Sudoku: the rules, the levels, and the techniques that solve them

The rules

• The grid: nine rows and nine columns, cut into nine boxes of three by three. Some digits are given; the rest are yours to find.
• The goal: fill every empty cell so that each row, each column and each box holds the digits 1 to 9 once each.
• One solution: every puzzle here has exactly one solution, so it can always be found by logic alone.

The six levels

Puzzles are made in the game and graded by the techniques a person needs to solve them:

Choose the level on the home screen; your choice is kept for the next game.

The techniques

Naked single

A cell where only one digit can go: every other digit is already in its row, column or box.

Hidden single

A digit that can go in only one cell of a row, column or box, even if that cell could take other digits.

Naked pair

Two cells of a row, column or box that can only take the same two digits: those digits go nowhere else in it.

Pointing

When a digit's places in a box all lie in one row (or column), it cannot go anywhere else in that row (or column).

Claiming

The other way round: when a digit's places in a row (or column) all lie in one box, it cannot go anywhere else in that box.

Hidden pair

Two digits that can go in only the same two cells of a row, column or box: those cells can take nothing else.

Naked triple

Three cells of a row, column or box with only three digits between them: those digits go nowhere else in it.

X-wing

A digit that can go in only two cells in each of two rows, and those cells lie in the same two columns: the digit is in one of each pair, so it goes nowhere else in those columns (and the same with rows and columns swapped).

Swordfish

The X-wing with three rows and three columns.

XY-wing

A cell with two candidates, A and B, that sees one cell with A and C and another with B and C: whichever it is, one of those two is C, so a cell that sees both of them cannot be C.

Pencil marks, hints and mistakes

• Notes turns pencil marks on: the digits you press go in small, as candidates. Writing a digit takes its pencil marks off the cell's row, column and box for you.
• Fill notes writes every candidate in every empty cell at once (the digits that can still go there as the grid stands), so you can start straight on the pairs; Undo takes it back in one step.
• Hint fills one cell with its digit: one you could have found now, when there is one, and says why in a line over the grid (the only digit that fits the cell, or the only place left for the digit in its row, column or box).
• Show mistakes (on the home screen) shows a wrong digit in red as soon as it is written. Two equal digits in a row, column or box are always shown.
• Undo takes moves back, as many as you like; Erase clears a cell.
• Mistake limit (in Settings, off unless you turn it on) ends a game at the third mistake, as in many Sudoku apps: you get a Second chance (the wrong digit taken back, play on), or start the puzzle again.
• Digits: large (in Settings) draws the digits and pencil marks bigger and bolder, for easier reading. Settings → Theme has a night colour scheme too, with a dark grid and light digits.

The daily puzzle

Every day there is one daily puzzle, at medium, the same for everyone in the world. Your best time on it is kept, and the home screen ticks it once you have solved it. A new puzzle comes at midnight UTC. "Share" sends your time with a link that opens the same puzzle.

The calendar button beside "Daily puzzle" shows the month: every day whose puzzle you have solved has a crown, and the days in a row with a crown are your daily streak. A day you missed can still be played from the calendar, from October 2026 on, and earns its crown too.

In Halloween week, 26 October to 1 November 2026, each day's crown is a pumpkin. The first pumpkin unlocks the Halloween theme (a night colour scheme) in Settings → Theme, and it stays yours after the week is over.

In Christmas week, 21 to 27 December 2026, each day's crown is a present. The first present unlocks the Christmas theme (an evergreen colour scheme) in the same place, and it stays yours too.

Achievements and statistics

There are 20 achievements to collect: from your first win to a hundred, a 6×6 puzzle, each level up to master, wins in a row, the daily puzzle, an easy puzzle in five minutes or less and a hard one in fifteen, wins without a mistake, without a hint, or a hard puzzle without a single pencil mark, three pumpkins in Halloween week and three presents in Christmas week. "Achievements" on the home screen lists them all. They are kept on your device.

"Statistics", beside it, shows your games played and won, your win rate, your current and longest streak, and your best time: for all your games together, or for each level and the daily puzzle apart.

Otto, an owl with round glasses and a pencil, hops in beside the box at the big moments: a puzzle solved (more for one without a mistake or at expert or master), a new best time, the daily puzzle, a first win at a level, and a word of comfort at the mistake limit. Never while you play, and a little less often after the first few times; tap him to send him off, or turn him off in Settings ("Otto: on/off").

Tips for solving faster

• Scan digit by digit. Take the digit that appears most, and look for the boxes still missing it: often only one cell is left.
• Look at the crowded rows first. A row, column or box with seven or eight digits is quick to finish.
• Use pencil marks on the hard ones. Mark the candidates of a box or row, then look for pairs.
• Never guess. Every puzzle here can be solved by logic; a guess that turns out wrong costs more time than looking again.
• Tap a digit you have placed to see all its places lit up: the gaps show where it is missing.

Controls

Tap a cell, then a digit. On a computer, type the digits, move with the arrow keys, Backspace to erase, N for notes, A to fill all notes, H for a hint, Ctrl+Z to undo and Esc for the menu (the game is kept).

Questions and answers

Is Sudoku free?

Yes. It is free to play, with ads in the browser version. There is nothing to buy.

Do I need to download anything?

No. It plays in any modern browser on a phone, tablet or computer. If you like, you can install it as an app: on Android and in Chrome on a computer, choose "Install app" on the home screen; on an iPhone or iPad, use Safari's "Add to Home Screen". Once installed, it starts even without a connection.

Is my game saved?

Yes. The game in hand and your records are kept on your device, without an account. Leave in the middle of a puzzle and "Continue" on the home screen picks it up again.

Will I run out of puzzles?

No. Every new game makes a new puzzle: there are more than anyone could play.

Which languages does it speak?

41, from English, Chinese, Hindi and Spanish to Turkish, Swahili and Vietnamese. It starts in your browser's language when it has it, and you can change it on the home screen.

Something is wrong, or I have an idea

Write to support@brookmoon.com.