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 Samurai Sudoku?
Samurai Sudoku is five ordinary sudoku grids laid out in a cross, each corner grid sharing one 3×3 box with the grid in the middle. That overlap is the whole puzzle: 5 × 81 comes to 405, but the four shared boxes are counted twice, so a samurai is 369 cells, not 405.
- every row, column and 3×3 box of each of the five grids holds 1 to 9 exactly once;
- the four shared boxes belong to two grids at the same time, and must satisfy both;
- there is exactly one answer, and no guessing is needed to reach it.
In Japan the five-grid layout is called Gattai-5 — “five merged” — and samurai is the name it picked up in The Times and the Australian papers that ran it. You will also see it sold simply as five-grid or overlapping sudoku. All the same puzzle.
Sudoku logic works on houses: nine cells that must hold 1 to 9 once each. In an ordinary sudoku every cell sits in exactly three of them — its row, its column, its box. The 36 cells inside a samurai’s shared boxes sit in six: a row, column and box in the corner grid, and another row, column and box in the centre. That is the entire difference between a samurai and five sudoku printed side by side.
Where do you start on a samurai?
In the shared boxes, and not because they look interesting: they are measurably the tightest part of the board. Every cell in them is squeezed by two grids at once, so they start with fewer candidates than anywhere else.
The gap holds at every level. Across 160 boards from this page, the cells in the four shared boxes averaged 1.4 candidates each on Easy against 2.0 elsewhere, and 3.7 against 4.2 on Expert — roughly two thirds of a candidate lighter per cell, all the way up. Clear the overlaps first and the digits you place there are worth double, because each one prunes a row, a column and a box in two grids.
What is a cross-grid deduction?
It is the move a samurai has and five separate sudoku do not: a cell that one grid cannot settle and its neighbour can. Because the shared cells belong to both grids, a digit forced by the centre lands inside a corner grid as a free clue — and often unlocks cells well away from the overlap.
This is not a flourish for hard boards; it is how a samurai is meant to be solved. We took the 200 individual grids inside 40 Expert boards and asked whether each one could be finished from its own givens alone. Two could. The other 198 need something their neighbours know — which is also why solving one grid to the last cell before looking at the others is the slowest way through.
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 | Grids that can be solved on their own |
|---|---|---|---|---|---|
| Easy | 210 | 100% | 0% | 0% | 85% |
| Medium | 165 | 93% | 0% | 8% | 53% |
| Hard | 130 | 83% | 10% | 8% | 13% |
| Expert | 105 | 5% | 15% | 80% | 1% |
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 200 individual 9×9 grids in those boards, asked whether their own givens alone pin them down. “Beyond our solver” boards still have exactly one answer.
Easy and Medium are ordinary sudoku spread over five grids — naked and hidden singles finish every Easy board and nine Medium boards in ten. Hard starts to want locked candidates and the odd naked pair. Expert is a different animal, and the next section is about why.
The generator starts from a finished samurai and takes digits away one at a time, keeping a removal only while the puzzle still has exactly one answer — and refusing it otherwise. As a second opinion we re-solved 160 fresh boards and all 164 daily puzzles with a separate solver written for the purpose: every one had exactly one answer. A board takes about 15 to 45 milliseconds to build, so it is ready before the page has finished loading.
Which level should you play?
Hard, for most people. It is the level where the samurai part of the puzzle earns its place: only 13% of its grids can be finished alone, so you are genuinely working across the overlaps, and 83% of boards still fall to singles once you do.
Expert is deliberately near the edge. We measured where the floor is by stripping a finished samurai as far as it would go while keeping one answer: it bottoms out around 95 givens, and Expert stops at 105. Four Expert boards in five need more than singles, locked candidates, pairs, triples and the X-Wing — the techniques our solver knows — so expect long chains, and expect to lean on the shared boxes for every digit they will give you. It has one answer and it is reachable by logic; it is not a puzzle to learn the variant on.
What goes wrong on a samurai
- Forgetting that a shared cell has two of everything. A digit in an overlap must clear six houses, not three. Most samurai mistakes are a placement that is perfectly legal in the grid you were looking at.
- Finishing one grid before looking at the rest. On Expert only one grid in a hundred can be finished alone, so grinding at a single corner is usually a sign you have stopped using the puzzle’s one real advantage.
- Pencilling all 369 cells. Notes are worth more here than on a 9×9, but filling the whole board before you have cleared the overlaps buries the shapes you need. Do the shared boxes first, then mark up.
What do the buttons do?
Select a cell, then a digit from 1 to 9; the page opens on Medium. Under the board:
- Notes turns pencil marks on and off. A correct digit clears itself from the notes of its row, its column and its box — in both grids, when you are in a shared 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 all 369 cells 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, and the arrows step over the empty space between the arms rather than stopping at it. The board is saved in this browser after every move, timer included, so a puzzle this size does not have to be finished in one sitting.
Daily, printable and solvable elsewhere
The daily Samurai 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 Samurai Sudoku page. Working through one from a book and want to know whether it holds together? The Samurai Sudoku solver takes all five grids and checks the overlaps with them.
If the overlap is what appeals, Jigsaw Sudoku changes the shape of the boxes instead of the number of grids, and X-Sudoku adds two diagonals to an ordinary grid. Killer Sudoku keeps one grid and replaces the given digits with sums.