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 Letter Sudoku?
Letter Sudoku is sudoku written in letters. Fill the 9×9 grid with the letters A to I so that:
- every row holds each of the nine letters once;
- every column does too;
- and so does every 3×3 box.
That is the whole rule. Nothing in sudoku adds or compares numbers, so the nine digits were only ever names, and any nine names work. A board here plays exactly like a classic one, and everything you know about solving sudoku carries over letter for letter.
Alphabet Sudoku deals the same puzzle in nine letters drawn at random from A to Z, so the set changes with every board, and Wordoku uses the nine different letters of a word that runs down the diagonal of the finished grid. Symbol, emoji, colour and shape sudoku do it with pictures. Each has its own page here, and underneath they are all this puzzle.
How do you find where a letter goes?
One letter at a time. Pick a letter, find every copy 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:
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, write the letters each empty square can still hold in your pencil marks 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 letters 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 twelve days. From 1 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. Each level only had a number of letters to leave, so the names meant less than they said. Expert was meant to leave 22 and managed it on 4 boards in 40, leaving a median of 25, much the same as Hard’s 26. 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 — though the page promised that no guessing was ever needed.
The generator was rewritten in September 2026. 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, and a new board appears in well under a tenth of a second at the median.
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: one Hard board could not be finished without locked candidates, and three Expert boards not without 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.
If letters feel slower than digits at first, give each one a digit in your head, A for 1 through I for 9, or sweep them in alphabetical order so none gets skipped. Hard and Expert then 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 a wrong letter is marked the moment it goes in. Under the grid:
- Notes turns pencil marks on and off. 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 at the level you are on; the level buttons deal one at theirs.
From a keyboard: the letters A–I, or 1–9 for A to I, the arrow keys, Backspace or Delete and Ctrl+Z. Browser shortcuts such as Ctrl+C are left to the browser. The board is saved in this browser after every move, so it is waiting when you come back.
Where to go next
The same puzzle in pictures: Symbol Sudoku, Emoji Sudoku, Color Sudoku and Shape Sudoku.