Sudoku Samurai Sudoku Solver for Five Overlapping Grids
This free Samurai Sudoku Solver is designed for the classic Gattai-5 layout: five standard 9x9 Sudoku grids arranged like a cross. There is one central Sudoku and four corner Sudokus. Each corner grid overlaps the centre grid in one shared 3x3 box, so the same nine cells must satisfy two different Sudoku grids at the same time.
That overlapping structure is what makes Samurai Sudoku harder to solve by hand and harder to check with a normal Sudoku solver. A single 9x9 solver only sees one grid. A proper Samurai Sudoku solver has to track five row systems, five column systems, five sets of 3x3 boxes, and four shared overlap zones. This page is built for that exact job: enter the clues, solve the complete Samurai grid, reveal a step, import a puzzle string, export your work, and check whether the puzzle has a unique solution.
A standard Samurai Sudoku board is drawn on a 21x21 layout. Not every position is active: the usable puzzle contains 369 active cells, arranged across five overlapping 9x9 Sudoku grids. The inactive spaces are only there because the five grids do not fill the whole square.
How to Use the Samurai Sudoku Solver
Start by copying the givens from your puzzle into the grid above. You can click a cell and type a digit, or use the number buttons. Shared overlap cells only need to be entered once. The solver treats those cells as belonging to both grids automatically, so a digit placed in an overlap affects the relevant corner grid and the centre grid together.
- Enter every clue from your printed or online Samurai Sudoku puzzle.
- Check the grid for obvious typing mistakes before solving.
- Use Step if you want help with one placement rather than the whole answer.
- Use Solve when you want the full Samurai Sudoku solution.
- Use Import if you already have a puzzle string.
- Use Export to save or share the puzzle you entered.
- Read the solver message to see whether the puzzle is solved, invalid, or not unique.
If you are using the solver to learn rather than simply get the answer, the Step button is usually the best place to start. It reveals one confirmed placement at a time, which lets you compare the solver's progress with your own notes. If you only want to verify a finished puzzle, fill in all givens or your completed grid and let the solver check the structure.
Samurai Sudoku Rules
Samurai Sudoku follows the ordinary rules of Sudoku inside each 9x9 grid. Every row must contain the digits 1 to 9 exactly once. Every column must contain the digits 1 to 9 exactly once. Every 3x3 box must contain the digits 1 to 9 exactly once. The difference is that there are five grids, and four of the 3x3 boxes are shared between a corner grid and the centre grid.
Those shared boxes are the heart of the puzzle. If a digit is forced in the top-left overlap, it affects both the top-left Sudoku and the central Sudoku. A mistake in one overlap can make two grids impossible at once. That is why Samurai Sudoku often feels manageable at first, then suddenly becomes tangled in the middle. The solver keeps those relationships synchronized so you do not have to check the same overlap manually in two separate places.
Why a Normal Sudoku Solver Is Not Enough
A normal 9x9 Sudoku solver can be useful if you isolate one of the five grids, but it cannot solve the full Samurai puzzle correctly. If you solve a corner grid alone, you may place digits in the overlap that break the centre grid. If you solve the centre grid alone, you may break one of the corners. The five grids are not independent; they form one combined logic puzzle.
A dedicated Samurai Sudoku solver understands the full board. It knows which 369 cells are active, which 72 positions are inactive on the 21x21 canvas, and which overlap cells must be shared. It can detect contradictions that only appear when two grids are considered together. It can also test whether a puzzle has one solution or multiple possible completions.
Importing 369-Cell and 441-Cell Puzzle Strings
The import box accepts two common formats. A 369-cell string includes only the active Samurai Sudoku cells, read left to right and top to bottom. This is compact and ideal when your source already understands the Samurai layout. A 441-cell string represents the full 21x21 board, including inactive spaces. In both formats, use digits 1-9 for clues and use 0 or a dot for blanks.
If an import fails, check the length first. A missing character, an extra space, or a copied line break can shift the entire puzzle. If the length is correct but the solver reports a contradiction, recheck the original givens around the four overlap boxes. Most Samurai Sudoku entry mistakes happen where two grids touch, because it is easy to count a shared 3x3 box as part of only one grid.
What the Solver Checks
The solver checks the same constraints a careful human solver would check: rows, columns, boxes, shared overlap cells, and active board positions. It does not treat inactive cells as part of the puzzle. It does not allow a digit to satisfy one grid while breaking the other grid that shares the same overlap. When possible, it also reports whether the puzzle is unique.
Uniqueness matters because a good Samurai Sudoku should have one logical solution. If a puzzle has multiple solutions, you may still be able to fill a valid grid, but the clues are not enough to force a single answer. That can make a puzzle frustrating, especially if you are solving from a newspaper, a puzzle book, or a copied image and you suspect a missing clue.
