How to Play Sudoku

Updated 23 August 2026

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 a digit from 1 to 9 so that no digit repeats in any row, column or box. A well-formed Sudoku always starts with some cells already filled in — the givens, or clues — and always has exactly one valid solution reachable by logic alone, with no guessing.

The grid itself never changes: nine rows, nine columns and nine boxes, eighty-one cells in total. What changes from puzzle to puzzle is only which cells start filled and which digits they hold, and it is that starting layout of givens that determines how much logic is needed to reach the one valid solution.

The three rules of Sudoku

Every Sudoku rule is really the same constraint applied to three different kinds of unit. Learn the constraint once and you already know all three rules.

• Row rule. Each of the nine rows must contain the digits 1 to 9 exactly once each.
• Column rule. Each of the nine columns must contain the digits 1 to 9 exactly once each.
• Box rule. Each of the nine 3×3 boxes must contain the digits 1 to 9 exactly once each.

A finished grid satisfies all three at the same time. That overlap is what makes Sudoku solvable by logic: a digit that is already ruled out by its row, its column and its box has nowhere left to go, which is the basis of almost every solving technique. Every named technique on this site, from the simplest single to the longest chain, is ultimately just a more efficient way of tracking that overlap across many cells at once instead of one cell at a time.

Solving your first cells

Do not scan for the hardest cell first. Start by looking for the easiest possible placements, because clearing those first makes every later cell easier too.

The very first thing to check is whether a box, row or column is missing only one digit. If eight of the nine cells in a unit are already filled, the ninth cell must take the digit the unit is still missing — no candidates to compare, just subtraction.

After that, look at a single blank cell and cross off every digit that already appears in its row, its column and its box. If only one digit survives, that is a naked single, and it can be placed immediately. If a cell still has several candidates left, check whether one of those candidates cannot go anywhere else in the same unit — that is a hidden single, and it is just as safe to place. Together these two patterns solve most of an easy puzzle without any deeper reasoning.

Notes and candidates

Once the easy placements run out, tracking candidates by hand becomes essential. A candidate, sometimes called a pencil mark, is a digit that a cell could still legally hold based on what its row, column and box already contain. Writing every candidate into a cell — on paper as small numbers in the corner, or with the notes mode in an app — turns a puzzle that looks stuck into one where patterns become visible: pairs, triples and quads of candidates that repeat across a unit are exactly what techniques like naked pairs and hidden pairs are built to spot. The full method for every pattern lives in the sudoku techniques index, and every term used across these guides is defined in the sudoku glossary.

Sudoku difficulty levels

Suuudokuuu’s difficulty ladder has six tiers, each requiring a wider set of techniques than the last.

• Newbie. Solvable with full houses and naked singles alone.
• Easy. Adds hidden singles, still no candidate tracking required.
• Medium. Always stalls on singles and always yields to intersections and subsets — pointing pairs, box line reduction, naked and hidden pairs through quads.
• Hard. Always stalls on subsets and always yields to a fish or a wing pattern.
• Nightmare. Always stalls on fish and wings and always yields to chain or coloring logic.
• Hell. Drawn from a bundled, verified 17-clue corpus — the minimum number of clues a Sudoku can have.

Every level is free to play, with no ads and no sign-up. Play a puzzle now at any of the six levels, or read the full breakdown of clue counts and required techniques on the Sudoku difficulty levels hub.

Getting started

1. Scan for missing unitsCheck every row, column and box for one that already has eight of its nine cells filled, and place the missing digit in the empty one.
2. Cross off candidates cell by cellFor each remaining blank cell, rule out every digit already used in its row, column and box. If exactly one digit is left, place it.
3. Look for hidden singlesFor each unit with several blank cells, check whether a digit that is still missing from the unit only fits in one of them, even if that cell has other candidates too.
4. Write in candidates once the easy cells run outPencil in every remaining candidate for the blank cells that are left, then work through the technique index in difficulty order to keep clearing them.