Sudoku

Play Sudoku with notes, hints and difficulty levels.

FAQ

Are the puzzles randomly generated?

Yes. Every "New game" first builds a complete valid grid with randomized backtracking, then removes numbers for the chosen difficulty and runs a solver that counts solutions, so the puzzle only starts once it is confirmed to be unique.

What do the three difficulties change?

Easy, medium and hard remove about 40, 48 and 54 numbers respectively: the more numbers removed, the more steps of logic you need. All three keep a unique solution and are generated instantly in your browser.

How does notes mode work?

Press the "Notes" button next to the number pad, then tap 1–9 to record candidates in the current cell; tap the same digit again to remove it. Entering a real digit automatically clears matching notes from the same row, column and box.

What happens if I enter a wrong number?

Digits that disagree with the solution are highlighted in red and counted as mistakes. Use "Undo" to step back (the last 40 moves are kept), "Erase" to clear the current cell, or "Check" to see the current conflict count: and "New game" always gives you a fresh puzzle.

How to use

Pick a difficulty and start
Choose Easy, Medium or Expert and press New game to generate a puzzle with exactly one solution. The given numbers cannot be changed, and the timer starts right away.
Select a cell and enter a digit
Click the cell you want to fill first. Then tap 1 to 9 on the number pad or press the number keys. Cells sharing a row, column or box with your selection are highlighted.
Take notes on candidates
Turn on Notes mode and the digits you enter become small 3x3 pencil marks inside the cell. Use them to record possible candidates, then switch Notes off to enter a final answer.
Use hints and undo
When you are stuck, press Hint to fill one correct digit, or Undo to step back. A digit that contradicts the solution counts as a mistake and turns red so you can correct it early.
Fill every cell to finish
The puzzle is complete when every row, column and 3x3 box contains the digits 1 to 9 exactly once. The header tracks your time and mistakes, and Play again deals a fresh puzzle.

About this tool

Sudoku began as Number Place, designed in 1979 by the American architect Howard Garns and published in Dell magazines. The Japanese publisher Nikoli introduced it in 1986 under the name Sudoku, meaning single number, and it spread worldwide from there. A standard grid is 9 by 9. You must place 1 to 9 so that no digit repeats in any row, column or 3x3 box. Puzzles here are generated in your browser and checked to have a single solution.

It suits people who like quiet, deliberate thinking. There is no clock pressure and no reaction speed involved, and you can walk away mid-puzzle and return later. One or two puzzles a day help keep concentration sharp, and many players use them to unwind away from a busy feed. Beginners should start on Easy to learn how rows, columns and boxes constrain each other before trying Expert grids.

How it works

The grid descends from Latin squares, the arrangements Euler studied where every row and column contains each symbol once. Sudoku adds the stronger 3x3 box constraint, and that extra rule cuts the candidate pool hard. The two basic moves are the naked single and the hidden single. A naked single reads the cell: its row, column and box already hold eight digits. A hidden single reads the digit: only one legal home is left in a unit. Both are constraint propagation, because every digit you place shortens the candidate lists around it, and easy puzzles are simply chains of such steps.

A puzzle needs a floor on clues to stay unique. A 2012 exhaustive search proved that 16 clues can never pin down one solution, so 17 is the proven minimum. Puzzles here are generated and checked for a single solution. Difficulty is a ladder of techniques, not a head-count of givens. Easy boards fall to naked and hidden singles alone, while hard boards ask for notes, tentative assumptions and clean rollbacks. The length of the reasoning chain, not the number of hints, is what sets the difficulty.

When to use it

A daily focus ritual
No speed pressure and no luck: one fixed puzzle a day works like maintenance for attention. Notes and undo are always available, interruptions are harmless, and the habit suits early mornings or winding down.
Quiet metro minutes
Nothing to install and nothing to sign into: the page opens on a fresh grid. Bumping trains do not disturb pencil marks, one puzzle fills two or three stops, and an unfinished one costs nothing tomorrow.
Teaching kids the rules
One sentence covers the whole rule set: no repeats in any row, column or box. Children can check their own answers at once, and candidate notes turn invisible thinking into something they can point at.

Tips & tricks

Start with the last remaining digit: when a cell already sees eight different numbers in its row, column and box, only one value is left.
Then hunt for hidden singles: if a digit fits in only one cell of a row, column or box, place it there at once.
Write candidates as notes, and when two cells block each other, test one assumption and be ready to undo it quickly.

Things to watch

Guessing without notes
Candidates kept in your head collapse the moment two cells interact, and a coin flip becomes the only move. Pencil marks spread the options onto the grid, so assumptions and rollbacks stay honest.
Forgetting the 3x3 boxes
Beginners police rows and columns and let the box constraint slide, then wonder why a legal-looking digit is wrong. Sweep the box once more after every entry: the boxes are half of the difficulty.
Building on a mistake
One wrong digit quietly poisons a whole region, and every later deduction inherits it. Fix red-marked errors at once, because rolling back three steps beats pushing ahead on a broken board.