About Sudoku
Play Sudoku free online with multiple difficulty levels. Fill the 9x9 grid so every row, column, and box contains 1-9. Notes mode and error checking included.
How to use
- Pick a difficulty before starting: Easy begins with 40-50 given clues (solvable with simple scanning), Medium gives 30-35 clues and requires basic candidate elimination, and Hard exposes 25-29 clues and forces you to use intermediate techniques like hidden singles, naked pairs, and pointing pairs.
- Click any empty cell to select it. Selected cells highlight, and the matching row, column, and 3x3 box also subtly highlight so you can scan for conflicts at a glance. Cells with thick borders contain givens — the puzzle's pre-filled clues — and cannot be changed.
- Enter a digit using the on-screen number pad or your physical keyboard (keys 1-9). The game flags conflicts immediately by highlighting any duplicate in the same row, column, or 3x3 box. Press Delete, Backspace, or 0 to clear a cell if you want to revise.
- Toggle Pencil Mode (the button in the controls panel) to enter small candidate numbers instead of a final answer. For each empty cell, pencil in every digit 1-9 that doesn't conflict with its row, column, or box. As you solve surrounding cells, eliminate candidates until only one possibility remains — this technique is called a naked single.
- Look for hidden singles: a digit that can only fit in one cell of a given row, column, or box even if that cell still has multiple candidates penciled in. Hidden singles are the workhorse technique for Medium puzzles. For Hard puzzles, learn naked pairs (two cells in a unit that share the same two candidates eliminate those candidates from the rest of the unit) and pointing pairs.
- Use the Hint button (you start with 3 hints per game) when you're truly stuck — it reveals the correct value for the easiest unsolved cell. The Undo button takes back the last placement, useful for testing a hypothesis without committing to it.
- Fill every row, column, and 3x3 box with the digits 1 through 9 (each appearing exactly once) to win. Every well-formed Sudoku has exactly one unique solution reachable through pure logic — if you ever feel forced to guess, you've missed a deduction somewhere. Press the timer-paused difficulty buttons any time to start a new puzzle.
Frequently asked questions
How do you play Sudoku?
Sudoku is a logic-based number placement puzzle played on a 9x9 grid divided into nine 3x3 boxes. Some cells start with given numbers (clues), and your goal is to fill every empty cell with a digit from 1 to 9 such that each row, each column, and each 3x3 box contains all nine digits exactly once. No math is required — Sudoku is purely a game of logical deduction. You examine the existing numbers in a cell's row, column, and box to determine which digits are still possible. Every well-formed Sudoku has exactly one unique solution.
What difficulty levels are available?
Multiple difficulty levels are available ranging from Easy to Expert. Easy puzzles have many given numbers and can be solved using only naked singles (cells where only one number is possible). Medium puzzles require hidden singles and basic elimination. Hard puzzles demand techniques like naked pairs, pointing pairs, and box-line reduction. Expert puzzles may require advanced strategies such as X-Wings, Swordfish, XY-Wings, and chains. Each level generates fresh puzzles, so you can practice at any difficulty indefinitely.
What is the best strategy for solving Sudoku?
Start by scanning each row, column, and box to find cells where only one number is possible — these are called naked singles and are the easiest deductions. Next, look for hidden singles: a digit that can only go in one cell within a particular row, column, or box, even if that cell has multiple candidates. Use pencil marks to track candidates in harder puzzles. As you progress, learn pattern-based techniques like naked pairs (two cells in a unit that share the same two candidates, eliminating those candidates from other cells in the unit) and X-Wings (a pattern across rows and columns that eliminates candidates).
Do you ever need to guess in Sudoku?
No — a properly constructed Sudoku puzzle can always be solved through pure logical deduction without any guessing. If you feel stuck and think you need to guess, it means there is a logical technique you have not yet applied. However, some very difficult puzzles require advanced techniques (like chains, coloring, or uniqueness-based methods) that can feel like guessing if you are not familiar with them. Learning these techniques in order of difficulty is the path to becoming a confident Sudoku solver at any level.
Where did Sudoku originate?
The modern Sudoku puzzle was created by Howard Garns, an American architect, and first published in 1979 in Dell Magazines under the name "Number Place." The puzzle gained massive popularity in Japan in the 1980s when publisher Nikoli adopted it under the name "Sudoku" (meaning "single number" or "digits must be single"). It became a worldwide phenomenon in 2004-2005 when newspapers globally began publishing Sudoku puzzles daily. Despite its Japanese name, the core concept is American. The underlying mathematics relates to Latin squares, studied by the Swiss mathematician Leonhard Euler in the 18th century.
What are pencil marks and why are they important?
Pencil marks (also called candidates or notes) are small numbers written in a cell to track which digits are still possible for that cell. For example, if a cell could be 2, 5, or 8 based on what is already placed in its row, column, and box, you write all three as pencil marks. As you solve other cells, you eliminate candidates — when only one candidate remains, that is the cell's value. Using pencil marks is essential for any puzzle beyond Easy difficulty, because your working memory cannot reliably track all the possibilities across 81 cells simultaneously.
How many valid Sudoku puzzles exist?
There are approximately 6.67 sextillion (6.67 x 10^21) valid completed Sudoku grids, a number calculated by Bertram Felgenhauer and Frazer Jarvis in 2005. When accounting for symmetries (rotations, reflections, digit relabeling, and row/column permutations), this reduces to about 5.47 billion essentially different grids. The number of valid Sudoku puzzles (partially filled grids with a unique solution) is much larger and depends on how many clues are given. The minimum number of clues needed for a unique solution is 17 — no valid Sudoku with 16 or fewer clues has been found.
Is this game free?
Part of ToolFluency’s library of free online tools for Games & Fun. No account needed, no data leaves your device.