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 X Sudoku?
X Sudoku is ordinary sudoku with two extra lines to satisfy. Fill the grid so that:
- every row, column and 3×3 box holds 1 to 9 exactly once;
- and so does each main diagonal — the one running corner to corner each way.
The two diagonals are what make the X, and they are marked on the board so you can see them. It is the same puzzle as Diagonal Sudoku and Sudoku X; the names are interchangeable and this page is all three.
Sudoku logic works on houses: nine cells that must hold 1 to 9 once each. In ordinary sudoku every cell sits in three — its row, its column, its box. The 17 cells on the X sit in four, and the centre cell sits in five, because both diagonals cross there. Nothing else about the puzzle changes, and no new technique is needed: the diagonal is simply another house to scan.
Where do you start on an X Sudoku?
On the X. Those 17 cells are the most constrained on the board, for the reason the box above gives — an extra house each — and it shows up the moment a puzzle is dealt.
The gap holds at every level. Across 160 boards from this page, cells on a diagonal averaged 1.8 candidates each against 2.6 elsewhere on Easy, and 3.8 against 4.6 on Expert — about eight tenths of a candidate lighter, all the way up. Clear the X first and the digits you place there are worth more than usual, because each one prunes four houses instead of three.
What does the diagonal actually add?
Treat each diagonal as a house and it places digits like any other. Here the row, the column and the box between them leave three possibilities, and the diagonal settles it:
How much the diagonals matter depends on the level, and on Easy the honest answer is: often not at all. We counted every board twice, once under X rules and once ignoring the diagonals. On Easy, 40% of boards still have exactly one answer without them — an ordinary sudoku with two lines drawn on it. From Medium up that all but disappears: 97% of Medium boards and every Hard and Expert board we generated need the diagonals to pin down a single answer.
What do the four levels actually need?
We ran 40 fresh boards per level through a solver that knows one technique at a time, then took techniques away to see which boards stopped:
| Level | Given digits | Singles alone finish it | Needs locked candidates or a subset | Beyond our solver | Boards where the diagonals are load-bearing |
|---|---|---|---|---|---|
| Easy | 38 | 100% | 0% | 0% | 60% |
| Medium | 30 | 93% | 3% | 5% | 97% |
| Hard | 25 | 38% | 15% | 48% | 100% |
| Expert | 21 | 0% | 8% | 93% | 100% |
40 boards per level from this page’s generator, fixed random seed, rounded. Each board counts under the hardest step it needed, from a solver that knows singles, locked candidates, naked pairs and triples, and the X-Wing. The last column is the share of boards that stop having a single answer if you ignore the two diagonals. “Beyond our solver” boards still have exactly one answer.
Easy and Medium are gentle — naked and hidden singles finish every Easy board and nine Medium boards in ten, with the diagonal doing its work quietly as one more thing to scan. Hard brings in locked candidates and naked pairs, and then some.
The generator takes a finished grid and removes digits one at a time, keeping a removal only while the puzzle still has exactly one answer under X rules. We re-solved 160 fresh boards and all 165 daily puzzles with a separate solver written for the purpose: every one had exactly one answer. A board takes about 2 to 4 milliseconds to build, so it is ready before the page has finished loading.
Which level should you play?
Medium is the sweet spot, and Hard if you want a fight. Medium is where the diagonals start being load-bearing — 97% of its boards need them — while nine boards in ten still fall to singles, so the variant teaches itself.
Above that it gets steep quickly, and it is worth being plain about how steep. Hard sends 48% of its boards past everything our solver knows, and Expert, at 21 given digits, sends 93% past it. Those boards all have exactly one answer and none of them needs guessing, but reaching it takes chains longer than singles, locked candidates, pairs, triples and the X-Wing. If a board feels impossible rather than hard, that is why — drop a level rather than doubting yourself.
What goes wrong on an X Sudoku
- Placing a digit that is legal everywhere except the diagonal. This is nearly every X Sudoku mistake. The row, column and box all look fine; the cell was on the X and you scanned it as though it were not.
- Forgetting the centre. R5C5 is on both diagonals, so it answers to five houses — it is usually the single most constrained cell on the board, and it is easy to leave for later.
- Scanning the diagonals only when stuck. They are tightest at the start, when the most candidates are still in play. Read them first, not as a last resort.
What do the buttons do?
Select a cell, then a digit from 1 to 9; the page opens on Medium. 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; a wrong one leaves your notes alone.
- Erase clears the selected cell, and pressing the same digit twice does the same.
- Undo takes back the last change to a cell, though not the notes a correct digit cleared elsewhere, and not the Mistakes count.
- Hint fills one cell with its answer, picked at random rather than the cell 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.
Digits 1–9, the arrow keys, Backspace or Delete and Ctrl+Z all work from a keyboard. The board is saved in this browser after every move, timer included, so it is waiting when you come back. The cells of both diagonals are tinted, and selecting one highlights its row, column and box so you can see all four houses it belongs to at once.
Daily, printable and solvable elsewhere
The daily X Sudoku gives everyone the same board each day, from the same generator, with the whole archive there to play. On paper, each level is a free PDF with answers: Easy, Medium, Hard and Expert, listed on the printable X Sudoku page. Checking one from a book, or wondering whether it has a single answer at all? The X Sudoku solver applies the diagonal rule as well as the usual three.
X Sudoku is the gentlest of the “one more house” variants, because the extra house is one you already know how to read. If it suits you, Jigsaw Sudoku bends the boxes into irregular shapes, Samurai Sudoku overlaps five grids at the corners, and Killer Sudoku replaces the given digits with sums.