How to play Sudoku: the rules and your first moves
Sudoku is a logic puzzle on a 9×9 grid. You never need to guess and you never need maths: every digit can be worked out from the ones already there.
The one rule
Fill the grid so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. That's it. The puzzle starts with some digits already placed (the clues), and there is exactly one way to complete it.
The grid has 81 cells in 9 rows, 9 columns and 9 boxes. Each row, column and box is called a unit, so every cell belongs to three units, and each of them needs a different digit in that cell.
Naming cells
Guides (this site included) name a cell by its row and column: R4C3 is row 4, column 3, counting from the top-left. Boxes are numbered 1 to 9, left to right and top to bottom, so box 5 is the centre.
Your first move: hidden singles
Take one digit, say 5. Look at a box that doesn't have a 5 yet. Every 5 already on the board rules out its whole row and column. If only one empty cell in the box is left, the 5 must go there. That's a hidden single, and it's the move that solves most of an easy puzzle.
Sweep the digits 1 to 9 this way, box by box, then do the same for rows and columns.
Your second move: naked singles
Now look at it from the cell's side. For an empty cell, list which digits its row, column and box already contain. If eight of the nine are taken, the last one is the answer. That's a naked single.
Pencil marks
Once the singles run out, write the possible digits (candidates) small in each empty cell, and rub them out as you place digits. Every technique after the singles works on these marks: naked pairs, pointing pairs and the rest. Most "stuck" moments come from a mark that should have been removed.
Three habits that help
- Never guess. If you can't prove a digit, it isn't time to place it. A guess that works teaches you nothing.
- Go back to the singles after every new digit. Each placement can open up a new single somewhere else.
- Learn one technique at a time. Puzzles rated by technique, like Ninebloom's levels, let you practise exactly the move you're learning. How the levels work.
Questions
Do you need maths to play Sudoku?
No. The digits are just nine different symbols; you never add them up. Classic Sudoku is pure logic. (Killer Sudoku is the variant that adds sums.)
Is there always only one answer?
In a proper puzzle, yes. A well-made Sudoku has exactly one solution and can be solved by logic alone, without guessing. Every Ninebloom puzzle is checked for this.
Where should I start on a new puzzle?
Pick a digit that already appears often, say a 1, and for each box without a 1, check which cells the existing 1s rule out. When only one cell is left, that's where it goes.