Free X Sudoku Solver

X Sudoku Solver for Diagonal Sudoku Puzzles

Our X Sudoku Solver helps you solve diagonal Sudoku puzzles online, check difficult grids, and understand how the two extra diagonals change the logic. X Sudoku, also called Diagonal Sudoku or Sudoku X, uses the normal 9x9 Sudoku rules plus one important addition: both long diagonals must contain the digits 1 to 9 exactly once. That extra rule looks small, but it can completely change the solving path.

Use this tool when you have an X Sudoku from a book, newspaper, app, or handmade puzzle and want to verify the answer. You can type the givens into the grid, paste an 81-character puzzle string, reveal the full solution, step through one cell at a time, and check whether the puzzle has a unique solution. The solver runs in your browser, so the puzzle does not need to be uploaded anywhere.

How to Use the X Sudoku Solver

Start by entering the givens exactly as they appear in your puzzle. Leave blank cells empty, or use 0 and dots if you are importing a puzzle string. After the grid is entered, click Solve to fill the whole answer, or use Step mode if you want to reveal one logical placement at a time without spoiling the entire puzzle.

If you are checking a puzzle you created, pay attention to the uniqueness message. A diagonal Sudoku can be valid but still have multiple solutions if the clues are too weak. A good published X Sudoku should normally have one solution, and the solver can help you confirm that before you share or print it.

X Sudoku Rules Explained

X Sudoku begins with classic Sudoku. Each row must contain digits 1 to 9, each column must contain digits 1 to 9, and each 3x3 box must contain digits 1 to 9. The X rule adds two diagonal units. The main diagonal runs from the top-left cell to the bottom-right cell. The anti-diagonal runs from the top-right cell to the bottom-left cell. Each diagonal must also contain every digit once.

The centre cell belongs to both diagonals, so it is often strategically important. A digit placed on one diagonal can remove candidates from the rest of that diagonal, even when it has no direct row, column, or box relationship with those cells. This is what gives X Sudoku its distinctive feel: familiar rules, but with extra long-range pressure.

Why Diagonal Constraints Matter

In ordinary Sudoku, a digit affects its row, column, and box. In X Sudoku, a diagonal digit may affect cells across the entire board. That means a puzzle can be solved through deductions that do not exist in classic Sudoku. A candidate might look possible by row, column, and box, but disappear because the same digit is already forced on a diagonal.

The diagonals also create new hidden singles. If a digit can only appear in one cell along a diagonal, that placement is forced even if the row and box still look open. When you are stuck, scanning the two diagonals as full units is often the fastest way to restart progress.

Solving Strategy for X Sudoku

Begin with normal Sudoku basics: scan rows, columns, and boxes for singles and obvious placements. Then do a dedicated diagonal pass. List which digits are missing from each diagonal and compare them with the row, column, and box restrictions on diagonal cells. This often reveals candidates that are impossible only because of the X rule.

After every diagonal placement, update related candidates immediately. A number placed on a diagonal affects eight other cells along that diagonal, plus its row, column, and box. That is a lot of impact from one move, and missing it can make the puzzle feel harder than it is.

Using Step Mode as a Learning Tool

Step mode is useful when you want help without losing the whole puzzle. Instead of revealing the entire solution, ask for one cell and then study why it fits. Check the row, column, box, and diagonal constraints around that cell. If the cell is on a diagonal, ask whether the diagonal rule was the decisive factor.

This is one of the best ways to improve at diagonal Sudoku. Over time you will start seeing diagonal singles, diagonal pairs, and diagonal eliminations before using the solver. The tool becomes a coach rather than just an answer button.

Checking Handmade X Sudoku Puzzles

If you create your own puzzles, an X Sudoku solver is almost essential. It is easy to design a grid that looks elegant but has more than one solution. The extra diagonal rules can make a puzzle stronger, but they can also hide ambiguity if the clues are not balanced across the grid.

After entering your handmade puzzle, check that it solves and that the solution is unique. If there are multiple solutions, add or adjust clues near the weak area. If there is no solution, look for a copied digit, a repeated diagonal value, or a clue that conflicts with a row, column, box, or diagonal.

Importing and Exporting Puzzle Strings

An 81-character string is a compact way to store or share a 9x9 Sudoku puzzle. Each character represents one cell, reading left to right and top to bottom. Digits represent givens, while 0 or a dot represents a blank cell. This format is useful when copying puzzles between tools or saving a puzzle for later.

