The only rule
Fill the grid so that every row, every column and every 3×3 box contains each of the digits 1 to 9 exactly once. That is the whole game. There is no arithmetic and nothing to add up — the digits are just nine distinct symbols.
A properly made puzzle has exactly one solution and can be reached by reasoning alone. If you find yourself guessing, either the puzzle is faulty or there is a move you have not spotted.
Your first placements
Pick a digit that already appears several times on the board — say 5. For each box that has no 5 yet, look at the rows and columns that pass through it. Every row that already contains a 5 rules out three cells of that box; so does every such column. Often only one cell survives, and that is where the 5 goes.
Only one cell in the marked box can take this digit — every other empty cell in the box is ruled out by the same digit in a crossing row or column.
This is called a hidden single, and on an easy puzzle it will carry you a long way.
The other question
Instead of asking "where can this digit go?", ask of a single empty cell: "which digits could go here?" Cross off everything that already appears in its row, its column and its box. If exactly one digit survives, it belongs there. That is a naked single.
Those two questions — one about a digit, one about a cell — solve every easy puzzle on this site.
Pencil marks
When scanning stops producing placements, start writing the possible digits into the empty cells. Small marks in the corners are fine. On paper, mark up one box or one line at a time rather than the whole grid; on screen, the notes button does the bookkeeping for you and clears a candidate from every affected cell when you place a digit.
Four mistakes beginners make
- Guessing. A guess that turns out wrong costs more time than any move you could have found honestly.
- Marking up everything first. Filling all 81 cells with candidates before making a placement is slow, and most of it is wasted work.
- Stale pencil marks. A candidate you forgot to erase will send you down a false path. Clear as you go.
- Only scanning rows. Boxes and columns hide just as many singles, and most stuck positions are stuck because one direction was never checked.
Where to go next
Play an easy puzzle and use the hint button whenever you are stuck — it names the technique it used and points at the cell, which is how the moves become familiar. When easy puzzles stop being interesting, the strategy guides take you through the rest in order.
Common questions
What are the rules of Sudoku?
Every row, every column and every 3×3 box must contain each digit from 1 to 9 exactly once. That is the only rule, and no arithmetic is involved.
Is there always only one solution?
In a properly constructed puzzle, yes. Every grid on this site is verified to have exactly one solution before it is published.
Do I need to guess in Sudoku?
No. A well-made puzzle can always be solved by reasoning. If you feel forced to guess, there is a move on the board you have not found.
How long does it take to learn Sudoku?
The rule takes a minute and the first two techniques take about five. Most people finish their first easy puzzle within half an hour of starting.
Is Sudoku good for your brain?
It is good practice at systematic attention and pattern recognition, and it is enjoyable, which matters more. Claims that puzzles prevent cognitive decline go well beyond what the evidence supports.