How to play Sudoku
Sudoku is a logic puzzle played with the numbers 1 to 9 on a 9×9 grid divided into nine 3×3 boxes. Some cells start filled in, and your job is to fill in the rest. It takes no arithmetic and no luck, only careful looking, which is why so many people solve one over their morning coffee or on the way to work.
Play Sudoku online here for free, in your browser on a phone, tablet or computer, with no download or sign-up. There are 4,000 puzzles in four difficulties, from easy to expert, and every one has a single solution that logic alone will reach, without guessing.
Pencil in notes, undo any move, and ask for a Hint that shows the next step and the logic behind it. Every day brings a new puzzle of each difficulty, the same for every player. In Settings you choose whether numbers are checked as you play, whether three mistakes end the game and what the board highlights, and pick an ivory or a dark board. Sign in with just an email to keep your statistics, challenge a friend to the same puzzle and climb the season’s ranks. Your game is kept in this browser, and once the page has loaded it works without an internet connection too.
The goal
Fill every empty cell so that each row, each column and each 3×3 box holds every number from 1 to 9 exactly once. The puzzle is solved when the last cell is filled and every number is right.
The rules
There is really just one rule, and it applies to rows, columns and boxes alike:
- The grid. Its 81 cells form 9 rows and 9 columns, and thicker lines split it into nine 3×3 boxes. Every row, column and box has nine cells.
- Given numbers. The numbers a puzzle starts with stay put: you can’t change or erase them. The ones you add are blue.
- Each number once. No number may appear twice in the same row, column or box. As each has nine cells, each ends up with every number from 1 to 9.
- Logic, not maths. The numbers are just nine different symbols: you never add them up. Every number you place should follow from those already on the board.
Playing on this page
- Placing a number. Tap or click a cell, then a number under the board, or type it on the keyboard. The arrow keys move between cells.
- Notes. Turn on Notes, or press N, and the numbers you tap go into the cell as small notes: the candidates it could still take. Tap a note again to remove it.
- Erase and Undo. Erase clears the selected cell (Backspace or 0 on a keyboard). Undo takes your moves back one at a time, all the way to the start (Ctrl Z, or ⌘Z on a Mac).
- Hint. Shows the next step: a wrong number first, if there is one, otherwise the next number logic can place, with the reasoning that leads to it.
- Mistakes. A number that doesn’t match the solution turns red and counts as a mistake. After the third, the game ends: start the puzzle again or keep playing without the limit. Both the checking and the limit can be turned off in Settings.
- Pause. The timer starts with your first move. The button beside it pauses the game and hides the board until you come back.
Difficulty levels
There are 1,000 puzzles of each difficulty. What makes one hard is not how many numbers it starts with but the hardest technique you need to solve it; you’ll find them all in the next section.
- Easy (IQ 75–89). The two simplest techniques, “only one place” and “one number left”, are all you need, and they are quick to spot: 33 to 41 numbers are given. Just right for a first game or a short break.
- Medium (IQ 90–106). The same two techniques, but with only 25 to 34 numbers given, the cells take longer to find. New players start here.
- Hard (IQ 107–124). Those two alone will get stuck: somewhere you’ll need a pointing pair, box-line reduction, or a naked or hidden pair. Notes make all the difference here.
- Expert (IQ 125+). Needs a triple or an X-wing, the hardest techniques used here, even though it gives as many numbers as a hard puzzle. Still, every expert puzzle can be solved without guessing.
What the IQ means
The IQ badge rates a puzzle’s difficulty on an IQ-style scale, the one every game on the site uses, where 100 is average. To rate a puzzle, the site solves it the way a person would, one step at a time and always with the easiest technique that works, and adds up how hard each step is to spot. The hardest technique sets the difficulty; within it, the more effort a puzzle takes, the higher its IQ. It rates the puzzle, not you.
How to solve Sudoku: the techniques
Every step in a Sudoku follows one of a handful of patterns. The Hint calls them by these names and lights up the cells it means on the board. From the easiest:
Only one place (hidden single)
Take a row, column or box and a number it still lacks. If every other empty cell there already sees that number in its own row, column or box, the number has only one place left. Start with this: it solves most easy puzzles on its own.
One number left (naked single)
Now look at a single cell. If its row, column and box already hold eight different numbers between them, only the ninth can go there.
Pointing pair or triple
If, within a box, a number can only go in cells of one row, it must land in that row inside this box. So no other cell of that row, outside the box, can have it. The same goes for a column.
Box-line reduction
The other way round: if, in a row or column, a number can only go in cells of one box, you can rule it out of the rest of that box.
Naked pair
Two cells of the same row, column or box whose notes are the same two numbers. Those two numbers will fill those two cells, one each, so no other cell of that row, column or box can have either of them.
Hidden pair
Two numbers that fit in only the same two cells of a row, column or box. Those cells must hold them, so any other notes in the two cells can go.
Naked and hidden triples
The same ideas with three cells and three numbers. In a naked triple, each cell needn’t have all three numbers: what counts is that between them they can only be those three.
X-wing
Find a number that, in two different rows, can only go in the same two columns. The four cells form a rectangle, and the number must take two opposite corners. You don’t know which two yet, but either way each of the two columns gets it in one of those rows, so no other cell of those columns can have it. It works the same with rows and columns swapped.
Tips
Three tips for beginners
One number at a time
Pick a number that is already on the board a lot and visit each box that still lacks it. Copies of it in the neighbouring rows and columns rule cells out, and often only one is left.
Watch the keys
The small number on each key shows how many of that number are still to place. Start with the ones that have one or two left: they are the quickest to finish.
