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 Asterisk Sudoku?
Asterisk Sudoku is sudoku with one more region. Fill the 9×9 grid with the digits 1 to 9 so that:
- every row, every column and every 3×3 box holds each digit once, as usual;
- and the nine shaded squares of the asterisk hold each digit once too.
The asterisk’s squares are R2C5, R3C3, R3C7, R5C2, R5C5, R5C8, R7C3, R7C7 and R8C5: one in each box, spread like a star around the centre. They are not spread evenly over the lines, though. Row 5 and column 5 each hold three of them, rows and columns 3 and 7 hold two, rows and columns 2 and 8 hold one, and the rest hold none — which is where the asterisk does most of its work, as the figures below show.
Take its rule away and most boards here stop having one answer. We checked 40 boards per level as if they were plain sudoku: 23 Easy boards, 37 Medium boards and every Hard and Expert board had more than one solution. You cannot finish these boards without using the asterisk.
How does the asterisk place a digit?
Exactly as a row does. The asterisk needs every digit once, so pick a digit it is missing and look at its empty squares: each one that already sees that digit in its own row, column or box is ruled out. When one square is left, the digit goes there. That is a hidden single, found in the tenth region:
Sweep the asterisk along with the rows, columns and boxes every time you look for a digit, and remember the other direction too: a digit already in one asterisk square is ruled out of the other eight, even where they share no row, column or box. Between them, those hidden singles and the squares down to one candidate finish every Easy and Medium board on this page.
What do Hard boards add?
Locked candidates, and the asterisk makes more of them. When singles run out, put the digits each empty square can still hold in its pencil marks. If a digit’s places in one house all lie inside another house, the first house’s copy of that digit is the second’s too, so it leaves the rest of the second. With the asterisk, that works between the asterisk and the lines that cross it more than once: row 5 and column 5, which hold three of its squares, and rows and columns 3 and 7, which hold two. A box holds only one asterisk square, so a box never locks with it:
It works the other way round as well: when a row’s or a column’s places for a digit all lie on the asterisk, the digit comes out of the rest of the asterisk. Hard boards are dug so that singles and locked candidates finish them.
And Expert boards?
Naked pairs. Two squares of one house that can each hold only the same two digits take those two between them, so no other square of that house can have either — and the asterisk is a house like any other:
Expert boards are dug so that singles, locked candidates and naked pairs finish them, and they start with the fewest digits.
Were the boards on this page fair?
Not for their first eleven days. From 2 April 2026 the generator took digits out without checking anything, and 133 of 160 boards we dealt from that first version have more than one answer. From 13 April it checked every digit 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. A solver that knows singles, locked candidates, pairs, triples and the X-Wing, using the asterisk as a house, could not finish 58 of the 160 boards — 3 Medium, 19 Hard and 36 of the 40 Expert — though the page promised that no guessing was ever needed. And the full search was slow: in Chrome, an Expert board took a median of nearly a second to deal and up to six seconds, with the page frozen while it did.
The generator was rewritten in October 2026, the same fix as this site’s letter, word and 8×8 sudoku had, with the asterisk as one more house. It now takes a digit out only while the level’s own steps still solve the board, which also proves the board has exactly one answer, and in Chrome a new board appears in a few hundredths of a second at every level.
How do the four levels differ?
By how many digits 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 digits | Dug for | Singles alone finish it | Old boards beyond our solver |
|---|---|---|---|---|
| Easy | 38 | singles | 40 of 40 | 0 of 40 |
| Medium | 30 | singles | 40 of 40 | 3 of 40 |
| Hard | 25 | + locked candidates | 20 of 40 | 19 of 40 |
| Expert | 21 | + naked pairs | 14 of 40 | 36 of 40 |
40 boards per level, fixed seed; every step counts the asterisk as a house. Every Expert board reached its 21 digits. On the new boards nothing past naked pairs was ever needed: 8 Hard boards could not be finished without locked candidates, and of the Expert boards 8 needed locked candidates and 8 needed pairs.
Here the levels really do differ in what they ask. Easy and Medium fall to singles alone; half the Hard boards and about two thirds of the Expert boards need more, and each level starts with fewer digits than the last — 38, 30, 25, 21. The old Hard and Expert were harder mainly because 55 of their 80 boards needed more than any level here teaches.
A digit comes out only while the level’s steps — singles on Easy and Medium, plus locked candidates on Hard, plus naked pairs on Expert, the asterisk counted in each — still fill the grid, and each step only strikes digits 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 the asterisk is new to you, or on Medium, where the page opens, if you know sudoku well. Easy gives 38 digits and Medium 30, and both fall to singles alone once you treat the asterisk as a tenth house.
The habit to build is the sweep: for each digit, check the asterisk as well as the rows, columns and boxes, and watch row 5 and column 5, where three asterisk squares sit. Selecting an asterisk square highlights the rest of the asterisk along with its row, column and box, which makes the tenth house easy to see. Hard and Expert then add the two moves above.
What do the buttons do?
Select a square, then a digit; the page opens on Medium. Selecting a square highlights its row, its column and its box, and the whole asterisk if the square is on it. A wrong digit is marked the moment it goes in, and a digit that repeats one in the same row, column, box or asterisk is flagged as a clash. Under the grid:
- Notes turns pencil marks on and off. A correct digit clears itself from the notes of its row, its column and its box, and of the asterisk if it sits on it; a wrong one leaves the other squares’ notes alone.
- Erase clears the selected square, and pressing the same digit twice does the same.
- Undo takes back the last change to a square, though not the notes a correct digit 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: 1–9, the arrow keys, Backspace or Delete and Ctrl+Z; shortcuts such as Ctrl+1 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
Other sudoku with extra regions: Hyper Sudoku adds four 3×3 windows, NRC Sudoku is the same puzzle under another name, and X-Sudoku makes both long diagonals hold 1 to 9.