How to Play Sudoku: A Step-by-Step Guide for Beginners
Knowing the rules of Sudoku does not tell you where to put your pen. This is the order that works, using one puzzle throughout.
Step 1: look where the board is already crowded
Every empty cell sits in a row, a column and a box. If those three groups between them already use eight different digits, the ninth is the only thing that fits, and you can write it in.
So do not start at the top-left corner. Start wherever the board is most filled in. The centre cell is a good example.
Read the highlighted column: 7, 9, 6, 2, 1 and 8 are all there. Read the highlighted row: it adds 4 and 3. That is eight digits between them. The only one left is 5, and nothing else could go in that cell.
A cell with exactly one possible digit is a naked single. Finding them is the whole of an easy puzzle.
Step 2: cross-hatch a box
Sooner or later no naked single is left. The next question has a different shape. Instead of asking what can go in this cell, ask where a given digit can go in this box.
Take the top-middle box and the digit 8.
Three 8s do the work, all of them outside the box:
- The 8 in the fourth column rules out that whole column inside the box.
- The 8 in the fifth column rules out that one too.
- Only the sixth column of the box is left, and the 8 in the third row rules out its bottom cell.
The middle cell of that column is already filled, so exactly one cell survives and the 8 goes there. Worth noticing: if you had looked at that cell on its own, four different digits would still have fitted. The answer only appears when you look at the whole box.
This is cross-hatching, and a digit found this way is a hidden single. Working one digit at a time across all nine boxes is the most productive scan there is.
Step 3: finish off units that are nearly full
Any row, column or box with a single empty cell is free — the missing digit is whatever the unit does not already have. Sweep for these every few placements, because your own answers keep creating them.
Step 4: start writing pencil marks
When scanning stops producing anything, stop scanning and start recording. In a cell you cannot resolve, write the small digits that are still possible. Most apps call these notes or pencil marks.
This is bookkeeping rather than a technique on its own, and what it buys you is visibility: two cells in a unit sharing the same two candidates, a digit that appears in only two places in a row, and so on. Do not fill in every cell from the start. Do one unit at a time, when that unit is where you are stuck.
Step 5: do not guess
If you are down to picking one of two digits to see what happens, stop. One of two things is true.
Either the puzzle needs a technique you have not learned yet, in which case a guess gets you a finished grid and teaches you nothing. Or you made a mistake several placements ago and the board no longer has a solution at all.
Every properly built puzzle has exactly one solution and a path to it that never requires a coin flip. A guess is a signal, not a move.
Mistakes beginners actually make
Filling in a digit because it fits. It has to be the only digit that fits. “Nothing stops a 4 here” is not a reason.
Forgetting the box. New solvers scan rows and columns and skip the third unit, which is where most early answers come from.
Trusting a wrong digit. One bad placement propagates quietly and surfaces much later as a contradiction. When a unit ends up with two of the same digit, the error is behind you, not in front of you.
Writing pencil marks too early. Eighty-one cells of notes on a puzzle that three scans would have cracked is a lot of work you did not need to do.
Where to go next
The rules themselves are in What is Sudoku. Pencil marks covers step 4 properly, and naked singles and hidden singles names the two patterns steps 1 and 2 rely on.