Sudoku is a logic puzzle played on a 9×9 grid subdivided into nine 3×3 boxes. The goal is to fill every cell with a digit from 1 to 9 so that each row, each column, and each box contains all nine digits exactly once. No arithmetic is involved — pure deduction. The ten tips below will take you from complete beginner to confident intermediate solver, covering everything from the most basic scanning technique to intermediate elimination strategies.
Key Definitions
- Grid — the full 9×9 Sudoku playing field, divided into nine 3×3 boxes.
- Row, column, box — the three units every digit 1–9 must appear in exactly once. Solving Sudoku means satisfying all three constraints simultaneously for every cell.
- Candidate — any digit still legal for a given empty cell. Most solving techniques revolve around tracking and eliminating candidates.
- Naked single — a cell with only one remaining candidate after all constraints are applied. It can be filled immediately.
- Hidden single — a digit that can only go in one cell within a row, column, or box, even though that cell may have other candidates.
1. Scan Rows, Columns, and Boxes Constantly
The most fundamental technique is also the most powerful: scan each unit (row, column, box) for digits that are almost complete. If a row already contains 1, 2, 3, 4, 6, 7, 8, and 9, the missing digit is 5, and you only need to find which empty cell in that row can legally hold it. Beginners skip this scan and miss free fills constantly.
2. Hunt for Naked Singles First
A naked single is a cell where eight of the nine digits are already ruled out by the row, column, and box constraints. The remaining digit is the only legal fill. After every move, scan for naked singles — they are the easiest deductions in the game and often cascade into a chain of further singles.
3. Look for Hidden Singles in Each Box
A hidden single is subtler: a digit may have only one legal cell in a box even though that cell has several other candidates. For example, if the digit 7 can only fit in the top-left cell of a particular box (because the other empty cells in that box share a row or column with an existing 7), then 7 must go there, regardless of what other candidates that cell appears to have. Box-level hidden singles are the most common and the easiest to spot.
4. Mark Candidates in Tough Puzzles
When scanning stops yielding fills, start marking candidate digits in each empty cell (lightly, in pencil, or via the app's note mode). Once candidates are visible, you can apply elimination techniques that are invisible to the unaided eye. Most intermediate puzzles are unsolvable without candidate marks; most easy puzzles never need them.
5. Eliminate With Naked Pairs
If two cells in the same unit both have exactly the same two candidates (say, {2,5} and {2,5}), then 2 and 5 must occupy those two cells in some order, and you can eliminate 2 and 5 from every other cell in that unit. This is called a naked pair, and it often unlocks a stuck puzzle.
6. Eliminate With Hidden Pairs
A hidden pair is the inverse: two digits that can only appear in the same two cells of a unit. Even if those cells have other candidates, those other candidates can be eliminated — the two digits must occupy those two cells, so no other digit can. Spotting hidden pairs takes practice but becomes second nature.
7. Use the Pointing-Pair Technique
Sometimes a candidate digit in a box is restricted to a single row or column. If, for example, all candidate 4's in a box lie along the same row, then no other cell in that row (outside the box) can be a 4, because the box already claims one. Eliminate 4 from those out-of-box cells. This is called a pointing pair (or pointing triple) and it is one of the most productive intermediate moves.
8. Apply Box-Line Reduction
The mirror of pointing pairs: if all candidates for a digit in a row or column fall inside a single box, then that digit cannot appear elsewhere in the box. Eliminate it from the other cells in the box. Together with pointing pairs, this technique clears many intermediate puzzles.
9. Work the Most Constrained Units First
When you do not know where to look, find the row, column, or box with the fewest empty cells. Heavily filled units offer the most constraints and the fewest remaining possibilities, so deductions are easier there. A box with only two empty cells is a goldmine compared to a row with six empties.
10. Never Guess
Properly constructed Sudoku puzzles have a unique solution reachable by pure logic. If you feel tempted to guess, you have missed a deduction — go back and re-scan, mark candidates if you have not already, and apply the elimination techniques above. Guessing leads to contradictory states and wasted time, and it teaches you nothing about how to solve the next puzzle.
Building Sudoku Habits
Sudoku is a skill, not a talent. The first hundred puzzles feel slow because you are training pattern recognition; the next hundred get faster as the techniques become automatic. For more logic-and-pattern games, see our guides on Wordle, chess for beginners, and Guess Who? strategy.