How to Play Sudoku: Rules, Strategy, and Tips for Beginners

A complete beginner's guide to sudoku rules and solving technique, with a free puzzle waiting at the end.

Sudoku looks intimidating the first time you see a blank 9×9 grid, but the rules are genuinely simple, and the puzzle rewards patience rather than luck or math skill. If you've never solved one before, this guide walks through the rules, a repeatable solving method, and the mistakes that trip up most beginners.

The Rules of Sudoku

A sudoku grid is 9 cells wide and 9 cells tall, divided into nine 3×3 boxes. Some cells start filled in ("fixed" cells); the rest are empty. Your job is to fill every empty cell with a digit from 1 to 9 so that:

Each row contains every digit from 1 to 9, with no repeats
Each column contains every digit from 1 to 9, with no repeats
Each of the nine 3×3 boxes contains every digit from 1 to 9, with no repeats

That's the entire rule set. There's no arithmetic involved — the digits 1-9 are just nine distinct symbols, and the puzzle would work identically with nine letters or colors instead.

How to Solve a Sudoku Puzzle, Step by Step

Every valid sudoku has exactly one solution that can be reached through logic alone, without guessing. The most reliable beginner method is scanning:

Pick a digit, say 5, and look at each 3×3 box in turn.
If a box already contains a 5 somewhere, skip it — that box is done for the digit 5.
For the remaining boxes, check which rows and columns already contain a 5, and rule out those positions within the box.
If only one empty cell in the box is left as a valid position, you've found where the 5 goes.

Repeat for each digit, then move on to individual cells: for any empty cell, check what digits are already used in its row, column, and box. If only one digit is left that doesn't conflict with any of the three, that's your answer for that cell.

Common Beginner Mistakes

Guessing too early. Every proper sudoku is solvable through deduction — if you're stuck, you've likely missed a scan somewhere, rather than reached a genuine 50/50 guess.
Only checking rows or only checking columns. The box constraint is just as important, and it's the one beginners forget most often.
Not re-scanning after each placement. Filling in one cell often unlocks the next one — go back over digits you've already placed instead of always moving forward.

Sudoku Difficulty Levels Explained

Difficulty in sudoku comes down to how many starting digits are given — fewer starting digits mean more deduction is required before you can make your first confident move. On casualgames.dev's sudoku, Easy starts with 46 filled cells (35 removed from a full grid), while Extreme starts with only 16 (65 removed). If you're new to sudoku, start on Easy or Medium — the scanning method above becomes much faster to apply once you've practiced it on a grid with more given digits to work from.

Play Sudoku Online for Free

Ready to try it? Play sudoku for free directly in your browser — no installation, no account, no ads blocking the board. Pick a difficulty, use Check to see if your entries are correct so far, and Solve if you want to see the finished grid. If you enjoy the logic of sudoku, casualgames.dev also has nonogram and minesweeper — two more free logic puzzles that reward the same kind of careful deduction.