Sudoku
Fill the 9×9 grid so every row, column, and box holds 1–9 once.
How to play Sudoku
Fill every empty cell so each row, column, and 3×3 box contains the digits 1–9 with no repeats. Click a cell, then tap a number on the pad (or press 1–9); use Notes to pencil in candidates, Erase to clear, Undo to step back, and Hint if you're stuck. Selecting a cell highlights its row, column, box, and every matching number; conflicts turn red. Pick Easy, Medium, or Hard, and solve it as fast as you can.
How to solve Sudoku
Every proper Sudoku has exactly one solution reachable by pure logic. You never have to guess. Start by scanning, sometimes called cross-hatching. Pick a digit, say 5, and look at where the fives already sit. Each five rules out its whole row, its whole column and its 3x3 box, so within a given box you can often find just one empty cell where a five is still allowed. Place it, then repeat for the next digit. Selecting a cell in this version highlights its row, column, box and every matching number, which makes scanning far quicker, and any digit that breaks a rule turns red so mistakes surface immediately.
Where Sudoku came from
The modern puzzle was designed by Howard Garns, a retired American architect, and first printed in 1979 by Dell Magazines as Number Place. It found its true home in Japan, where the publisher Nikoli gave it the name sudoku, a contraction meaning roughly that the digits must stay single. The worldwide craze arrived in 2004, when Wayne Gould wrote a program to generate puzzles and persuaded The Times of London to print them. The nine-by-nine grid ultimately descends from the Latin squares studied by the mathematician Leonhard Euler in the 1700s; Sudoku's extra 3x3-box rule is what turns it into a deduction puzzle.
Solving techniques that always work
When plain scanning stalls, switch to reasoning about the candidates left in each cell.
- Naked single: a cell where only one digit remains possible after eliminating everything in its row, column and box. Fill it in.
- Hidden single: a digit that can legally go in only one cell of a row, column or box, even if that cell still shows other candidates.
- Use the Notes tool to pencil every candidate into a cell, then let pairs and triples reveal themselves.
- Naked pair: two cells in the same unit sharing the same two candidates lock those digits, so you can strike them from every other cell in that unit.
- Re-scan after each placement; one new digit often triggers a chain of forced moves.
Pairs, pointing, and boxes that talk to rows
Medium and Hard grids are built to survive plain scanning, and this is where candidate logic earns its keep. The workhorse is the pointing pair. When the only cells that can hold a digit inside a box fall on a single row or column, that digit is locked to the line: suppose the top-left box still needs a 7, and the only two cells that can take one sit in the box's bottom row. Wherever the 7 ends up, it lives on that row, so you can erase 7 from that row's cells in the two neighboring boxes, and one of those erasures frequently collapses a cell down to a single candidate. The same logic runs in reverse, called box-line reduction: if every candidate for a digit on a row falls inside one box, the box claims the digit and its other six cells drop it. Hidden pairs are the sneaky cousin of naked pairs: two digits whose candidates appear in only the same two cells of a unit. Those cells are spoken for, so every other pencil mark inside them can be wiped, and what remains is often a naked single next door. On the hardest grids, hunt for an X-Wing: a digit whose only candidates in two different rows sit in the same two columns, forming a rectangle. The digit must occupy opposite corners of it, which removes that digit from everywhere else in both columns.
The habits of fast solvers
The most common mistake is guessing when stuck. Because the solution is unique, a guess that happens to be wrong silently corrupts the whole grid, and the error may not surface for many moves. If you cannot find a move, you have simply missed one: change digits and scan again, or use Hint. The second killer is stale notes: a pencil mark you forgot to erase will eventually convince you a cell has two options when it really has one, so after every placement, strip that digit from the notes of every cell it sees. Fast solvers also work in sweeps rather than staring: run the digits 1 through 9 in order, place whatever is forced, and let the matching-number highlight do the looking for you. Speed comes from rhythm, not rush; the clock rewards the player who never needs Undo, and cells turning red is a safety net you should almost never touch.
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Sudoku: frequently asked questions
How is the puzzle generated?
Each game builds a complete solved grid by backtracking and then removes cells, so you get a fresh medium puzzle every time.
Is my best time saved?
Yes. Your fastest solve time is stored in your browser and shown as your Best. Log in to put it on the leaderboard.
Do I ever have to guess in Sudoku?
No. A properly made Sudoku has a single solution that logic alone can reach. If you feel forced to guess, it means there is a deduction you have missed. Scan a different digit or pencil in candidates and a legal move will appear.
What is the single most useful technique for beginners?
Scanning, also called cross-hatching. Take one digit at a time, see which rows, columns and boxes already contain it, and look for a box where only one empty cell can legally hold that digit. It solves a surprising amount of any easy or medium puzzle.
What is the difference between a naked single and a hidden single?
A naked single is a cell that has only one candidate left once you remove everything seen in its row, column and box. A hidden single is a digit that fits in just one cell of a unit even though that cell might still list other candidates. Both give you a certain placement.
How should I use the Notes feature?
Pencil the possible digits into each empty cell, then look for patterns: a cell down to one note, a digit that appears as a note in only one cell of a unit, or two cells sharing an identical pair of notes. Notes turn vague scanning into precise elimination.
Why did my grid suddenly become unsolvable?
Almost always an earlier entry was wrong. Because the puzzle has one solution, a single bad digit eventually leaves a cell with no legal option. Watch for cells that turn red, and use Undo to step back to where the contradiction began.
What actually changes between Easy, Medium, and Hard?
Harder puzzles start you with fewer given digits and are built so plain scanning runs dry sooner, forcing pencil-mark techniques like pairs and pointing. The rules never change, and every grid still has exactly one solution reachable by logic alone.
Is Sudoku free to play?
Yes. Sudoku is completely free on Inkplay, with no download, no sign-up, and no in-game purchases. Open the page and play instantly on desktop or mobile.
Can I play Sudoku unblocked?
Yes. Sudoku runs entirely in your browser with no download, install, or plugins, so it loads instantly and keeps working even on school or work networks where bigger game sites get blocked.
More games to play
In the mood for more? Try 2048, Memory Match, Lights Out, Minesweeper, or browse all puzzle games and the full games list on Inkplay. Sudoku also appears in Logic Puzzles.