How to Play Sudoku
Sudoku is a logic puzzle played on a 9×9 grid divided into nine 3×3 boxes. The goal is to fill every empty cell with digits 1 through 9 so that no digit repeats in any row, column, or 3×3 box. You never need guessing if you apply the right techniques—only logical deduction. This guide walks you from the basic rules to techniques used by regular solvers, so you can enjoy puzzles on Sudoku Online with confidence.
Basic Rules
- Each row must contain the digits 1–9 exactly once.
- Each column must contain the digits 1–9 exactly once.
- Each of the nine 3×3 boxes must contain the digits 1–9 exactly once.
- The puzzle starts with some cells already filled (the "givens"). Those cannot be changed. Your job is to deduce the rest.
Core Solving Techniques
Most puzzles—especially easier ones—can be solved by repeatedly finding cells where only one digit is possible. As you get faster, you will recognise patterns without naming them. Here are the ideas behind those patterns.
Naked single
A naked single is the simplest move: in one empty cell, eight digits are ruled out by the numbers already present in its row, column, and box. The only digit left is the answer for that cell.
To find it, look at an empty cell and list which digits 1–9 are already used in the same row, same column, and same 3×3 box. If eight digits appear among those three groups, the ninth digit must go in that cell. Scan rows, columns, and boxes that are nearly full—those often hide naked singles.
Hidden single
A hidden single means: for a particular digit, there is only one cell in a row, column, or box where that digit can legally go—even if that cell still has other candidates mentally.
Example: in a row, digit 7 might be blocked in eight of the nine cells by existing 7s or box constraints. The ninth cell is the only place 7 can go in that row, so you place 7 there. Hidden singles are easiest to spot when you focus on one digit at a time and scan each row, column, and box.
Pairs (and looking ahead)
When two cells in the same row, column, or box can only be the same two digits (for example 3 and 5 in both cells), those digits are "locked" to those two cells—you can eliminate 3 and 5 from other cells in that row, column, or box. Similar ideas extend to triples and beyond on harder puzzles.
On Sudoku Online, Easy and Medium puzzles rarely require advanced patterns; Hard and above may. If you are stuck, use notes to track candidates—then naked and hidden singles become easier to see.
Step-by-Step Solving Process
Scan the grid for rows, columns, or 3×3 boxes that already contain many digits. Fewer empty cells mean fewer possibilities.
For each empty cell, ask which digits 1–9 are not present in its row, column, or box. If only one digit remains, enter it (naked single).
If no naked single is obvious, pick a digit and check each row, column, and box: can that digit only go in one cell there? If yes, place it (hidden single).
Use notes (pencil marks) for cells with two or more candidates so you do not lose track as the grid fills in.
Repeat. Avoid guessing: if you place a wrong number, the puzzle may still "look" fine until much later. Logic keeps the puzzle fair.
Using notes (pencil marks)
Notes let you write small candidate digits in a cell—typically all values that are still possible for that cell after checking its row, column, and box. They are essential on harder grids and useful for learning on easier ones.
Good habit: update notes after each placement—remove the digit you placed from every related cell in the same row, column, and box. On our site, use the game's note mode so you can toggle candidates quickly.
- Start with obvious eliminations before writing every candidate in every cell.
- If a cell's notes shrink to one digit, that is a naked single—place it.
- Do not over-rely on hints; they often reduce your score on the leaderboard, which rewards cleaner solves.
Common mistakes to avoid
- Guessing when you are stuck—one wrong digit can cascade into a long wasted effort. Step back and scan for singles instead.
- Forgetting to check all three constraints (row, column, and box) before placing a number.
- Ignoring notes on medium and hard puzzles; candidate tracking is how you spot hidden singles and patterns.
- Rushing on timed runs; a few seconds spent verifying a placement beats a full restart.
Mini example (how logic works)
Picture a 3×3 box where eight of the nine cells are filled, and only digit 4 is missing. That empty cell must be 4—you do not need to look at the whole grid. Similarly, if in one row eight different digits already appear, the ninth cell must hold the only digit not yet in that row.
Larger puzzles are just more of the same idea: keep eliminating impossible digits until only one choice remains for a cell, or only one cell remains for a digit in a unit.
Open a game on our Play page and try to solve the first few cells using only these rules before using hints—you will internalise the logic faster.
Pro tips for faster solves
- Develop a consistent scan order (e.g. digits 1–9 across each box, or row-by-row) so you do not miss easy placements.
- Prioritise constrained areas: rows, columns, or boxes with the fewest empty cells.
- Practice one difficulty level until you are comfortable before moving up—speed comes from pattern recognition.
- Use the leaderboard as motivation, not stress: compare against yourself week over week.
You now have a solid foundation for Sudoku rules and basic techniques. The best next step is practice: start an Easy or Medium game on Sudoku Online, use notes when needed, and revisit this page when you want to refresh a concept. Happy solving!