Sudoku Solver with Hints (scan from photo)

When a child is stuck on a sudoku from a puzzle book, type the grid or photograph the page, and the tool checks that the puzzle has exactly one solution, then shows the next cell that can be filled and the rule that fills it. It works for 9×9 puzzles and for the 4×4 and 6×6 grids in children's books, and it can print the puzzle with an answer page.

How to use

1. Choose the "Grid size" (4×4, 6×6 or 9×9). To try the tool first, press one of the "Example puzzles" such as "Easy" or "Kids 4×4"; pressing it again loads the next example.
2. To type a puzzle, choose "Puzzle digits" under "Typing enters", tap a cell and press a digit on the keypad or keyboard. Arrow keys move between cells and "Erase" or Backspace empties a cell.
3. To read a photo instead, press "Read from a photo", then "Choose a photo" or "Turn on camera" and "Take this picture". Drag the four yellow handles onto the outer corners of the grid if they are not already there, and press "Read digits".
4. Cells the reader is unsure about are marked ?. Compare each with the paper, tap it and press the right digit, or "Erase" if the cell is empty. Hints wait until no ? is left.
5. Read the message under the grid. It says whether the puzzle has exactly one solution, more than one (orange cells), or none (red cells), so a copying mistake shows up before anyone starts solving.
6. Press "Next cell". One cell is highlighted with the rows, columns or boxes that decide it, and a sentence explains the rule. When candidates must be ruled out first, those steps are listed and the removed candidates are struck out in red. Press "Fill this cell" to write it in.
7. Choose "My answers" to enter the child's own digits, then press "Check my answers" to mark wrong cells. "Show solution" fills every empty cell, and "Hide solution" takes it away again.
8. Use "Copy link" to share the puzzle, or "Print" and "Save as PDF" for a puzzle page and, with "Include an answer page" ticked, a second page with the solution.

How it works

• The solver combines constraint propagation (a cell with one candidate, or a digit with one place in a row, column or box, is filled at once) with backtracking on the cell with the fewest candidates. It keeps searching after the first solution; finding a second one means the puzzle is not unique.
• Hints use these techniques in this order: naked single, hidden single, pointing pair or triple, box-line reduction, naked pair, naked triple, hidden pair, X-wing. A hint always ends in exactly one filled cell. When none of them is enough, the tool says guessing would be needed and shows the cell with the fewest candidates from the solution.
• Photo reading finds the grid, straightens it with a perspective correction, decides empty or digit by the amount of ink in the middle of each cell, and recognises digits with an OCR engine that runs in the browser. The first reading downloads the engine (about 15 MB). Printed digits read well; handwriting and faint pencil less so.
• Photos are processed on this device and are not uploaded. The current grid and settings are kept in this browser only. A shared link contains only the puzzle digits.

How to solve a sudoku step by step: the rules the hints use

Every empty cell has candidates: the digits not yet used in its row, its column and its box. If only one candidate is left, that is a naked single. Often it is easier to look the other way round: pick a digit and a box (or row, or column) and ask where that digit can still go. If only one cell is left, that is a hidden single. Most easy and medium puzzles are solved with these two rules alone.

Harder puzzles need a step that removes candidates without filling a cell. Example: inside one box, all the places for 5 lie in the same row. The 5 of that box must be in that row, so no other cell of the row outside the box can be 5 (a pointing pair). Naked pairs work the same way inside one house: if two cells can only hold 2 and 7, those two digits are used up there. The hint lists such steps first and then names the cell they decide, so a parent can explain the reason, not only the answer.

Sudoku has no solution or more than one: finding the copying mistake

A puzzle from a book has exactly one solution. If the tool reports none, a digit was most often copied into the wrong cell or misread. A digit that appears twice in a row, column or box is shown in red straight away. Otherwise the message names a cell where no digit fits, a digit that has no place in a row, column or box, or a small set of cells that cannot all be right together; one of those is usually the mistake.

If the tool reports more than one solution, a given digit is probably missing. The orange cells are where two solutions differ, which shows the area to compare with the paper. The same check helps with a child's own work: in "My answers" mode, "Check my answers" marks the cells that do not match the solution.

Reading a sudoku from a photo: getting a clean scan

Hold the phone straight above the page so the grid fills most of the picture, in even light without the shadow of the phone or a hand. The grid is found as the largest connected set of lines; if the yellow handles land elsewhere, drag them onto the four outer corners, and use "Rotate photo 90°" when the page is sideways. The yellow overlay shows where each cell will be read.

Pencil marks in the corners of a cell are ignored, because only ink near the middle counts. A digit the engine is unsure of, an unexpected digit such as 8 in a 6×6 grid, or a faint mark gets a ?. Even when nothing is marked, compare the grid with the paper once, because a misread digit changes the solution.

Questions and answers

Can it solve a sudoku from a photo?

Yes. Choose a photo or use the camera, adjust the four corner handles, and press "Read digits". After you confirm the cells marked ?, the solution and the hints work the same as for a typed puzzle.

How do I get a hint for just the next step instead of the whole answer?

Press "Next cell". It reveals one cell and the reason for it, and "Fill this cell" writes it in. The full answer appears only when you press "Show solution".

Does it work for 4×4 and 6×6 kids' sudoku?

Yes. Choose 4×4 (four 2×2 boxes) or 6×6 (six boxes, 3 cells wide and 2 tall) under "Grid size". The hints use the same rules with the digits 1–4 or 1–6.

What does it mean when it says the puzzle has more than one solution?

The digits entered do not decide every cell. Usually a given digit was left out when copying; the orange cells show where solutions differ.

Why does the hint say guessing is needed?

Some very hard puzzles need techniques beyond X-wing, such as chains. At that point the tool shows one cell from the solution, the one with the fewest candidates, and the next hints continue from there.

What is a hidden single in sudoku?

A digit that has only one possible cell in a row, column or box, even though that cell may still have other candidates. The hint marks the other empty cells of the house with × and highlights the digits that block them.

Can I check my child's answers without showing the solution?

Yes. Choose "My answers", enter the child's digits and press "Check my answers". Wrong cells turn red and the rest of the solution stays hidden.

Can I print the puzzle with the answer?

Yes. "Print" and "Save as PDF" make an A4 or Letter page with the puzzle and, if "Include an answer page" is ticked, a second page with the solution, given digits in bold.