How does Excel number its columns?

Excel uses bijective base-26: A=1, B=2, up to Z=26, then AA=27, AB=28, and so on. There is no zero digit, which is why it is called bijective rather than ordinary base-26.

What is the formula for letters to a number?

For each letter from left to right, multiply the running total by 26 and add the letter's position (A=1 … Z=26). So AB is 1×26 + 2 = 28.

How do you convert a number back to letters?

Repeatedly subtract 1, take the remainder modulo 26 as the next letter from the right, then integer-divide by 26. This adjustment handles the missing zero digit correctly.

What is the maximum Excel column?

Modern Excel worksheets stop at column XFD, which is index 16384. This converter accepts larger values too, since the same bijective base-26 math extends without limit.

Are lowercase letters accepted?

Yes. Column labels are case-insensitive, so ab and AB both convert to 28. The output label is always shown in uppercase to match spreadsheet conventions.

Excel Column ↔ Number Converter

Why Excel columns are not ordinary base-26

Spreadsheet column labels look like base-26, but they use a quirk called bijective numeration. There is no digit for zero: the symbols run A through Z (values 1 through 26), so after Z comes AA rather than BA. That single difference is why naive base-26 conversion gives the wrong answer for multi-letter columns.

How it works

To turn letters into a number, scan left to right and accumulate:

total = total * 26 + (letterValue)   where A=1 … Z=26

So AB is 1 * 26 + 2 = 28, and ZZ is 26 * 26 + 26 = 702.

To turn a number back into letters, peel off one letter at a time from the right, subtracting one before each step to account for the missing zero:

n = n - 1
letter = 'A' + (n mod 26)
n = floor(n / 26)

Repeat until n reaches zero, then reverse the collected letters. This converter implements exactly that algorithm, so it round-trips perfectly with Excel and Google Sheets.

Example and tips

Column index 16384 returns XFD, the last column in a modern Excel sheet. Going the other way, the label AAA works out to 1×676 + 1×26 + 1 = 703. Because the math is purely positional, the tool handles arbitrarily long labels, which is handy for generating CSV headers or validating data-export pipelines.