How to Play Sudoku: Rules, Strategy & a Step-by-Step Beginner's Guide
Quick answer: Sudoku is a logic puzzle played on a 9x9 grid. You fill every empty cell so that each row, each column, and each 3x3 box contains the numbers 1 to 9 exactly once. No math is required — only logic and deduction.
Despite being made of numbers, Sudoku never asks you to add, multiply, or calculate anything. The digits are just nine distinct symbols. All you actually need is logic, patience, and a few minutes of quiet.
This guide takes you from the bare rules to solving a complete puzzle on your own.
What are the rules of Sudoku?
A classic Sudoku is a 9x9 grid — 81 cells — divided into nine 3x3 sub-grids called boxes (or blocks, or regions). Some cells arrive already filled. These are the givens, or clues, and they never change.
To solve the puzzle you fill every remaining cell while obeying three constraints:
- Every row must contain the digits 1 through 9 exactly once.
- Every column must contain the digits 1 through 9 exactly once.
- Every 3x3 box must contain the digits 1 through 9 exactly once.
That is genuinely the whole game. A digit can never repeat within the same row, column, or box, and everything you will ever deduce follows from that one restriction applied in three directions at once.
Two things the rules do not say are worth stating plainly, because they trip up almost every new player:
- Nothing has to add up to anything. Sudoku rows do not sum to a target. (You may be thinking of Killer Sudoku, a variant where cages do carry sums.)
- Diagonals do not matter in classic Sudoku. Only rows, columns, and boxes. The variant that constrains diagonals is called X-Sudoku, and it is one of several Sudoku types.
Is Sudoku a 3x3 or a 9x9 puzzle?
Both, and this confuses a lot of beginners searching for one or the other. The full board is 9x9. That board is carved into nine 3x3 boxes. So a “3x3” in Sudoku is a region inside the 9x9 grid, not a separate kind of puzzle.
Smaller grids do exist for children and beginners — 4x4 and 6x6 Sudoku work identically with fewer digits — but “Sudoku” with no qualifier means the 9x9 board.
How do you start a Sudoku puzzle?
The best opening move is not to look for an empty cell you can fill. It is to look for a digit that is already nearly finished.
Scan the grid and find whichever digit appears most often among the givens — say there are five 7s. That digit is your best lead, because each existing 7 eliminates an entire row and an entire column for every other 7 on the board. The more of a digit already placed, the more constrained the remaining placements are.
Work that digit first, then move to the next most common one. Beginners who start with the emptiest region instead usually stall within two minutes.
The two techniques that solve an easy puzzle
Almost every easy Sudoku can be finished with just these two moves. Neither requires writing anything down.
Crosshatching
Pick a digit and a box that is missing it. Trace along each row and column that passes through that box. Any row or column already containing your digit rules out all three cells it crosses inside the box.
Often you will eliminate every cell but one. That last cell is where the digit goes — not as a guess, but as the only survivor.
This is the workhorse of Sudoku. Most players spend the majority of an easy puzzle crosshatching, and it stays useful at every difficulty.
Counting
If a row, column, or box already holds eight digits, the ninth is whatever is missing. Count 1 through 9, find the gap, fill it in.
It sounds trivial, and it is — but every digit you place through crosshatching creates new nearly-complete regions, so counting keeps paying off throughout the solve. The two techniques feed each other.
Your first puzzle, step by step
Here is the loop to run, in order, until the grid is full:
- Survey the board. Which digit appears most? Start there.
- Crosshatch that digit through every box still missing it. Place whatever becomes certain.
- Check for completions. Did that placement leave any row, column, or box with exactly one gap? Count it and fill it.
- Move to the next most common digit and repeat steps 2 and 3.
- When you run dry, cycle through all nine digits once more. Every placement you made has changed the constraints, so a digit that gave you nothing on the first pass will often give you something on the second.
- Only when scanning genuinely stalls, start writing pencil marks.
Steps 1 through 5 will carry you through any easy puzzle and most medium ones.
When to start using pencil marks
Pencil marks are the small candidate digits you write into a cell to record what could still legally go there. They are the bridge from beginner to intermediate play.
Do not reach for them too early. On an easy puzzle, filling in candidates everywhere is slower than just scanning, and the clutter hides the obvious moves. Bring them out when plain crosshatching has stopped producing placements — usually partway through a hard puzzle.
Once your candidates are written down, two patterns appear immediately:
- Naked single — a cell with only one candidate left. That candidate is the answer.
- Hidden single — a digit that appears as a candidate in only one cell of a row, column, or box, even though that cell has several candidates. It is easy to miss and enormously useful.
Hidden singles are the single highest-value pattern for a player leaving the beginner stage. They get their own walkthrough in hidden singles and pairs.
The mistakes that cost beginners the most
Guessing. A proper Sudoku has exactly one solution reachable by pure logic. If you place a digit you cannot justify, you are no longer solving — and the contradiction typically surfaces twenty moves later, by which point you cannot tell which placement was wrong.
Writing a digit you are only fairly sure about. Same failure, quieter. If you cannot state the reason, leave the cell empty.
Forgetting the third constraint. New players check the row and the column and forget the box, or check the box and forget the column. Every placement must satisfy all three.
Grinding one region. If a section will not yield, the information you need is somewhere else on the grid. Move.
How hard should your first puzzles be?
Start on easy and stay there until you can finish one without stalling. Easy puzzles are not merely “puzzles with more givens” — they are constructed so that scanning alone is always enough, which is exactly what you are practising.
Difficulty in Sudoku is defined by the hardest technique required, not by the number of clues, which is why a 26-clue puzzle can be easier than a 30-clue one. Sudoku difficulty levels explained covers how the tiers actually work.
When easy puzzles stop resisting, move to medium, then pick up pencil marks and intermediate strategies — naked pairs, pointing pairs, box/line reduction. Beyond those sit hard and evil puzzles and techniques like the X-Wing.
Why play Sudoku at all?
Beyond the puzzle itself, Sudoku is unusually good at holding attention. It asks for exactly one thing at a time, it rewards patience rather than speed, and it has a definite end. Many players describe a solve as closer to a few minutes of meditation than to a game — which is measurably good for you.
Ready to try? Play a free easy puzzle now, take on today’s daily Sudoku, or print a page of four if you would rather work on paper.
