Sudoku Made and maintained by Brian Hamilton and Karan Hamilton.
Brian builds the playable puzzle tools, while Karan helps test clarity, difficulty feel, and the player experience.
Playable puzzles and controls are tested in the browser for rule accuracy, usability, and a clear solving experience. See how we make and test puzzles, our editorial standards, or report a problem.
What is Alphabet Sudoku?
Alphabet Sudoku is sudoku played with nine letters instead of nine digits, and a different nine for every board: each new game draws them at random from the whole alphabet. Fill the 9×9 grid so that:
- every row holds each of the nine letters once;
- every column does too;
- and so does every 3×3 box.
The buttons below the grid show the nine in play, in alphabetical order. Sudoku never adds or compares numbers, so nothing is lost in the swap: every technique that works on a classic board works here letter for letter. Letter Sudoku is the same puzzle with A to I every time, and Wordoku hides a nine-letter word in its letters.
There are 3,124,550 ways to choose nine letters from twenty-six, so the same set seldom comes round twice, though nothing stops it. The letters are saved with the board, so a game you come back to keeps them. A set can hold letters that look alike, such as O and Q, I and J or M and N; the buttons always show which nine are in play, and the logic never depends on which they are.
How do you find where a letter goes?
One letter at a time. Pick a letter, find every copy of it already on the board, and look at a box that still needs it: each copy rules out its own row and column. When only one empty square of the box is left, the letter goes there, whatever else that square could hold on its own. That is a hidden single:
Select a square holding the letter and every copy of it on the board lights up, which makes that sweep quick. The same sweep works along a row or a column: a letter with one place left in a line goes there. And a square whose row, column and box already hold eight different letters takes the ninth. Between them, those two moves finish every Easy and Medium board on this page.
What do Hard boards add?
Locked candidates. When singles run out, put the letters each empty square can still hold in its pencil marks — the Notes button draws them small — and look inside one box at a time. If a letter’s places in the box all lie in one row or column, the box’s copy of that letter is that line’s copy too, so it comes out of the rest of the line:
It works the other way round as well: a letter confined to one box within a row or column comes out of the rest of that box. Hard boards are dug so that singles and locked candidates finish them.
And Expert boards?
Naked pairs. Two squares of one row, column or box that can each hold only the same two letters will take those two between them, whichever way round, so no other square of that row, column or box can have either:
Expert boards are dug so that singles, locked candidates and naked pairs finish them, and they start with the fewest letters.
Were the boards on this page fair?
Not for their first five days. From 8 April 2026 the generator took letters out without checking anything, and 140 of 160 boards we dealt from that first version have more than one answer. From 13 April it checked every letter it took out with a full search, and we re-counted 160 of its boards with a separate counter to confirm it: each had exactly one answer. What it never checked was how a board would be solved, though the page said every puzzle’s solution was “reachable through pure logical deduction — no guessing required”.
Each level only had a number of letters to leave, so the names meant less than they said. The page promised Expert “22–25 clues”, but the generator aimed at 22, managed it on 4 boards in 40 and left a median of 25, much the same as Hard’s 26; Easy, Medium and Hard always had exactly 40, 32 and 26, not the ranges the page gave. It said Hard “Demands intermediate strategies like naked pairs, pointing pairs, and box/line reduction” and Expert “Requires advanced techniques such as X-Wing, Swordfish, and XY-Wing”, yet singles alone finished 22 of 40 old Hard boards and 15 of 40 old Expert ones. And a solver that knows singles, locked candidates, pairs, triples and the X-Wing could not finish 26 of the 160 boards — 3 Medium, 12 Hard and 11 Expert.
The generator was rewritten in October 2026, the same fix as Letter, Color, Symbol, Emoji and Shape Sudoku’s, which dealt from the same code. It now takes a letter out only while the level’s own steps still solve the board, which also proves the board has exactly one answer. Two keyboard faults went at the same time, both because any letter can be in play here: with C among the nine, Ctrl+C typed a C instead of copying, and with Z among them, Ctrl+Z typed a Z instead of undoing. Shortcuts are now left to the browser, and Ctrl+Z always undoes. The page also said there were “over 1.9 million ways” to choose the nine letters; there are 3,124,550.
How do the four levels differ?
By how many letters are given, and by the steps a board is dug for. We dealt 40 fresh boards per level from the new generator, and 40 per level from the old one:
| Level | Given letters | Dug for | Singles alone finish it | Old boards beyond our solver |
|---|---|---|---|---|
| Easy | 40 | singles | 40 of 40 | 0 of 40 |
| Medium | 32 | singles | 40 of 40 | 3 of 40 |
| Hard | 26 | + locked candidates | 37 of 40 | 12 of 40 |
| Expert | 22–25 | + naked pairs | 31 of 40 | 11 of 40 |
40 boards per level, fixed seed. Expert digs toward 22 letters as far as its steps allow and keeps the fewest of up to six tries: 22 to 25, median 23. On the new boards nothing past naked pairs was ever needed: 3 Hard boards could not be finished without locked candidates, and of the Expert boards, 6 needed locked candidates and 3 naked pairs.
At the top the new levels are easier than the old ones, and fair all the way down. Most Hard and Expert boards now fall to singles alone, and what makes them harder is having fewer letters to start from — 26, and about 23 — against Medium’s 32. The old Hard and Expert were harder mainly because more than a quarter of their boards needed more than any level here teaches.
A letter comes out only while the level’s steps — singles on Easy and Medium, plus locked candidates on Hard, plus naked pairs on Expert — still fill the grid, and each step only strikes letters that cannot go where they are struck. We re-counted all 160 new boards with a separate counter written for the purpose: every one had exactly one answer, the stored one.
Which level should you play?
Start on Easy if sudoku is new to you, or on Medium if you know it with numbers. Easy gives 40 letters and Medium 32, and both fall to singles alone. The page opens on Medium.
Read the nine buttons before you start: they are the board’s alphabet, in order, and it is quicker to sweep for one letter at a time in that order than to read the grid square by square. Hard and Expert add the two moves above, on boards with fewer letters to start from.
What do the buttons do?
Select a square, then a letter; the page opens on Medium. Selecting a square highlights its row, its column and its box, and every copy of the letter in it. A wrong letter is marked the moment it goes in, and so is one that repeats a letter already in its row, column or box. Under the grid:
- Notes turns pencil marks on and off; they show as small letters. A correct letter clears itself from the notes of its row, its column and its box; a wrong one leaves the other squares’ notes alone.
- Erase clears the selected square, and pressing the same letter twice does the same.
- Undo takes back the last change to a square, though not the notes a correct letter cleared elsewhere, and not the Mistakes count.
- Hint fills one square with its answer, picked at random rather than the square you could work out next.
- Solve asks first, then fills the grid and stops the timer.
- New Game deals another board, with nine new letters, at the level you are on; the level buttons deal one at theirs.
From a keyboard: the letters in play, in either case, or 1–9 for the letters in button order; the arrow keys, Backspace or Delete and Ctrl+Z. A letter that is not in play types nothing, and shortcuts such as Ctrl+C are left to the browser. The board is saved in this browser after every move, letters and all, so it is waiting when you come back.
Where to go next
The same puzzle in pictures instead of letters: Symbol Sudoku has card suits, a note and a few shapes, Emoji Sudoku nine emoji, Color Sudoku nine colours and Shape Sudoku nine shapes.