When importing an X Sudoku string, remember that the string itself may not say it is diagonal. The same givens can behave differently under classic Sudoku rules and X Sudoku rules. Always use the X Sudoku solver when the two diagonals are part of the puzzle.

Common X Sudoku Mistakes

The most common mistake is forgetting one of the diagonals. Solvers often check the visible row, column, and box, then place a digit that quietly duplicates a value on a diagonal. Another mistake is treating the centre cell like an ordinary square. Because it belongs to both diagonals, it can be more constrained than it first appears.

When the solver says a puzzle has no solution, check the diagonal clues carefully. A repeated digit on either diagonal is enough to break the puzzle. Also confirm that you did not import a standard Sudoku into the X Sudoku solver by accident, because some classic puzzles are not valid diagonal puzzles.

When to Use a Solver and When to Keep Solving

A solver is most helpful when it supports your reasoning. Use it to verify a finished puzzle, check whether a puzzle is valid, or get one hint when you are completely stuck. If you want the satisfaction of solving, try a fresh diagonal scan before revealing the full answer.

For learning, use the solver after you make a deduction. If your placement matches the solution, your logic was probably sound. If it does not, backtrack and find the first point where a row, column, box, or diagonal rule was missed.

Worked Example: How a Diagonal Clue Changes the Grid

Imagine the main diagonal is missing 2, 5, and 8. One empty diagonal cell already sees 2 in its row and 5 in its box, so the diagonal rule forces that cell to be 8. In a normal Sudoku, that same cell might still appear to have several options. The extra diagonal unit turns a vague candidate list into a confirmed placement.

That placement can then ripple outward. Once 8 is placed on the main diagonal, every other cell on that diagonal loses 8 as a candidate. The row, column, and 3x3 box also lose 8. This is why a single diagonal deduction can unlock several ordinary Sudoku deductions immediately afterwards.

Diagonal Candidate Patterns to Watch For

The most useful X Sudoku pattern is a diagonal hidden single: a digit missing from a diagonal can only fit in one diagonal cell. Diagonal pairs are also powerful. If two cells on the same diagonal can only contain the same two digits, those digits can be removed from every other cell on that diagonal.

Look for intersections between diagonals and boxes too. A diagonal passes through three 3x3 boxes in a visible way, and the centre box contains the shared centre cell. If a digit is restricted to diagonal cells inside one box, it may create eliminations both along the diagonal and inside the box. These overlaps are where X Sudoku often becomes easier than it first looks.

Classic Sudoku Solver vs X Sudoku Solver

Use the classic Sudoku solver when your puzzle only follows row, column, and 3x3 box rules. Use this X Sudoku solver when the puzzle explicitly marks both diagonals or is described as Sudoku X, X Sudoku, or Diagonal Sudoku. The same givens can lead to different results depending on which rule set is active.

If a puzzle fails here but works in a classic solver, it probably was not designed as an X Sudoku. If it works here but has multiple classic solutions, the diagonal rules may be exactly what make it unique. Choosing the right solver prevents false errors and confusing results.

Creating Better Diagonal Sudoku Puzzles

When designing X Sudoku, spread clues across the whole grid instead of loading only the diagonals. The diagonals are powerful, but they should interact with rows, columns, and boxes rather than carry the entire puzzle alone. A balanced puzzle gives solvers several entry points and avoids a grid that collapses too quickly.

After removing clues, test the puzzle repeatedly. Check that it has one solution, then solve it manually or use step mode to see whether the route feels fair. If the puzzle requires guessing, add a clue near the ambiguous area. If it solves too easily, remove a clue that was doing too much work.

X Sudoku Solver FAQ

Is X Sudoku the same as Diagonal Sudoku?

Yes. X Sudoku, Sudoku X, and Diagonal Sudoku usually mean the same variant: classic 9x9 Sudoku with both long diagonals also containing 1-9.

Can I paste a puzzle string?

Yes. Use an 81-character string with digits for givens and 0 or dots for blanks. The solver then applies diagonal Sudoku rules to the imported grid.

Why does a normal Sudoku fail in this solver?

A classic Sudoku may not satisfy the two diagonal rules. If the puzzle was not designed as X Sudoku, it can have no solution under diagonal constraints.

Does the solver check uniqueness?

Yes. Uniqueness checking helps identify whether the puzzle has exactly one answer or multiple valid completions.

Does the puzzle leave my browser?

No. The solving happens locally in your browser, so the grid does not need to be uploaded to a server.