Beginner 9 min read
How to play sudoku
Three rules, one board, and the first move. Ten minutes from here, you can pick up any puzzle in a newspaper and start.
The three rules
Fill the grid so that every row, every column, and every 3×3 box contains the digits 1 to 9, each exactly once. That is the entire rule set. There is no arithmetic, no sequence, no comparison of sizes — the digits could be nine colours and the puzzle would be identical.
The grid is 9×9, divided by thicker lines into nine boxes of 3×3. Those nine boxes, nine rows and nine columns are collectively called units, and almost every sentence in every sudoku guide is about what a unit can and cannot contain.
Some squares already hold digits. Those are givens — they are part of the puzzle and never change. Your job is the empty ones.
The board you are looking at
A real puzzle, the kind that appears in a newspaper. Nothing has been solved yet.
the digit it producescells it acts on
The mistake almost everyone makes on their first puzzle is to start at the top-left corner and work along, square by square. That is how you solve a crossword. A sudoku is not solved by squares; it is solved by digits.
The first move
Pick the digit that already appears most often on the board — it has the fewest squares left to fill. Then look, box by box, for a box that does not contain that digit yet.
In the board above, look at the 9. It appears in the top-left box, in the top-right box, and in the middle box. Now look at the bottom-right box: where can a 9 go?
- The bottom row of that box is blocked by the 9 in the middle box's bottom row.
- The right column is blocked by nothing yet.
- The middle column of that box is blocked by the 9 in the top-right box.
Three of the nine squares are eliminated by rows and columns, and one more is taken by a given. What is left is a single square — and that is where the 9 goes, with no guessing and no doubt.
That is a hidden single: one digit, one unit, exactly one square left for it. It is the first move you learn, and on an easy puzzle it is the move you will make most often. There is a full guide to hidden singles once you have played a few boards.
The loop after that
- Place a digit. Choose the digit with the most copies already on the board, and find its only possible square in some box, row or column.
- Re-scan only what you touched. That placement just changed one row, one column and one box. Look at those three units for the same digit again — often the same digit now has only one home in the neighbouring box too.
- Move to the next digit when the current one stops producing placements.
- Repeat until the board is full.
Step 2 is where beginners lose time. Re-scanning the whole board after every placement feels thorough and costs about five times as much attention as it needs to. The board did not change everywhere; it changed in twenty squares.
What not to do
Do not guess. Every digit on a valid sudoku is reachable by logic. A guess that happens to be right teaches you nothing, and a guess that is wrong is invisible for twenty moves — by which point you will be re-deriving the whole board. If you are stuck, the answer is a technique you have not learned yet, not a coin.
Do not start writing candidate marks yet. Those faint 3×3 pencil grids are essential later, when the puzzle hinges on pairs and triples. On your first few boards they are busywork: the digits that have only one home are visible without them, and marking 81 cells before making a single placement is the fastest way to decide sudoku is tedious.
Do not check your work by re-counting every unit. When you want to verify, check one row at a time and stop at the first conflict. The stuck-board checklist is a fixed order for exactly this, and it is faster than an anxious full re-read.
Where to go next
Play a board. The game on the front page has five difficulty levels and a hint button that explains the reasoning instead of handing over a number — that button is the single most useful thing on this site for a beginner.
When the easy level stops requiring thought, read naked single and hidden single properly, then come back. Those two techniques will carry you through every easy puzzle ever printed, and about half of the medium ones.
If you would rather work on paper, the easy printable sheet generates puzzles that need nothing beyond the loop above — verified, not guessed at.
Questions people actually ask
Do I have to use all nine digits in every row?
Yes. Every row, every column and every 3×3 box must contain 1 through 9 exactly once. A digit that appears twice in any of those is a mistake, even if the rest of the board still works.
Can I guess when I get stuck?
You can, but you will not know you were wrong until much later, and by then you will have built twenty moves on a false premise. Everything on a sudoku board is derivable by logic. Guessing turns a puzzle into a coin flip.
Do I need to write tiny numbers in the corners?
Not at first. Candidate marks help once you are stuck on pairs and triples. For your first few puzzles, scanning for digits that have only one home is faster than marking all 81 cells.
How long should an easy puzzle take?
Fifteen to twenty-five minutes for a first-timer, ten once the scanning habit sets in. If you are taking an hour on the easy setting, you are probably re-checking the whole board instead of only the cells the last placement touched.
Is sudoku maths?
No. The digits could be nine symbols. Nothing is ever added, subtracted or compared by size — the only property used is whether two cells hold the same symbol.