How to Play Sudoku – A Complete Beginner's Guide

Sudoku looks intimidating the first time you see one: a 9x9 grid, most of it empty, and no obvious place to start. In reality it needs no arithmetic and no guessing. Every puzzle can be solved with pure logic, and once you know what to look for, the first few numbers usually fall into place within a minute. This guide walks through everything from the basic rules to your first complete solve.

The Grid

A sudoku grid is 9 cells wide and 9 cells tall — 81 cells in total. Thicker lines divide it into nine 3x3 boxes. That means every single cell belongs to three groups at once: one row, one column, and one box. Understanding that overlap is the key to the whole game, because a number placed in one cell rules itself out in all three of those groups simultaneously.

The Only Rule You Need

Each row, each column, and each 3x3 box must contain the digits 1 through 9, each exactly once. That's it. There is no addition, no multiplication, and no hidden trick. The numbers are just nine distinguishable symbols — you could play with nine colours or nine letters and the puzzle would be identical.

Because each group must contain all nine digits with no repeats, a digit that already appears in a row cannot appear anywhere else in that row. The same goes for its column and its box. Every number you place eliminates possibilities elsewhere, and solving a sudoku is really just following that chain of eliminations to its conclusion.

Starting Numbers and Why They Matter

The digits printed at the start are called givens or clues. They can never be changed. A well-formed sudoku always has exactly one valid solution, which means you never have to guess: if you're stuck, there is always some cell somewhere on the board whose value can be proven with the information already visible.

The number of givens is what makes a puzzle easy or hard. An easy puzzle might start with 36 or more clues, leaving lots of nearly-full rows to work from. A hard puzzle might start with 26 or fewer, which forces you to chain several deductions together before a single cell can be filled.

Making Your First Move

The fastest opening technique is called scanning. Pick a digit — say 5 — and look at where the 5s already are. Now pick a 3x3 box that doesn't have a 5 yet. Every row that already contains a 5 somewhere else on the board is now off-limits inside that box, and so is every such column. Cross out those rows and columns mentally, and see what's left. If only one empty cell in the box survives, that cell must be a 5.

This works surprisingly often at the start of a puzzle, especially for digits that appear four or five times among the givens. Work through the digits one at a time rather than jumping around the board at random — it's far less tiring and you'll spot more.

Playing on Sudoku.io

On this site, tap or click any empty cell to select it, then choose a number from the on-screen keypad below the board. On desktop you can simply type the number on your keyboard, and press Backspace or Delete to clear a cell. The cell you've selected is highlighted so you never lose your place.

Correct answers build a combo: chain them together without a mistake and each one is worth progressively more points, so steady accurate play scores far better than fast guessing. A wrong answer costs you a life and breaks the combo. You also get one hint per puzzle, which reveals the correct number for whichever cell you currently have selected — useful when you're genuinely stuck rather than as a shortcut, since hinted cells don't score points.

Five Mistakes Beginners Make

• Guessing when stuck. A guess that turns out wrong can send you twenty moves down a dead end before the contradiction shows up. There is always a logical next step somewhere — move to a different part of the board and come back.
• Only looking at rows and columns. Beginners often forget the 3x3 boxes entirely, which is where most of the early deductions actually come from.
• Filling in the first plausible number. "Plausible" isn't good enough. Only place a digit when you can say exactly why no other digit fits.
• Ignoring nearly-complete groups. A row with eight digits filled in has exactly one possible answer for the ninth. These are free points and they're easy to miss when you're focused elsewhere.
• Rushing the easy levels. The scanning habits you build on easy puzzles are exactly what carry you through hard ones. Speed comes from pattern recognition, not from hurrying.

Where to Go Next

Once scanning stops being enough — usually somewhere around the Hard difficulty — you'll need techniques that work with candidates, the set of digits still possible in each cell. Our sudoku strategies guide covers naked singles, hidden singles, naked pairs, pointing pairs and the X-Wing, in the order they're worth learning.