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Classic Sudoku
Free sudoku, Easy to Fiendish. Never a guess.
How to play classic sudoku
Fill the grid so that every row, every column and every 3x3 box holds the digits 1 to 9 exactly once. That is the whole rule set, and it is enough for puzzles that range from a five-minute warm-up to a long fight.
Our classic puzzles are graded by the hardest technique the solve needs. Easy uses only singles. Medium adds pointing pairs and box/line reductions, Hard adds naked and hidden pairs and triples, Expert adds X-Wings, Skyscrapers, kites and wings, and Fiendish adds coloring and chains. None of them need a guess.
At a glance
- Every row, column and 3x3 box holds 1 to 9 once.
- A proper puzzle has exactly one solution.
- At least 17 givens are needed for a unique solution.
- Difficulty comes from the techniques, not the number of givens.
The technique ladder
Each level adds techniques on top of the ones before it. A puzzle's level is the hardest step its solve needs, so a Hard puzzle never asks for an X-Wing. Every technique links to a lesson with practice boards.
| Level | Techniques it adds |
|---|---|
| Easy | Full houseHidden singleNaked single |
| Medium | Pointing pairBox/line reduction |
| Hard | Naked pairHidden pairNaked tripleHidden triple |
| Expert | X-WingSkyscraper2-String KiteXY-WingXYZ-WingW-WingEmpty RectangleUnique RectangleSwordfishNaked and hidden quads |
| Fiendish | JellyfishSimple ColoringX-ChainXY-ChainAIC |
How to start a classic sudoku
Find the full houses
A row, column or box with one empty cell takes the missing digit. Early on there are few, but they snowball.
Cross-hatch one digit at a time
Pick a digit and look at each box: the rows and columns that already hold it rule cells out. When one cell is left, that is a hidden single.
Look for naked singles
A cell that sees eight different digits in its row, column and box can only be the ninth.
Add candidates when singles run out
Pencil marks show pointing pairs and naked pairs, the next rungs up. The app can fill candidates for you from the More menu.
Ask for a nudge, not the answer
Our hints name the technique first, then the houses, then the pattern. Most of the time the name is enough.
Common mistakes
Guessing to get unstuck
Every puzzle here can be solved by logic. If you're guessing, there's a technique you haven't spotted yet, and a hint will name it.
Letting pencil marks go stale
After placing a digit, clear it from the candidates of every cell that sees it. Stale marks hide the next pair. The “Remove notes when placing a digit” setting does it for you.
Judging difficulty by givens
A 24-given puzzle can be Fiendish and a 30-given puzzle Easy. Check the level instead.
Five levels, no guessing
Every puzzle has one solution, proved when it is set, and our solver finishes it by logic alone. The level is the hardest step the solve needs. Here is what each level asks of a classic puzzle:
- EasyFull houses and singles. Nothing else.Play Easy
- MediumAdds pointing pairs and box/line reductions.Play Medium
- HardAdds naked and hidden pairs and triples.Play Hard
- ExpertAdds X-Wings, Skyscrapers, kites, wings, rectangles, Swordfish and quads.Play Expert
- FiendishAdds Jellyfish, coloring, X-Chains, XY-Chains and AICs. Never a guess.Play Fiendish
A new classic sudoku comes out every day, Easy on Monday and Fiendish on Sunday. Past dailies
Beyond classic
Once classic feels easy, the variants are the next step. Each adds one rule to the same grid. Thermo and killer are the gentlest starts. Got a puzzle from a book or newspaper? Type it into the puzzle maker and play it here with hints.
Questions
How many clues does a sudoku need?
At least 17 for a unique solution. Clue count says little about difficulty, though: some 17-clue puzzles are easy and some 30-clue puzzles are very hard. We grade by the techniques the solve needs instead.
What should I do when I'm stuck?
Fill in candidates (the More menu can do it for you) and look for a pair or a pointing pair. If that fails, a hint names the next technique without giving the answer away.
Is there a daily sudoku?
Yes. A new classic sudoku comes out every day, harder through the week, and past days stay in the archive.
How many sudoku grids are there?
About 6.67 sextillion (6,670,903,752,021,072,936,960) valid completed 9x9 grids. That's why a generator never runs out.
What's the hardest sudoku technique?
In our ladder, chains: X-Chains, XY-Chains and AICs, which follow a string of linked candidates. Our Fiendish puzzles use them, and the chains lesson walks through each.
Can I play a sudoku from a newspaper or another site?
If it's on SudokuPad or f-puzzles, paste the link into the app and it opens with full pencil marks and hints. For a printed puzzle, the puzzle maker lets you type it in.
More ways to play
Killer SudokuCages that add upThermo SudokuDigits climb from the bulbArrow SudokuThe arrow sums to its circleKropki SudokuWhite dots, black dotsXV SudokuX is 10, V is 5Sandwich SudokuSum between the 1 and the 9German Whispers SudokuNeighbors differ by 5 or moreDiagonal SudokuBoth diagonals hold 1 to 9Anti-Knight SudokuNo repeat a knight's move apartAnti-King SudokuTouching cells never repeat