How to use the sudoku solver
Using the solver is simple. The empty 9×9 grid above works just like a normal sudoku board — tap a square, then tap a number on the pad (or use your keyboard). Enter all the "given" numbers from your puzzle, leaving the rest blank.
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Enter the givens
Tap each square and type the number that is already printed on your puzzle. If you make a mistake, tap the same number again to remove it, or use Backspace / Delete.
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Check for conflicts
The solver highlights duplicate numbers in the same row, column or box in red. Fix any conflicts before solving — they usually mean a number was entered in the wrong square.
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Tap Solve
Hit the Solve button. The algorithm fills in every empty square in milliseconds. Numbers you entered are shown on a yellow background; numbers the solver found appear in green.
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Read the result
You can now see the complete solution. Compare it with your half-finished puzzle to find where you went wrong, or use it to check your answer.
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Start over
Tap Clear to wipe the grid and enter a different puzzle. Or tap Example to load a sample puzzle and see the solver in action.
How does a sudoku solver work?
Our solver uses a technique called backtracking — the same core algorithm that powers most sudoku-solving software. Here is a plain-English explanation of what happens when you press Solve:
The algorithm scans the grid from top-left to bottom-right and finds the first empty square. It tries placing the number 1. If 1 does not violate any sudoku rule (no duplicate in the row, column or 3×3 box), the algorithm moves on to the next empty square and repeats. If no number from 1 to 9 fits, the algorithm "backtracks" — it goes back to the previous square and tries the next number. This process continues until every square is filled or every possibility has been exhausted.
Backtracking is a type of depth-first search. In the worst case it could explore billions of combinations, but because sudoku rules aggressively prune the search tree, even the hardest 9×9 puzzle is typically solved in a few milliseconds on a modern device.
More advanced solvers add extra logic on top of backtracking. Techniques like naked singles, hidden singles, naked pairs, pointing pairs, box/line reduction and X-Wing can fill in cells before the brute-force search even starts. Our solver keeps things fast and simple by relying on pure backtracking — it is more than fast enough for any newspaper or website puzzle.
Backtracking algorithm — step by step
If you want to understand the algorithm at a deeper level, here is exactly what our solver does under the hood:
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Find the next empty cell
Scan every cell from row 1 to row 9, left to right. The first cell with a value of 0 (empty) is the target.
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Try each candidate
For that cell, try the numbers 1 through 9 in order. For each number, check whether it already appears in the same row, the same column, or the same 3×3 box.
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Place a valid number
If the number does not conflict with any existing values, place it in the cell and move on to the next empty cell (back to step 1).
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Backtrack if stuck
If none of the numbers 1–9 are valid for the current cell, the puzzle is in a dead end. Remove the last placed number (set the cell back to 0) and return to the previous cell to try its next candidate.
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Repeat until solved
Keep going until there are no more empty cells — the grid is complete and you have found the solution. If the algorithm backtracks all the way to the very first cell and runs out of candidates, the puzzle has no solution.
When should you use a sudoku solver?
A solver is a tool — like a calculator for number crunching or a spell-checker for writing. Used wisely, it helps you learn. Here are some good reasons to use one:
- **Checking your answer** — you have finished a puzzle and want to make sure every number is correct before you look at the official answer key.
- **Finding where you went wrong** — you are stuck and suspect an earlier mistake. Comparing the solved grid with your progress reveals the error instantly.
- **Learning new techniques** — solve the puzzle yourself as far as you can, then use the solver to finish it. Work backwards from the solution to understand what logic you missed.
- **Verifying a handmade puzzle** — if you create sudoku puzzles yourself (or for a class), the solver confirms that the puzzle has a valid, unique solution.
- **Solving a printed puzzle** — got stuck on a printable sudoku worksheets sheet or a newspaper puzzle with no answer key? Type the givens in and get the solution.
How humans solve sudoku differently
A computer solver uses brute-force backtracking. Humans cannot check millions of combinations per second, so they rely on logical techniques instead. Understanding these techniques will make you a much better sudoku player:
- **Naked single** — a cell has only one candidate left after eliminating all numbers that appear in its row, column and box. This is the most basic technique and the one beginners learn first.
- **Hidden single** — a number can only go in one cell within a row, column or box, even though that cell may have multiple candidates. Look for the number that has no other home.
- **Naked pair / triple** — two cells in the same unit share the exact same two candidates. Those numbers are "locked" to those cells and can be eliminated from every other cell in the unit. The same idea extends to triples.
- **Pointing pair** — within a box, a candidate number appears only in one row (or column). That means the number cannot appear in that row (or column) outside the box.
- **Box/line reduction** — the opposite of a pointing pair. Within a row (or column), a candidate appears only inside one box, so it can be removed from the rest of that box.
- **X-Wing** — a candidate appears in exactly two cells in two different rows, and those cells share the same two columns. The candidate can be eliminated from all other cells in those columns.
Easy puzzles can be solved entirely with naked and hidden singles. Medium puzzles usually require pairs or pointing pairs. Hard and expert puzzles may need X-Wings or even more advanced chains. Try our easy sudoku and medium sudoku levels to practise the basics, then move up to hard sudoku and expert sudoku when you are ready for tougher logic.
