How to play Sudoku
Sudoku is a logic puzzle on a 9×9 grid, and it needs no arithmetic: the digits are just nine different symbols. Every puzzle here is freshly generated, has exactly one solution, and gets a little harder or easier depending on how your last solve went. Play the Classic grid, the Hyper variant with four extra boxes, or let Random choose for you.
The rules
- Fill every empty cell with a digit from 1 to 9.
- Each row and each column must contain every digit exactly once.
- Each of the nine 3×3 boxes, marked by thick lines, must contain every digit exactly once.
- Hyper mode only: four extra tinted 3×3 regions must also contain 1 to 9. They cover rows 2–4 and 6–8 crossed with columns 2–4 and 6–8, overlapping the normal boxes.
Click a cell, then press a number button or type a digit. Arrow keys move the selection and Backspace clears a cell. A wrong digit turns red and adds to the Errors counter, and once all nine copies of a digit are correctly placed, its button is crossed out.
Getting started
Pick one digit and check every box. Say 7 already appears in row 1 and row 3, both within the top band of boxes. In the top-right box, the 7 must then go in row 2. If two of that box's three cells in row 2 are filled, only one spot remains, for example row 2, column 8. Place it and move on to the next digit.
Then look for nearly full units. If row 5 is missing only 4 and 9, and column 2 already holds a 9, then row 5, column 2 must be the 4 and the other gap is the 9. Selecting a cell highlights its row, column and box, and selecting a filled cell highlights every matching digit, which speeds up these scans.
Strategies and tips
- Cross-hatching. Trace a digit's existing copies along their rows and columns. Inside a box, every cell on those lines is ruled out; if one cell survives, that is where the digit goes.
- Naked singles. For one empty cell, list the digits already in its row, column and box. If eight are excluded, the ninth is the answer.
- Pointing pairs. If within a box a digit can only sit in cells sharing one row, it cannot appear elsewhere in that row outside the box. This often unlocks the neighbouring box.
- Naked pairs. When two cells in one unit can each only be the same two digits, those digits are claimed and can be removed from the rest of that row, column or box.
- Use the hyper regions. In Hyper mode the tinted regions are extra units to scan. They often settle cells near the centre early.
- Note candidates on paper. This page has no pencil-mark mode, so on sparse grids jot candidates for tricky cells on scrap paper rather than guessing. Guesses cost errors, and errors affect your rating.
How the difficulty adapts
Your rating decides how many starting clues you get. In Classic mode the generator aims for 50 clues at rating 1000 and removes one clue per 80 points, down to 20; the five difficulty buttons jump to 50, 42, 34, 29 or 25 clues. Hyper starts from 40 clues and moves one clue per 100 points, with levels from 40 down to 20. One rating is shared by both modes.
A clean solve, meaning three errors or fewer within the target time, earns +80 in Classic and +100 in Hyper: one clue fewer next time. More errors or running over time costs 160 (200 in Hyper), and each hint costs another 80 (100 in Hyper). The target time ranges from about 6 minutes on the fullest grids to 30 minutes on the sparsest, and the timer pauses while you are in another tab.
Why bother? A puzzle that stays slightly hard keeps you thinking, while one you solve on autopilot does little for you. Keeping the challenge just ahead of your skill is what turns play into training. Curious? Read what research says about puzzles and brain health.
A short history of Sudoku
Grids in which each symbol appears once per row and column go back to the Latin squares studied by mathematicians such as Leonhard Euler in the 18th century. The modern puzzle with 3×3 boxes appeared in American Dell puzzle magazines in the late 1970s as Number Place. In the 1980s the Japanese publisher Nikoli adopted it and named it Sudoku, short for a phrase meaning that each digit must appear only once. It became a worldwide craze around 2005 after British newspapers began printing it daily, and variants with extra regions, like Hyper, followed.
Frequently asked questions
Does every puzzle have a unique solution?
Yes. The generator only removes a clue if exactly one solution remains, so you never need to guess.
What does the Hint button do?
Select an empty or wrong cell and press Hint to reveal its correct digit in purple. Each hint lowers your rating, so the next puzzle gets roughly one extra clue per hint.
What does Random mode do?
Random flips a coin for every new puzzle, giving you Classic or Hyper. The subtitle above the grid tells you which one you are playing.
Is my rating saved?
When you are signed in, your rating and chosen mode are saved to your account. Without an account, the rating resets when you reload the page. You can always jump to a level with the difficulty buttons.