the alphabet, in order, and stops. Type into the box above and each letter comes back as its position.

CLICK becomes 3 12 9 3 11.

There's no rule to learn beyond the alphabet you already know, and no secret component. That's the whole reason it can't protect anything: the sender knows nothing the reader doesn't.

It does have one real weakness, and you'll hit it the first time you use the box above. 1 12 is A-L. Drop the spaces and 112 could be A-A-B, A-L or K-B, and nothing in the encoding tells you which. The numbers only work with separators, which is a problem the letters never had.

Then why does it matter?

Because a cipher can't do arithmetic on a letter. It has to have a number first.

Watch the Caesar cipher with this in mind and it stops looking like letter-substitution and starts looking like addition. Suetonius records Caesar's key as substituting D for A, which is a shift of three. In A1Z26 terms:

Letter	A1Z26	+3	Wraps to	Result
C	3	6	6	F
L	12	15	15	O
I	9	12	12	L
C	3	6	6	F
K	11	14	14	N

CLICK becomes FOLFN. The cipher is one addition per letter, and A1Z26 is the part that makes the addition possible.

That's the honest description of what this page is for: it's not a code, it's the ruler.

What happens at Z?

You wrap around to A, and that wrap is more interesting than it looks.

Add 3 to Y, which is 25, and you get 28. There's no 28th letter, so you subtract 26 and land on 2, which is B.

One pedantic note, since this page is about the arithmetic being visible: on a 1-indexed alphabet the tidy form is ((n - 1 + k) mod 26) + 1, not n + k mod 26. That off-by-one is exactly why implementations usually make A zero.

It matters because modulo is the notation real cryptography is written in. The wrap itself is not what makes anything hard: modular addition is undone by subtracting, and a shift cipher falls to 25 guesses.

The difference is the operation, not the wrap. Swap addition for exponentiation, and 26 for a number hundreds of digits long, and undoing it stops being subtraction and becomes a problem nobody has solved efficiently. That is our reading of the family resemblance, not a claim that this page is doing cryptography.

Is A1Z26 ever the right tool?

For hiding a message, no. For three other things, yes.

• Teaching. It makes the arithmetic under a cipher visible, which in our experience is why it tends to be the first thing anyone is shown.
• Puzzles. A string of numbers under 27 is a recognisable shape, and we read the appeal as the noticing rather than the cracking.
• Reading other people's ciphers. Once you see letters as numbers, a shift becomes an interval you can measure rather than a substitution you have to guess.

If you want the version with no key that at least looks scrambled, that's Atbash. If you want the smallest thing that has a key at all, that's Caesar. This page is what both of them are standing on.

Frequently asked questions

What is the A1Z26 cipher?

A letter-to-number encoding: A is 1, B is 2, through to Z is 26. It has no key, so it is not really a cipher. It changes how a message is written down rather than who can read it.

Is A1Z26 secure?

No. There is no key and no secret, so anyone who can count to 26 can read it instantly. Treat it as a puzzle format or a teaching tool.

How is A1Z26 related to the Caesar cipher?

A Caesar shift is A1Z26 plus one sum. Convert each letter to its number, add the shift, wrap at the end of the alphabet, and convert back. Suetonius records Caesar's shift as three, which in these numbers is just "+3" - on the modern 26-letter alphabet, since the Latin one ran to about 23 letters.

What happens when you go past Z?

You wrap to A. Y is 25, so adding 3 gives 28, and subtracting 26 gives 2, which is B. That is arithmetic modulo 26, offset by one because this alphabet starts at 1 rather than 0.

Why does A1Z26 need spaces between the numbers?

Because the numbers are variable length. Without separators 112 could be A-A-B, A-L or K-B, and nothing in the encoding says which. Letters never had that problem, and it is the one thing A1Z26 is worse at than the alphabet it replaces.

Type a word into the box above and read the numbers. Then go and add three to each one by hand, and you'll have done what Caesar did.

Sources

• Suetonius, Life of Julius Caesar 56 (for the shift of three), LacusCurtius, University of Chicago - retrieved 30 July 2026, https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Julius*.html