Don’t guess
If you’re not sure, leave the cell empty: one wrong number leads to more. There is always a step you can be certain of, and if you can’t find it, the Hint will show you where it is and why.
Tips for harder puzzles
From medium puzzles on, when scanning alone stops getting you anywhere:
Write every note
Before you look for pairs, triples or an X-wing, fill in the notes of every empty cell. These techniques work on notes, and one missing note can hide them. With “Clear notes when I place a number” on, the notes tidy themselves as you play.
Hunt for pairs
Run your eye along each row, column and box for two cells with the same two notes. A single pair often opens up a puzzle that seemed stuck.
Use the highlights
Select a cell with a number and every copy of it lights up, so you can see at a glance which boxes still lack it. Shading the row, column and box shows everything the selected cell sees.
Play without checking
For a real test, turn off “Check numbers as I play” in Settings: mistakes stay hidden, just as on paper. If you fill the grid and some numbers are wrong, the page tells you, and the Hint finds them.
Sudoku of the day
Every day there is a new puzzle of each difficulty, the same for every player, so you can race your friends. Pick Today’s puzzle in the New game menu to get today’s puzzle at the difficulty you are playing, and the label beside the title then reads “Daily” with the date. A new day starts at midnight where you are, and a day’s puzzle won’t come round again for 1,000 days.
Every puzzle also has a number, shown beside the timer. Enter it under “Choose a puzzle by number” to play the same grid again, or send it to friends so they can solve the very same puzzle.
Questions and answers
How do I fill in a number?
Tap or click an empty cell to select it, then tap a number under the board. On a computer you can also type the number, move between cells with the arrow keys and erase with Backspace. A number placed by mistake can be erased, or taken back with Undo.
How do notes work?
Notes are the small numbers you pencil into a cell to keep track of what it could still be. Turn on Notes with its switch or the N key, then tap numbers to add them to the selected cell, and tap them again to remove them. On a keyboard, Shift with a number does the opposite: a note while Notes are off, a number while they are on. When you place a number, it comes out of the notes in its row, column and box by itself, unless you turn that off in Settings.
What does the Hint do?
It shows the next step and explains it. If a number on the board is wrong, the Hint points that out first. Otherwise it finds the next number logic can place. On hard puzzles, getting there can take a few steps of ruling candidates out first: the Hint shows them one at a time and clears them from your notes, or you can skip straight to the number. The game counts how many hints you take.
What happens when I make a mistake?
While numbers are checked, as they are unless you change it, a number that doesn’t match the solution turns red and adds one to the mistakes beside the timer. The third mistake ends the game: you can start the same puzzle again or keep playing without the limit. You can turn off the limit, or the checking altogether, in Settings, but every wrong number still costs points, and a win with three or more pays no coins.
How is Sudoku scored?
Every number you get right scores the puzzle’s IQ minus 70: 5 points on the easiest puzzles, 75 on the hardest. A mistake or a hint takes one number’s worth back. Solve the puzzle within its par, 5, 10, 20 or 30 minutes from Easy to Expert, and half the points again are added; quicker, up to double. The total earns XP on your profile, a quarter as much for a puzzle you have solved before, and the win pays coins. After a win you also see what share of players you were faster than on that puzzle.
What is in the New game menu?
“Restart this puzzle” clears the board back to its given numbers. “Choose a puzzle by number” deals any of the 4,000 puzzles by its number. “Today’s puzzle” deals the puzzle of the day at your difficulty, and “Random puzzle” one of any difficulty. Below them, pick Easy, Medium, Hard or Expert for a random puzzle of that difficulty; each shows the IQ span its puzzles score in.
Does every Sudoku have only one solution? Will I need to guess?
A proper Sudoku has exactly one solution, and every puzzle here does. Each was also solved with the techniques on this page before it made the list, so you never need to guess or try a number to see what happens: there is always a next step that logic alone can find.
Are puzzles with fewer numbers harder?
Not necessarily. Difficulty comes from the techniques a puzzle needs, not from how many numbers it gives: here, expert puzzles start with 23 to 32 numbers, about as many as hard ones. The fewest numbers a Sudoku can start with and still have only one solution is 17, as a computer search proved in 2012.
Do I need to be good at maths?
No. Sudoku has nothing to do with sums: the numbers could just as well be nine letters or nine colours. All it takes is attention, patience and the techniques above, which anyone can learn.
Where does Sudoku come from?
It was most likely invented by Howard Garns, a retired American architect, and first appeared in 1979 in a US puzzle magazine under the name Number Place. In 1984 the Japanese publisher Nikoli took it to Japan and called it Sudoku, short for a phrase meaning “the digits must be single”. It went round the world in 2004 and 2005, after The Times in London began printing it.
Is it free? Do I need to sign up?
Yes, all 4,000 puzzles are free, and you can start straight away, with no sign-up and no download. Signing in is optional and only needed to keep your wins, XP and coins in your profile. An email address is enough: we send you a sign-in link.
Does it work offline? Can I install it as an app?
Yes. Once you have opened Sudoku, it keeps working without an internet connection, and your game is saved on the device with every move. To put it on your home screen like an app, choose Add to Home Screen in Safari’s Share menu on an iPhone or iPad, or Install app in Chrome’s menu on Android or a computer.
What happens to my game if I close the page?
Nothing is lost: your numbers, notes and time are saved in this browser with every move, so when you come back you carry on where you left off. A solved puzzle is finished, and your next visit deals a new one. Once you sign in, your solved puzzles, XP and coins are kept in your profile.
More games
If you like games where logic beats luck, try FreeCell: every card is face up from the first move, and nearly every deal can be won with a good plan. Mahjong is a calm game of clearing a board of tiles in pairs, and classic Klondike is the solitaire most people learn first.