When to Use Step Instead of Solve
Use Solve when you need the complete answer quickly. Use Step when you want a hint without spoiling the entire puzzle. Step solving is useful when you are stuck in one region but still want to finish most of the Samurai Sudoku yourself. It can also help you learn how the overlap zones influence the rest of the grid.
After each step, pause and ask why that digit was forced. Look at the row, column, 3x3 box, and any overlapping grid involved. If you can explain the placement, you have gained a technique rather than just an answer. If the placement depends on advanced search, use it as a checkpoint and continue with your own deductions from there.
Common Samurai Sudoku Mistakes
The most common mistake is entering a clue in the wrong copy of an overlapping box. The overlap is shared, so there is no separate version for the corner grid and the centre grid. Another frequent problem is importing a 441-position string into a system expecting 369 active cells, or the other way around. The board may look almost right but be shifted in later rows.
Players also sometimes assume that each 9x9 grid can be solved independently. In easier Samurai puzzles, a corner may make progress on its own for a while. In harder puzzles, however, a corner grid might need information from the centre, and the centre might need information from a different corner. The whole point of the format is that logic travels through the shared boxes.
Strategies for Solving Samurai Sudoku by Hand
Begin with the four corner grids. They usually have more ordinary Sudoku structure available before the overlaps become difficult. Fill obvious singles and mark candidates in the shared 3x3 boxes. Then move to the centre grid and see how those overlap candidates restrict central rows, columns, and boxes. Repeat this loop rather than trying to finish one grid completely before looking at the others.
Candidate notation is especially helpful. Write small digits in empty cells, and be disciplined about removing candidates when a digit appears in a shared row, column, or box. Look for naked singles, hidden singles, pairs, triples, pointing pairs, and box-line reductions. These classic Sudoku techniques still work, but the best opportunities often appear when a candidate is restricted by two grids at once.
If you get stuck, use this solver as a diagnostic tool. Enter the givens and compare the next step with your pencil marks. If your candidates disagree with the solver, revisit the overlap boxes first. A single mistaken candidate in a shared region can block progress across a surprisingly large part of the puzzle.
Good Uses for This Solver
This tool is useful for checking a Samurai Sudoku from a newspaper, validating a puzzle from a book, finishing a difficult online grid, converting a puzzle into a shareable string, or confirming that a puzzle you created has exactly one solution. Teachers and puzzle club organisers can use it to check handouts before printing. Solvers can use it as a hint tool rather than an answer machine.
If you want to practise before using the solver, try our Samurai Sudoku page. If you want paper puzzles, visit printable Samurai Sudoku. For standard 9x9 grids, the main Sudoku Solver is simpler and faster. For solving explanations and general techniques, the Sudoku Helper can be a useful companion.
What to Do If the Solver Finds a Problem
If the solver reports that a Samurai Sudoku is invalid, start by checking the givens rather than assuming the published puzzle is broken. Confirm every clue in the four shared 3x3 boxes, then check whether any digit has been typed into the wrong cell of a corner grid. Imported strings deserve the same care: make sure you used the right format, because a 369-cell active string and a 441-position full-board string are not interchangeable.
If the solver says a puzzle has more than one solution, the clues may be insufficient. That sometimes happens with hand-made puzzles or grids copied from images where one given was missed. Add the missing clue if you can identify it, or use the result as a warning that the puzzle may not be logically fair. A high-quality Samurai Sudoku should force a single final grid without guessing.
Samurai Sudoku Solver FAQ
Can this solve any Samurai Sudoku?
It is built for the standard five-grid Gattai-5 layout with four overlapping 3x3 boxes. It is not intended for every possible multi-Sudoku layout, such as huge custom Gattai puzzles with more than five grids.
How many cells are in Samurai Sudoku?
The full board is drawn as 21x21, which means 441 positions. Only 369 positions are active puzzle cells. The remaining positions are inactive spaces outside the five overlapping Sudoku grids.
Can I paste a puzzle string?
Yes. Paste either 369 active cells or a full 441-position board string. Use digits 1-9 for clues and 0 or dots for blanks. If the import looks wrong, check whether your source exported active cells or full-board positions.
Does the solver check overlapping grids?
Yes. Shared cells are checked against every 9x9 grid they belong to. A placement must be valid in the corner grid and the centre grid at the same time.
Can it tell me if my puzzle has no solution?
Yes. If the givens contradict the Samurai Sudoku rules, the solver can report that the puzzle is invalid or cannot be solved. Recheck copied clues, especially in overlap boxes, before assuming the original puzzle is broken.
Can it tell me if there is more than one solution?
When the search allows it, the solver reports whether the puzzle appears to be unique. A well-made Samurai Sudoku should have one solution; multiple solutions usually mean the puzzle needs more clues or one clue was copied incorrectly.