Sudoku solver

How to use the sudoku solver

1. Enter the puzzle. Tap a square, then a number. Or press Scan a photo and take a picture of the grid; on a computer, pick a screenshot or an image file. It can be blank, half done or finished: digits you wrote in show in colour and get checked.
2. Check the board. After a scan, compare every digit with your photo and tap any square to fix it. Clashing digits turn red.
3. Press Check and solve. The solver makes sure the puzzle has exactly one solution and tells you how hard it is.
4. Take one step or all of them. Next step fills one square and explains it. Show the answer fills the rest. You can also keep solving yourself; Next step fixes a wrong digit first if you made one.

Tips for a good photo

• Get the whole 9×9 grid in the frame, as straight on as you can.
• Use even light. A shadow across the grid is the most common reason for a misread digit.
• Hold the page flat. A curled newspaper page bends the rows.
• Pencil marks are fine: the scanner skips small candidate digits. Your own full-size digits are read too, and checked against the solution.

Step by step: the techniques it uses

The solver works the way a person does, from the simplest move to the hardest, and every step it shows is one you could find by hand:

• Naked single: a square where eight digits are already in its row, column or box, so only one is left.
• Hidden single: a digit that fits in only one square of a row, column or box.
• Pointing pair and box/line reduction: when a digit's places in a box all sit in one row or column, it can't go anywhere else in that row or column (and the other way round).
• Naked and hidden pairs and triples: two or three squares that share two or three candidates between them, so those digits are ruled out everywhere else in the unit.
• X-Wing and XY-Wing: patterns across two rows or columns, or across three squares, that rule a digit out of squares they all see.
• Swordfish: the X-Wing idea stretched over three rows and three columns.

How hard is your sudoku?

After the check, the solver rates the puzzle by the hardest technique it needs:

These are the same levels as our free sudoku games, so if your newspaper puzzle rated Expert, you know which level to play next.

An AI sudoku solver that shows its work

Two different things happen on this page. An AI image model reads your photo: it finds the grid, tells printed clues from digits written in by hand or in an app's colour, and skips small pencil marks. That is the only job the AI does. Everything after that, the check, the difficulty rating and each step of the solution, comes from a solver that follows the sudoku rules square by square, the way you would on paper.

We tested the reader on 14 real photos from newspapers, puzzle books and phone apps: blank, half done and finished, with shadows, curled pages and pencil marks. It read 10 of the 12 standard sudokus correctly and turned down both pictures that were not standard 9×9 sudokus. On the two it misread, the check stopped it before solving, so it never gave a wrong answer.

Sudoku checker: are my answers right?

Stuck halfway and not sure whether an earlier digit was a mistake? Scan the page as it is. The solver works out the true solution from the printed clues alone, then compares your digits with it. Wrong ones are marked, and Next step corrects the first of them and explains the right digit. If you have finished, it tells you whether the whole grid is right.

Why it checks before it solves

A solver that answers anything you give it will happily solve a mistyped puzzle, and the answer won't match the one in your newspaper. This one checks first. Clashing digits, a puzzle with no solution and a puzzle with several solutions all stop it, with a note about what to look for. When it does give an answer, that answer is the only one your clues allow.

Questions

How do I solve a sudoku from a photo?

Press Scan a photo, take a picture of the puzzle (or pick one you already have), and wait a few seconds. The digits appear on the board next to your photo. Check them, fix any square by tapping it, then press Check and solve.

Is the photo scanner always right?

Not always, which is why it shows you the board before solving. An AI image reader picks out the digits; our own solver then checks them. If two digits clash, or the puzzle has no solution or more than one, it stops and tells you where to look instead of giving a wrong answer.

Do you keep my photo?

No. The photo is shrunk on your device and sent to an AI image reader only to pick out the digits; we don't store it. The one exception is if you report a problem with a scan and tick the box to attach the photo; then it is kept for up to 90 days so we can fix the reader. Puzzles you type in never leave your device.

Why does it say my puzzle has no solution?

At least one digit is wrong: a misread photo or a typo. A 1 read as a 7, or a 3 as an 8, is the usual cause. Compare the board with your puzzle square by square.

Why does it say there is more than one solution?

A clue is missing. A proper sudoku has exactly one solution, and that takes at least 17 clues. Look for an empty square that should hold a digit.

Can it solve newspaper and NYT sudoku?

Yes. Any standard 9×9 sudoku works, from a newspaper, a puzzle book, an app screenshot or a website. Type it in or scan it.

Can it check my answers?

Yes. Photograph the puzzle half done or finished. The scanner keeps the printed clues apart from the digits you wrote in, solves the puzzle from the clues alone, then marks each of your digits right or wrong. If they are all right, it says so.

Can ChatGPT or another AI solve a sudoku?

Chatbots can read a puzzle from a photo, but working it out is where they slip: they write the answer as text, one guess at a time, and on a hard grid they often put a digit where its row, column or box already has it. This solver uses AI only to read the photo; the solving is done by a program that follows the rules, so every answer it gives has been checked.

Can I use it from ChatGPT, Claude, Gemini or Grok?

Yes. Ask the assistant to read your photo into 81 digits, row by row, with 0 for an empty square, and to open https://gridpuzzles.app/sudoku-solver/?g= followed by those digits: the puzzle opens here, already checked. An assistant that can fetch web pages can also call https://gridpuzzles.app/api/sudoku/solve?g= with the same digits and get back the checked solution, the difficulty and every step with its technique.

Can I get just a hint?

Yes. Next step fills one square and says why, naming the technique that proves it, so you can carry on by yourself.

Is it free?

Yes. Typing puzzles in has no limit. Photo scans are limited to 10 a day for each visitor, because every scan costs us a little.

Want a fresh puzzle instead? Play sudoku online from easy to evil, try today's daily sudoku, or print sudoku puzzles with answers.