The Rules of Sudoku, Explained Simply

Sudoku has one of the shortest rulebooks of any popular game. This page lays out the full rules, explains what makes a puzzle valid and fair, and clears up the misunderstandings that trip up new players.

The board

A standard Sudoku is played on a grid of nine rows and nine columns — 81 cells in total. Thicker lines divide the grid into nine smaller blocks of three-by-three cells. These blocks are called boxes, regions, or sometimes nonets. When a puzzle begins, some cells already hold numbers. These are the clues or givens, and they never change.

The three rules

Everything about Sudoku comes down to three constraints that must all be true at once:

1. Each row must contain every digit from 1 to 9, with no repeats. A row has nine cells, so each digit appears exactly once.
2. Each column must contain every digit from 1 to 9, with no repeats. Columns also have nine cells.
3. Each of the nine boxes must contain every digit from 1 to 9, with no repeats. A box is a three-by-three block, which is nine cells.

That is the complete rulebook. A finished grid is correct when all three conditions hold everywhere at the same time. Notice that the three rules reinforce each other: a single number placed in a cell affects its row, its column, and its box simultaneously.

What makes a valid puzzle

A properly made Sudoku has exactly one solution. This single-solution rule is what separates a real Sudoku from a random arrangement of numbers. Because there is only one answer, you can always reach it through logical deduction — you never have to guess, and you never hit a fork where two different numbers would both work.

Every puzzle generated on vSudoku is checked to confirm it has a unique solution before it ever reaches your screen. The number of clues we leave visible is what sets the difficulty: more clues make the logic shorter and easier, while fewer clues force longer chains of reasoning.

Winning the game

You win when every cell is filled and all three rules are satisfied. There is no score to beat and no opponent — the only measure, if you want one, is the time on the clock and the difficulty you chose. Many players simply enjoy the steady rhythm of filling the grid.

Common misconceptions

“You need to be good at maths.”

You do not. The digits are symbols, not quantities. Nothing is ever added or multiplied. Sudoku is a pure logic and pattern game.

“Diagonals have to contain 1 to 9 too.”

Not in standard Sudoku. Only rows, columns, and boxes carry the one-of-each rule. There is a variant called Diagonal (or X) Sudoku that adds the two main diagonals as extra constraints, but that is a different game with its own puzzles.

“Good solvers guess cleverly.”

The best solvers guess less, not more. Because a valid puzzle has one solution reachable by logic, every move can in principle be justified. Guessing is a sign that a logical step is waiting to be found.

A quick glossary

• Cell — a single square on the grid.
• Given / clue — a number provided at the start that cannot be changed.
• Candidate — a number that could still legally go in an empty cell.
• Box / region / nonet — one of the nine three-by-three blocks.
• Unit — any row, column, or box; the three groups the rules apply to.

Now put the rules to work

With the rules in hand, the next step is learning how to apply them efficiently. Our how-to-play guide walks you through your first solve, and the tips and strategy page covers the techniques that crack the harder levels. Or jump straight in and start a puzzle now.