How many clues does a sudoku need?
A valid 9×9 sudoku puzzle must have exactly one solution. Mathematicians have proven that the minimum number of givens (clues) required to guarantee a unique solution is 17. No 16-clue puzzle with a unique solution has ever been found — and in 2012, Gary McGuire's team at University College Dublin proved computationally that none exists.
In practice, most published puzzles have between 22 and 36 givens. An easy puzzle might show 36–40 numbers, leaving only about half the grid for the solver to fill in. A fiendishly hard puzzle might show just 22–24, requiring deep chains of logic to crack.
Our solver requires you to enter at least 17 numbers before it will attempt a solution. If you enter fewer, the puzzle almost certainly has multiple solutions, and the solver will ask you to add more clues.
Common mistakes when entering a puzzle
The most common reason the solver says "no solution found" is a data-entry error. Here are the usual culprits:
- **Transposed digits** — typing a 6 instead of a 9, or a 1 instead of a 7. Double-check any cells the solver highlights in red.
- **Wrong row or column** — placing a number one cell to the left or right of where it should be. Count carefully from the edge of the grid.
- **Missing a given** — skipping a number from the printed puzzle. If the solver produces a different answer from the one you expected, a missing given is often the cause.
- **Entering your own guesses** — only type the original given numbers, not numbers you have already filled in yourself. If any of your guesses were wrong, the solver will fail.
Sudoku solvers beyond the classic 9×9
The backtracking technique works for any constraint-based puzzle, not just the standard 9×9 grid. The same algorithm can solve:
- **4×4 and 6×6 grids** — smaller puzzles for younger children. Try our 4×4 sudoku and 6×6 sudoku games.
- **Killer Sudoku** — each cage must also add up to a target sum. Solving requires combining sudoku logic with arithmetic. Play Killer Sudoku.
- **Jigsaw Sudoku** — the boxes are irregular shapes instead of rectangles. The row and column rules still apply. Play Jigsaw Sudoku.
- **16×16 and 25×25 grids** — giant sudoku puzzles that use letters or hexadecimal digits alongside numbers. The same backtracking approach works, though solving time increases with grid size.
- **Samurai Sudoku** — five overlapping 9×9 grids that share corner boxes. Solvers handle these by treating shared cells as belonging to both grids simultaneously.
This solver is currently built for the standard 9×9 grid — the most popular size by far. We may add support for other variants in the future.
Want to play instead?
If you came here looking for a solver but actually want to practise, head over to our main game. We have easy sudoku, medium sudoku, hard sudoku and expert sudoku levels — plus Killer Sudoku and Jigsaw Sudoku variants for something different.
Every game includes hints, auto-notes, a check button and a friendly board designed for kids and adults alike. No signup, no ads in the way, and it works great on phones, tablets and laptops.
For screen-free play, check out our printable sudoku worksheets — free PDF worksheets you can download and print at home or school.
Frequently asked questions
Is this sudoku solver really free?
Yes — completely free, with no signup, no ads blocking the grid and no usage limits. Use it as often as you like.
How fast is the solver?
The solver uses a backtracking algorithm that typically finds the solution in under 10 milliseconds — essentially instant. Even the hardest known 9×9 puzzles are solved in a fraction of a second.
Can the solver handle any 9×9 sudoku puzzle?
Yes. If a valid solution exists, the solver will find it. If the puzzle has no solution (for example because of a data-entry error), it will tell you. If the puzzle has multiple solutions (too few givens), it will return one of them.
Why does the solver say "no solution found"?
Almost always this means a number was entered in the wrong cell. Check for conflicts highlighted in red, or clear the grid and re-enter the puzzle carefully.
What is the minimum number of clues needed?
A valid 9×9 sudoku with a unique solution requires at least 17 givens. This was proven mathematically in 2012. Our solver asks for at least 17 numbers before it will attempt to solve.
Does the solver work on mobile phones?
Yes. The grid and number pad are designed for touchscreens. Tap a cell, tap a number, and tap Solve — it works on any device with a modern browser.
Can I use this to check my own solution?
Absolutely. Enter only the original given numbers (not your own guesses), tap Solve, then compare the solver's output with your completed grid.
What algorithm does the solver use?
It uses backtracking — a depth-first search that tries numbers 1–9 in each empty cell and backtracks whenever it hits a constraint violation. This is the standard approach for sudoku solvers and is extremely fast for 9×9 grids.
Can it solve Killer Sudoku or Jigsaw Sudoku?
Not yet — this solver is built for the standard 9×9 grid. Killer and Jigsaw variants have additional constraints that require a modified algorithm. We may add variant solvers in the future.
Is using a solver cheating?
That depends on your goal. If you are learning, a solver is a great tool for checking answers and understanding where you went wrong. If you are competing or doing a timed challenge, using a solver would defeat the purpose. We recommend trying our Hint feature in the main game first — it reveals one cell at a time so you can keep solving the rest yourself.
Play sudoku online
Done solving? Try playing one of our free puzzles instead.