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Sudoku

Fill every row, column and box with 1–9 — Easy, Medium and Hard

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How to Play

  • The puzzle: You get a 9×9 grid partially filled with numbers. Your job is to fill in all the empty cells.
  • The rule (only one rule): Every row, every column and every 3×3 box must contain each digit 1–9 exactly once. No repetition anywhere.
  • Why it's a puzzle: Because the given numbers constrain every empty cell — you must use logic to deduce which digit is the only one that fits each empty space.
  • How to start: Find a row, column or box that is nearly full — the missing digit must go in the only empty spot.
  • How to place a number: Tap a cell to select it, then tap 1–9 on the pad (or type on keyboard).
  • Notes mode: Not sure yet? Toggle Notes and tap candidates — they appear as tiny numbers inside the cell. Eliminate them as you solve nearby cells.
  • Win: Fill all 81 cells correctly.
  • Lose: 3 wrong placements = Game Over.
  • Hint / Undo: Use Hint to reveal one cell. Ctrl+Z or Undo reverses your last move.

Free Sudoku Online — Easy, Medium and Hard

Play unlimited Sudoku puzzles directly in your browser — no download, no account, no time limit. Choose from Easy, Medium or Hard difficulty. Use pencil notes to track candidates, get a hint when stuck, and undo any move. Works perfectly on desktop and mobile.

Features

3 Difficulty Levels

Easy, Medium and Hard — each generated fresh every game. Easy has 41+ given numbers, Hard has as few as 26.

Pencil Notes

Toggle Notes mode to write candidate numbers in any cell. Notes appear as a tiny 3x3 grid inside the cell.

Hint System

Stuck? Use a Hint to reveal the correct value for one empty cell and keep making progress.

Unlimited Undo

Made a mistake? Undo any number of moves to backtrack through your history without starting over.

Pauseable Timer

Track how long each puzzle takes. Click the timer to pause it — useful when you need a break mid-game.

Mobile Ready

Full-width number pad designed for tapping. Grid scales to any screen size using fluid CSS.

Who Plays Sudoku?

Puzzle EnthusiastsA daily Sudoku is a classic brain workout — satisfying, portable and available in any difficulty.
Break-Time PlayersEasy puzzles take 5–10 minutes — perfect for a coffee break without losing track of where you were.
Students & TeachersSudoku develops logical deduction and systematic thinking skills used in mathematics and coding.
Older AdultsRegular Sudoku practice is associated with maintaining cognitive sharpness and problem-solving ability.

Common Questions

How do you play Sudoku?
Fill every row, column and 3x3 box with the digits 1 through 9, using each digit exactly once per row, column and box. No number may appear twice in the same row, column or 3x3 box.
What is the difference between Easy, Medium and Hard?
Easy puzzles provide more given numbers (41+), so most cells can be solved by simple scanning — finding the one row, column or box that only allows one value. Medium requires some elimination logic. Hard puzzles provide fewer givens and often require advanced techniques like pointing pairs, X-wings or forcing chains.
What are pencil notes and when should I use them?
Pencil notes (or candidates) are small numbers written in a cell to track which digits are still possible. Use them on Medium and Hard puzzles when you cannot immediately determine the correct value. Toggle Notes mode, then click numbers to add or remove candidates from the selected cell.
Does every puzzle have a unique solution?
Yes — our generator creates puzzles with exactly one solution using a backtracking solver that verifies uniqueness. A well-formed Sudoku must have a unique solution by definition — if multiple solutions exist, it is not a valid puzzle.

Pro Tip

Start every puzzle with a "naked single" scan — go through every empty cell and check if only one digit is possible given the existing numbers in its row, column and box. On Easy puzzles, naked singles alone solve the entire board. On harder puzzles, after placing all naked singles, look for "hidden singles" — a digit that can only go in one place within a row, column or box, even if that cell has multiple candidates.

Did You Know?

6.67×10²¹
Valid Sudoku Grids
There are exactly 6,670,903,752,021,072,936,960 valid completed Sudoku grids — approximately 6.67 × 10²¹. Despite this enormous number, when symmetry reductions are applied, there are only 5,472,730,538 essentially different Sudoku grids.
17
Minimum Clues for Unique Solution
In 2012, Gary McGuire proved that the minimum number of given clues required to create a Sudoku with a unique solution is 17. No valid 16-clue puzzle has ever been found — and this was computationally proven after checking billions of configurations.
1979
First Published Sudoku
The modern Sudoku was invented by Howard Garns, an architect from Indiana, and first published in Dell Magazines in 1979 as "Number Place". It became popular in Japan in 1986 under the name "Sudoku" (meaning "single numbers") and went global in 2004.

Difficulty Reference

DifficultyGiven CluesTechniques NeededAvg. Solve Time
Easy41–50Naked singles only5–10 min
Medium32–40Naked & hidden singles10–20 min
Hard26–31Pointing pairs, X-wings20–45 min
Expert17–25Forcing chains, bifurcation45+ min

You May Also Ask

Can I pause the timer?
Yes — click the timer display to pause it. The board is not hidden when paused, so this is just a convenience feature to stop the clock without losing your progress. Click again to resume.
What happens after 3 mistakes?
After 3 incorrect number placements, the game ends with a Game Over overlay. A mistake is counted each time you place a number that is not the correct value for that cell in the solution. Click New Game to start again at any difficulty.
What is a "naked single" in Sudoku?
A naked single is a cell where only one digit is possible — every other digit 1–9 is already present in the cell's row, column or 3x3 box. It is the simplest Sudoku solving technique and the foundation of all Easy puzzles. Always find all naked singles before trying harder techniques.

Common Mistakes

Guessing instead of deducing
Randomly filling numbers without logical justification leads to contradictions that are hard to untangle, especially without pencil notes to track what changed.
Only place a number when you can logically prove it is the only possibility for that cell.
Ignoring columns and rows when checking boxes
Players often focus on 3x3 boxes but forget to cross-check rows and columns. A number can be valid in a box but invalid because it already appears in that row or column.
Always check all three constraints: row, column AND box before placing a number.
Not updating notes after placing a number
When you place a number in a cell, it eliminates that number as a candidate from all cells in the same row, column and box. Forgetting to update notes leads to false candidates and wrong placements.
After placing any number, immediately remove it from the notes of all related cells.