Sudoku 9×9
The classic 9×9 Sudoku with multiple difficulty levels. Pure deductive reasoning. Fill the grid so every row, every column, and every 3×3 box contains the digits 1-9 exactly once each.
The rules of Sudoku
Sudoku is a 9×9 grid puzzle. The grid is divided into nine 3×3 boxes. The puzzle starts with some cells already filled in (the "givens"); your job is to fill the remaining cells so that each row, each column, and each 3×3 box contains every digit from 1 to 9 exactly once. A properly constructed Sudoku has exactly one solution, and that solution is reachable through pure deduction — no guessing required.
How a Sudoku is generated
The puzzles on this page are generated client-side by JavaScript. The generator first builds a complete valid 9×9 solution through randomized backtracking (filling cells one at a time, undoing when a contradiction is reached). It then removes cells in random order, checking after each removal that the puzzle still has a unique solution. The number of cells removed — and thus the difficulty — is controlled by the difficulty button you selected:
- Easy: ~38 givens (43 empty cells). Solvable by naked singles and straightforward scanning.
- Medium: ~30 givens (51 empty cells). Requires hidden singles and some box-row interactions.
- Hard: ~24 givens (57 empty cells). Requires more advanced techniques such as naked/hidden pairs or pointing pairs.
Worked example: a naked single
Consider a row that already contains 1, 2, 4, 5, 7, 8, 9 (in any order). The two missing digits for that row are 3 and 6. Now look at one of the empty cells in that row. If the column it sits in already contains a 3, then this cell cannot be 3 — so it must be 6. We've found a naked single: a cell where only one candidate is possible. This is the most common solving step at every difficulty level.
Worked example: a hidden single
A hidden single is a digit that has only one possible cell in a row, column, or box — even though that cell might have other candidates. For example, if a particular 3×3 box is missing the digit 5, and you scan every empty cell in the box and find that only one of them could legally hold a 5 (because every other candidate cell already has a 5 in its row or column), that cell must be 5 — even if the cell could in principle hold other digits. Hidden singles are the technique that unlocks most medium puzzles.
Does Sudoku make you smarter?
The honest answer, per the broader research literature, is that practicing Sudoku reliably makes you better at Sudoku and at closely related constraint-based puzzles, but there is no strong evidence that this skill transfers to general cognitive ability, working memory, or fluid intelligence. A 2018 cross-sectional study (Sala et al., PMC) on brain-training interventions — which would include puzzle sites like this one — found no significant transfer to untrained cognitive tasks.
What Sudoku does offer is a focused, satisfying mental challenge that engages attention, deductive reasoning, and short-term memory in a way that scrolling a feed does not. If you enjoy it, the engagement itself is the value — not a hypothetical IQ boost. For a fuller discussion, see our article Do Brain Games Actually Work?.