Sudoku

about this tool

How to play

Fill the 9 × 9 grid so that every row, every column and every 3 × 3 box holds the numbers 1 to 9 once each. The bold numbers are given and cannot be changed. Choose a cell by tapping it or moving with the arrow keys, then a number from the pad under the board or on your keyboard; the chosen cell is shaded and outlined. Tapping the same number again takes it out, and 0, Backspace or Delete erases a cell. The cells in the chosen cell's row, column and box are shaded, and every cell holding the same number is shaded a little darker, so you can see at a glance where a number can still go.

A number that clashes with another in its row, column or box is marked at once — in bold, the error colour and underlined, so it shows without colour too. Turn on Show mistakes and a number that does not match the solution is marked with a dotted underline instead, even before it clashes with anything. A number on the pad is struck through once all nine are placed with none of them clashing.

Notes

Switch Enter to notes, or press N to switch back and forth, and the numbers you choose are pencilled into the cell as small candidates instead. Entering a number clears the cell's notes, and also clears that number from the notes of every cell in its row, column and box, as most players would do by hand. Erasing a wrong number does not put those notes back; Undo does.

One solution, and real levels

Every puzzle here has exactly one solution. Each one starts as a full grid; numbers are taken out one at a time in a shuffled order, and a number stays out only while the puzzle still has a single solution, which is checked by solving it after every removal (for easy and medium, by solving it with singles alone, which is stricter). You never have to guess between two answers that both fit.

The level comes from how the puzzle can be solved:

| Level | Starts with | Can be solved by | | ------ | -------------- | ----------------- | | Easy | 38 numbers | singles alone | | Medium | 30 numbers | singles alone | | Hard | about 22 to 28 | more than singles |

A single is a cell where only one number fits, or a number that fits in only one cell of a row, column or box. Easy and medium puzzles fall to those alone, the easy ones with more to start from. A hard puzzle is cut down until no more numbers can come out, and is kept only if singles get stuck partway: from there it needs you to work with candidates — locked candidates, pairs, and often harder patterns than that.

Hints, undo and the clock

A hint puts the right number in the selected cell, replacing a wrong one if need be. If that cell is already right, or given, it goes to the empty cell with the fewest candidates instead, or to a wrong cell once none is empty. The number of hints is shown and goes into your result.

Undo takes back moves one at a time, all the way to the start, Clear included, and says what it took back and where; each puzzle keeps its own Undo while you play others. Undo puts back numbers and notes only: the clock and the hint count stay as they are, so a hint once seen is always counted. The clock starts at your first move and stops when the puzzle is solved; Clear takes every number and note off but leaves the clock running. Once a puzzle is solved, Undo is off, and Clear starts a fresh go with the clock and hints from zero — that one Clear cannot be undone.

Today's puzzle, and sharing

Today's puzzle gives everyone the same puzzle at each level on the same date, by your device's clock, read when you press the button. Any puzzle comes from a short code, so Copy link sends the very same one; Copy result shares how far you are, your time and your hints with that link. A link with only a level opens a new puzzle at that level. Nothing is stored or sent: every puzzle you move on from — to a new one, another level, the sample or today's — is listed under Put aside with its clock stopped, and Last game brings back the most recent, until you leave the page.

Keyboard and screen readers

The board is a grid: Tab into it and move with the arrow keys, which wrap round the edges. The top row of number keys works on any keyboard layout, U undoes and H gives a hint without leaving the board. A screen reader hears each cell's number (and whether it is given), or that it is empty and its notes, then whether it clashes or, with Show mistakes on, is not the solution, and what each move did and where — including which cell a new number clashes with, and how many numbers are wrong when you turn Show mistakes on. The number pad leaves focus on the board, so you can mix tapping and typing. Nothing on the board moves. For other puzzles, Minesweeper, 2048 and Word Guess share a game by link the same way.

questions

How do you play Sudoku?

Fill the 9 × 9 grid so that every row, every column and every 3 × 3 box holds the numbers 1 to 9 once each. Some numbers are given at the start and cannot be changed. Choose a cell, then a number, by tapping or with the keyboard; a number that clashes with another in its row, column or box is marked straight away.

Does every puzzle have one solution?

Yes. Each puzzle is made from a full grid by taking numbers out one at a time, and a number stays out only while the puzzle still has exactly one solution, which is checked by solving it every time. So there is never a point where you have to guess between two answers that both fit.

What makes a puzzle easy, medium or hard?

How it can be solved, not only how many numbers it starts with. Easy and medium puzzles can be finished with singles alone — a cell where only one number fits, or a number that fits in only one cell of a row, column or box — easy starting from 38 numbers and medium from 30. Hard puzzles start with fewer and cannot be finished by singles: they need you to reason with candidates: locked candidates, pairs or harder patterns.

Can I write notes, undo and get hints?

Yes. Switch to notes (or press N) to pencil small candidates into a cell; entering a number clears that number from the notes of every cell in its row, column and box. Undo takes back moves one at a time, Clear included (except the Clear that restarts a solved puzzle), and never lowers the clock or the hint count. A hint fills the selected cell with its right number, or the empty cell with the fewest candidates if the selected one is already right or given, and every hint is counted.

Is there a puzzle of the day, and does it save my game?

Today’s puzzle is the same for everyone at each level on the same date. Nothing is saved or sent: every puzzle you move on from is kept under Put aside, with its clock stopped, until you leave the page, and Copy link gives anyone the same puzzle. Copy result shares how far you are, your time and hints with that link.