Classic Sudoku: play sudoku online free

Classic sudoku is where it all starts: a 9×9 grid, nine 3×3 boxes and the digits 1 to 9. You can play sudoku online free here at four difficulty ranks, from a relaxed warm-up to genuinely hard sudoku with hints from Professor Hoot whenever you get stuck.

With no extra rules to lean on, every deduction comes from the grid itself, which makes a hard classic one of the purest logic puzzles there is. At the top rank every given is essential: remove any one and the puzzle can no longer be solved by logic alone.

The rules

• Fill every empty cell with a digit from 1 to 9.
• Each row contains every digit from 1 to 9 exactly once.
• Each column contains every digit from 1 to 9 exactly once.
• Each of the nine 3×3 boxes contains every digit from 1 to 9 exactly once.
• Every puzzle has exactly one solution, and you never need to guess to find it.

How to start

Scan for hidden singles

Pick a digit and a box, and ask: where can this digit go? Each copy of the digit already in the grid rules out its whole row and column. If only one cell in the box survives, that cell is a hidden single. It is the single most useful move in sudoku, and it is how most solvers begin.

Then look for naked singles

Turn the question round: pick a cell and ask which digits it could be. If its row, column and box already contain eight different digits, only one is left. That is a naked single. Hidden singles are easier to spot by eye; naked singles show up once you start keeping pencil marks.

When the singles run dry

Harder puzzles need you to reason about where a digit might go. If a box’s only candidates for a digit lie in one row, that digit can be removed from the rest of the row (pointing). If two cells in a house can only be the same two digits, those digits can be removed from every other cell in the house (a naked pair). Both are explained step by step in our solving techniques guide.

Common mistakes

• Placing a digit because it “looks right”. If you can’t say exactly why it must go there, it doesn’t go there yet.
• Forgetting to clear pencil marks after placing a digit. Stale marks create pairs that aren’t really there.
• Scanning only rows and columns. Boxes produce most of the early hidden singles.
• Guessing and backtracking. Every puzzle here has a logical path, so a guess only hides the step you missed.

How the ranks scale difficulty

Every puzzle type on Hoot’s Puzzles comes in four ranks, named after the stages of a mathematical idea. Lemma is the warm-up: about a dozen more digits are given than logic strictly needs, and singles plus the variant’s own rule will carry you through. Proposition needs a real idea or two, typically pointing and claiming. Theorem is near-minimal, so almost every clue is load-bearing and you may need pairs. Conjecture gives the fewest digits and sparser rule clues: expect to think hard.

In classic sudoku the givens are the only clues, so rank shows up purely in how many digits you start with and how deep the logic goes. That makes it the best place to learn the core techniques before the variants add their own twists.

There are 99,999 puzzles per rank, each with its own number, so you can replay one or share it. Every type also has a 30-puzzle collection that climbs from Lemma to Conjecture. Every puzzle is checked to have exactly one solution that you can reach by logic alone, with no guessing.

Keep exploring

• Killer SudokuCages with sums instead of given digits. Partition arithmetic and the rule of 45.
• Sudoku XBoth main diagonals are extra houses. Also called diagonal sudoku.
• WindokuFour shaded windows are extra 3×3 houses. Also known as hyper sudoku.
• Solving techniquesSingles, pairs, cage arithmetic, line logic and Latin squares, all in one guide.
• All puzzle guidesEvery puzzle type on Hoot’s Puzzles, with rules and tips.