How to Play Sudoku: The Complete Guide

Sudoku may look intimidating at first glance — a 9×9 grid full of empty squares waiting to be filled. But the rules are remarkably simple. Once you understand the three core constraints, you can solve any Sudoku puzzle with nothing more than logic and patience. This guide covers everything from the basic rules to practical strategies that will get you solving confidently.

The Sudoku Grid

A Sudoku puzzle consists of a 9×9 grid made up of 81 cells. These cells are grouped into nine 3×3 boxes (also called blocks, regions, or sub-grids), each outlined by a thicker border. The grid also contains 27 houses — nine rows, nine columns, and nine boxes — and every single one of them must obey the same set of rules.

When you start a puzzle, some cells are already filled with numbers. These are called givens or clues. You cannot change them. The number of givens varies by difficulty — an easy puzzle might start with 36 or more clues, while an extreme puzzle might have as few as 21. No matter how many clues you start with, every valid Sudoku puzzle has exactly one solution.

The Three Rules of Sudoku

Every Sudoku puzzle obeys three simple rules. Master these and you've mastered the game:

  1. Rule 1 — Rows: Each of the nine rows must contain every digit from 1 to 9 exactly once. No repeats allowed.
  2. Rule 2 — Columns: Each of the nine columns must contain every digit from 1 to 9 exactly once. No repeats allowed.
  3. Rule 3 — Boxes: Each of the nine 3×3 boxes must contain every digit from 1 to 9 exactly once. No repeats allowed.

That's it. If you fill the entire grid so that every row, column, and 3×3 box contains the digits 1-9 one time each, you've solved the puzzle. If even one house violates this rule, the solution is incorrect.

A critical corollary: because each of the 27 houses must contain the digits 1-9 exactly once, every cell belongs to three houses simultaneously — its row, its column, and its box. Any digit you place affects those three houses and eliminates that digit as a candidate in every other cell within those houses.

Step-by-Step: Solving Your First Puzzle

Step 1: Scan for easy wins

Look at the rows, columns, and boxes that already have the most numbers filled. In each of these houses, there are fewer empty spaces to work with — meaning fewer possibilities to consider. An easy puzzle often has rows or boxes with only 2-3 empty cells, making them trivial to complete.

Step 2: Use cross-hatching

Pick a number — say, 5 — that appears frequently on the board. Trace through the rows and columns where that 5 appears. The rows and columns containing a 5 cannot hold another 5. This lets you eliminate entire rows and columns within a 3×3 box, often leaving exactly one cell where that number can go. This technique is called cross-hatching and it's the most reliable way to make early progress.

Step 3: Pencil in candidates

When you hit a wall, switch to candidate mode. For each empty cell, mentally list (or use our Notes feature to mark) every digit that could possibly fit given the current state of the row, column, and box. A cell with only one candidate left is a naked single and can be filled immediately.

Step 4: Look for hidden singles

Within a house (row, column, or box), scan each digit 1-9 one at a time. If you find a digit that can only go in one cell of that house — even if that cell has multiple candidates — you've found a hidden single. Place it and update your candidates elsewhere.

Step 5: Cycle through techniques

As you fill cells, new opportunities emerge elsewhere. Alternate between scanning for naked singles, hidden singles, and cross-hatching. For harder puzzles, you'll need more advanced techniques like pointing pairs, naked pairs, and X-wings — all of which are covered in our techniques guide.

What Sudoku Is Not

Sudoku is not a math game. Despite involving numbers, no arithmetic is required. You could replace the digits 1-9 with letters, colors, or symbols and the logic would be identical. The numbers serve purely as distinct symbols.

Sudoku is also not a guessing game. Every properly constructed puzzle has exactly one solution reachable through pure deduction. If you find yourself guessing, you've probably missed a logical constraint somewhere on the board.

Common Mistakes Beginners Make

  • Forgetting a house: It's easy to check the row and column but forget to check the 3×3 box. Always verify all three houses before placing a digit.
  • Placing a number that creates an impossible situation later: A number might look valid for its row, column, and box in isolation, but it could force a duplicate elsewhere. This is why guessing is dangerous.
  • Neglecting to update candidates: Every time you fill a cell, that digit is eliminated from the row, column, and box. If you don't update your candidate notes, you'll work with stale information.
  • Rushing on easy puzzles: Easy puzzles reward careful, methodical scanning. Speed leads to oversight.

Difficulty Levels Explained

Puzzles-Sudoku offers six difficulty levels. Here's what changes between them:

  • Easy — 36-45 clues. Solvable with basic scanning and cross-hatching. No advanced techniques needed.
  • Medium — 30-35 clues. Requires candidate notes and occasional hidden singles.
  • Hard — 25-29 clues. Expect to use naked pairs, pointing pairs, and systematic candidate elimination.
  • Expert — 22-24 clues. Requires X-wing, swordfish, and other intermediate-to-advanced patterns.
  • Master — 21-23 clues. Heavy reliance on advanced patterns and multi-step chains.
  • Extreme — 20-22 clues. The hardest puzzles we generate. Requires the full arsenal of solving techniques.