Free Sudoku Solver

Stuck on a puzzle? Enter your 9×9 grid by hand or upload a photo — Sudoku Knacker cracks any valid Sudoku in milliseconds, right in your browser. No sign-up, no downloads.

Click a cell, then type 1–9. Move with ←↑↓→, erase with Backspace. Or load the .

Scan a puzzle from an image

Upload an image Drop a photo or screenshot here — JPG, PNG, or WebP. We find the grid and read the digits for you.

Controls

Click a cell and type a digit, or upload a photo of your puzzle.

Everything runs locally in your browser — your puzzles and photos never leave your device.

How to Use the Sudoku Solver

Three steps from stuck to solved.

Enter your puzzle

Click any cell and type the given digits, or upload a photo of the grid and let the built-in OCR fill it in for you.

Press Solve

The solving engine checks your grid and computes the complete solution in a fraction of a second — even for the toughest expert puzzles.

Review the answer

Your original clues stay bold while the solution appears in blue. Prefer just a nudge? Use Hint to reveal one cell at a time.

Why Sudoku Knacker?

Instant solving engine

A optimized backtracking solver with constraint propagation cracks any valid 9×9 grid — usually in under 10 milliseconds.

Photo recognition

Snap the newspaper or screenshot an app. The OCR pipeline locates the grid, splits the 81 cells, and reads each digit.

Live error checking

Duplicate digits in a row, column, or box light up in red the moment you type them, so bad input never reaches the solver.

Smart hints

Don't want the full answer? Hint reveals a single correct cell — perfect for getting past one stubborn bottleneck.

Light & dark themes

A crisp dark mode for late-night puzzle sessions, following your system preference with a one-click override.

Private by design

No accounts, no tracking pixels on your data. All solving and image recognition happens on your own device.

How Photo Recognition Works

From snapshot to solved grid in four automatic steps.

Grid detection

The image is converted to black and white, and projection analysis finds the 10 horizontal and 10 vertical grid lines.

Cell extraction

Each of the 81 cells is isolated, grid lines are stripped away, and the digit's exact bounding box is located.

Digit recognition

Every cell containing ink is passed to the OCR engine, which is trained to read the digits 1 through 9.

Your review

The recognized grid loads into the interactive board with a detection preview, so you can fix any cell before solving.

For best results, use a straight-on photo with the full grid in frame, even lighting, and no glare. Screenshots of puzzle apps work great.

Frequently Asked Questions

Is the Sudoku Knacker solver really free?

Yes — completely free, with no usage limits and no account required. The site is supported by unobtrusive ads, which is why it can stay free for everyone.

Can it solve every Sudoku puzzle?

It solves any puzzle that has at least one valid solution, from newspaper easies to the hardest expert grids. If your input has no solution — usually a typo — the solver tells you instead of guessing.

Does the photo of my puzzle get uploaded anywhere?

No. Image recognition runs entirely in your browser using WebAssembly. Your photos, screenshots, and puzzle data never leave your device.

Does it work on phones and tablets?

Yes. The board is fully responsive and touch-friendly, with an on-screen number pad in addition to keyboard input.

What's the difference between Solve and Hint?

Solve fills in the entire solution at once. Hint reveals exactly one correct cell — either the one you selected or a smart pick — so you can keep solving on your own.

Can I use the solver while playing on your site?

Absolutely. If you get stuck on a puzzle in our Play mode, you can transcribe it here (or screenshot it and use photo recognition) and get a hint or the full solution.

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