Sudoku

What is Sudoku?

Rocket Sudoku is the classic 9×9 number puzzle, free in your browser on phone, tablet or computer. Every day brings a new daily puzzle that's the same for everyone, and you can generate as many fresh puzzles as you like at four levels: Easy, Medium, Hard and Expert. Every puzzle is built on the spot, has exactly one solution and never needs guessing.

Difficulty comes from the logic a puzzle needs, not just how many clues it shows. Easy puzzles fall to singles, Medium needs pointing pairs and naked or hidden pairs, Hard brings in X-Wings and XY-Wings, and Expert calls for colouring, XYZ-Wings and chains. Pencil-mark notes, automatic candidate clean-up, undo and redo, a timer you can pause and hints that explain their reasoning are all built in, and your game is saved if you close the tab.

How to play Sudoku

1. The goal: fill every empty cell so each row, each column and each 3×3 box contains the digits 1 to 9 exactly once.
2. Tap or click a cell (or move with the arrow keys), then tap a number on the pad or type 1 to 9. The cell's row, column and box are highlighted, along with every cell holding the same digit.
3. Turn on Notes (or press N) to pencil in candidates. When you place a digit, it's removed from the notes in that row, column and box automatically.
4. Wrong digits turn red and add to the mistake counter, but there's no game over. Use Undo, Erase or Hint whenever you need them.
5. The pad shows how many of each digit are still to place, and a digit disappears once all nine are on the board. Fill the last cell to finish and see your time.

Sudoku tips and strategy

• Start with hidden singles: pick a digit and look for a box where it can only go in one cell. Scanning across rows and columns of boxes finds most early moves.
• Fill in notes once the easy singles run out. Candidates make pairs, pointing pairs and X-Wings visible.
• If two cells in a unit can only hold the same two digits (a naked pair), those digits can't go anywhere else in that unit.
• When a digit in a box is confined to one row or column, it's ruled out from the rest of that row or column. This pointing pair is the key move in most Medium puzzles.
• Stuck? Hint fills one cell and names the technique behind it, so you learn the move instead of just getting the answer.
• Try Auto-candidates in Settings on Hard and Expert puzzles to skip the bookkeeping and focus on spotting patterns.

Sudoku rules

A Sudoku grid has 81 cells in nine rows, nine columns and nine 3×3 boxes. Some cells start filled in (the givens). You complete the rest so that every row, every column and every box contains each digit from 1 to 9 exactly once. No arithmetic is involved: the digits could just as well be letters or colours.

A proper Sudoku has exactly one solution, so it can always be solved by logic alone. Every puzzle here is checked for that before you see it: the generator only keeps a puzzle when step-by-step reasoning finishes the whole grid, which also proves the solution is unique.

The four difficulty levels and the techniques they need

Easy (36 to 40 clues) can be solved with naked singles, where a cell has only one possible digit, and hidden singles, where a digit has only one possible cell in a row, column or box. Medium needs at least one step beyond singles: pointing pairs, box/line reduction, or naked and hidden pairs and triples.

Hard requires a fish or wing pattern such as the X-Wing, Swordfish or XY-Wing, or a quad. Expert needs advanced techniques: simple colouring along chains of conjugate pairs, the XYZ-Wing, the Jellyfish or an XY-Chain. The game grades each puzzle by solving it the way a person would, always using the simplest technique available, so a Hard puzzle really does need a Hard move somewhere.

The daily Sudoku

The daily puzzle changes at midnight and is the same for every player, so you can compare times with friends and share your result. Its difficulty follows the week: Monday is Easy, Tuesday, Wednesday and Friday are Medium, Thursday and Saturday are Hard, and Sunday is Expert. Solve the daily on its day to build a streak, which you can see in Statistics along with your best and average times for each level.

Sudoku FAQ