Sudoku is the world's most popular number puzzle. Published daily in newspapers and magazines across the globe, played by hundreds of millions of people, and available in a huge range of difficulty levels — Sudoku appeals to everyone from absolute beginners to competitive puzzle solvers. The rules are beautifully simple, but the challenge can be as deep as you want it to be. This guide will teach you the rules, the basic solving techniques, and the habits that will make you a confident Sudoku player.
The Rules of Sudoku
A Sudoku puzzle is played on a 9×9 grid, which is divided into nine 3×3 boxes (called "regions" or "boxes"). At the start of each puzzle, some cells are pre-filled with digits. Your task is to fill every empty cell with a digit from 1 to 9, following three simple rules:
- Each row must contain the digits 1–9, with no repetition.
- Each column must contain the digits 1–9, with no repetition.
- Each 3×3 box must contain the digits 1–9, with no repetition.
That's it. No maths required — Sudoku is purely a logic puzzle. The numbers 1–9 are just symbols; you could replace them with letters or shapes and the puzzle would work exactly the same way.
A correctly solved Sudoku has exactly one solution. Every puzzle, no matter the difficulty, has been designed so that logical reasoning (never guessing) is sufficient to reach the unique answer.
Difficulty Levels
Sudoku puzzles come in four main difficulty levels:
- Easy: Many cells are pre-filled and the solving techniques required are straightforward. Great for beginners.
- Medium: Fewer starting numbers. Requires a combination of basic techniques.
- Hard: Requires more advanced techniques and careful analysis of multiple cells simultaneously.
- Expert/Extreme: The fewest starting numbers, requiring the most sophisticated strategies.
On 456Lane, you can choose Easy, Medium, or Hard. Start with Easy until you're comfortable with the basic techniques, then progress.
Technique 1: Scanning (Cross-Hatching)
Scanning is the most fundamental Sudoku technique and it's where every beginner should start. Pick a digit — let's say 5. Look at all the rows and columns that already contain a 5. Each row or column containing a 5 eliminates all the cells in that row or column as possible homes for another 5. By eliminating rows and columns, you can often narrow down where the 5 must go in a given 3×3 box — sometimes to just one cell, which means you can fill it in immediately.
Work through all nine digits this way. In an Easy puzzle, scanning alone will often let you fill in the majority of the grid.
Technique 2: Lone Singles (Last Remaining Cell)
Look at each 3×3 box, row, and column. If eight of the nine cells in a unit are filled in, the remaining empty cell must contain the one missing digit. This is called a lone single and it's one of the easiest fills to spot.
Even in partially filled units, scan for cells where only one digit is possible given everything else in that row, column, and box. If a cell is at the intersection of a row, column, and box that between them contain eight of the nine digits, only one digit remains — and that's the answer.
Technique 3: Hidden Singles
A hidden single occurs when a digit can only go in one cell within a row, column, or box — even though that cell might appear to have multiple possibilities. For example, if the digit 7 can only fit in one specific cell of a 3×3 box (because all other cells in the box are eliminated by 7s in their row or column), that cell must be 7, regardless of what other digits could also fit there.
Hidden singles are a step up from lone singles and require you to think about digit placement within units rather than just counting empty cells.
Technique 4: Pencil Marks (Candidates)
As puzzles get harder, you'll encounter cells where multiple digits are possible and you can't immediately determine the answer. This is where pencil marks (also called candidates or notes) come in. In each empty cell, write down (or mentally note) all the digits that could possibly go there based on the current state of the puzzle.
Maintaining accurate pencil marks lets you spot patterns that aren't immediately obvious. For example, if two cells in the same row both have the candidates {3, 7} and only those two candidates, then 3 and 7 must go in those two cells — meaning you can eliminate 3 and 7 as candidates from all other cells in that row. This is called a "naked pair."
General Tips for Beginners
Never guess. Sudoku is a pure logic puzzle — every digit can be determined through reasoning. If you feel like you have to guess, look more carefully. There's always a logical step you're missing.
Work the whole grid, not just one area. Don't fixate on one box or row. Scan the whole grid regularly — sometimes a placement on one side of the board opens up a deduction on the other side.
Start with the most-filled units. A row, column, or box that already has 6 or 7 digits is close to complete and easy to finish. Start there to quickly fill in easy cells before tackling the harder, more open areas.
Check your work as you go. After each digit you place, quickly verify that it doesn't violate any row, column, or box rule. Catching errors early prevents a cascade of incorrect placements that can ruin the whole puzzle.
Take your time. Sudoku rewards patience and careful observation. Unlike arcade games, there's no time pressure (unless you're competing). Slowing down and looking carefully is always better than rushing and making mistakes.
Why Sudoku Is Great for Your Brain
Beyond being a fun challenge, Sudoku has real cognitive benefits. Studies suggest that regular Sudoku play improves logical reasoning, concentration, and short-term memory. It's also a great stress reliever — focusing on a puzzle pulls your mind away from worries and into a state of calm, focused problem-solving that many people find genuinely relaxing.