There is only one rule
A Sudoku grid is nine rows, nine columns and nine boxes of three by three. Each of those twenty-seven groups — we will call them units — must end up holding the digits 1 to 9, each exactly once.
That is the whole game. Every technique you will ever read about is a shortcut for spotting where the rule has already made your mind up for you.
The digits are not quantities. Nothing is added, and nothing is bigger than anything else. Replace them with nine colours and the puzzle is identical. If you catch yourself doing sums, you have wandered off.
Never guess
A properly made grid always has one answer, reachable by reasoning alone. If you are guessing, you have not run out of logic — you have run out of the logic you currently know. Every grid our dealer hands you is checked for a unique solution before it reaches the screen.
Scanning: let a digit find its own square
Beginners usually stare at an empty cell and ask “what goes here?”. That is the hard question. The easy question is the other way round: pick a digit, then ask where it can go.
Take a digit that already appears several times. Each copy blocks its entire row, its entire column and its entire box. Draw those lines in your head across a box that still needs that digit, and quite often only one square survives.
This is usually called cross-hatching, because you are sweeping lines across a box from two directions. It is the single most productive habit in beginner Sudoku, and on an easy grid it will carry you most of the way to the end on its own.
A practical order of work:
- Start with the busiest digit. Count which digit already appears most often on the grid. It has the most blocking power, so it is the most likely to be forced somewhere.
- Sweep all nine boxes for it before moving on. Finishing one digit everywhere is faster than hopping between digits.
- Every time you place one, sweep again. A new digit adds three new blocked lines, which often unlocks a box you just gave up on.
- Then take the next busiest digit and repeat.
The cell with nowhere left to hide
The other easy pattern runs in the opposite direction. Instead of a digit with one square left, you find a square with one digit left.
Solvers call this a naked single: the cell is not hiding anything, it simply has one candidate. Look for units that are seven or eight cells full; those are where singles live. Whenever you fill a cell, glance at its row, column and box, because you may have just created one.
Between cross-hatching and naked singles you have enough to finish an easy grid completely. Most people never need anything else to enjoy the puzzle.
Pencil marks and the hidden single
When scanning dries up, start writing candidates into the cells. For each empty cell, note every digit that is not already visible in its row, column or box. Our board does this with the notes toggle, or by holding Shift while you type.
Pencil marks cost time, so do not fill in the whole grid at once. Mark a single row, column or box that looks nearly finished and read it carefully. What you are hunting for is a digit that appears in the candidate list of exactly one cell in that unit.
This is a hidden single, and it is the pattern that most often gets people past their first hard grid. The word hidden is the point: the cell looks undecided, and the decision is visible only from the unit's point of view, not the cell's.
Keep your marks honest
Stale pencil marks are worse than none, because you will trust them. Every time you place a digit, rub it out of the candidates of every cell in the same row, column and box. On our board that happens automatically, which is most of the reason the notes toggle exists.
When the grid stops giving
On a harder grid you will reach a point where no square is forced and every cell has two or three candidates. This is normal and it is not a sign that you have made a mistake. Two things to try before anything clever:
- Pairs. If two cells in the same unit have the same two candidates and nothing else — say both can only be 3 or 7 — then between them they use up the 3 and the 7. Neither digit can appear anywhere else in that unit, so cross them off the other cells. Turning on Highlight pairs on the board marks these for you.
- Pointing. If the only places a digit can go inside one box all sit in the same row, then that digit is definitely somewhere in that row inside that box. It therefore cannot be anywhere else along the row, in either of the other two boxes.
Both are the same idea as a hidden single, applied to a group of cells instead of one. And both need accurate pencil marks, which is why the previous section matters.
If you are truly stuck, the most common cause is not a missing technique but a wrong digit placed twenty moves ago. Turn on conflict highlighting, and if the grid is clean, undo back to your last confident position rather than pushing on.
Four habits worth building
- Finish a digit before switching. Sweeping all nine boxes for one digit keeps the blocked lines in your head, where hopping between digits keeps clearing them.
- Re-scan after every placement. A single new digit changes three units. That is where the next answer usually is.
- Mark one unit at a time. Full-grid pencil marks turn a calm puzzle into bookkeeping.
- Stop when it stops being pleasant. The grid is stored in your browser and will wait. No streak breaks, nothing expires.
Difficulty on our board is set by how many digits you start with, from roughly forty-three down to twenty-five. The gap between the easiest and hardest setting is not really about the count — it is about how long you can keep going on scanning alone before pencil marks become unavoidable.
Try it on a real grid
Start on Easy, use nothing but cross-hatching, and turn notes on only when you get stuck. Conflicts and pairs can be toggled from the sidebar.