Sudoku
About Sudoku
Sudoku is a logic puzzle played on a nine by nine grid divided into nine three by three boxes. Some numbers are given at the start, and your task is to fill every remaining cell so that each row, each column and each three by three box contains the digits 1 to 9 exactly once.
Despite involving numbers, there is no arithmetic in Sudoku at all — the digits are just nine distinct symbols, and the puzzle would work identically with nine colours or nine letters. It is pure constraint satisfaction, which is why it appeals to people who dislike maths puzzles as well as people who love them. A properly constructed puzzle has exactly one solution reachable by logic alone, so guessing is never necessary; if you find yourself guessing, there is almost always a deduction available that you have not spotted yet. This version offers three difficulty levels, pencil marks for candidates, mistake checking and a timer.
How to Play Sudoku
- Pick a difficulty and select any empty cell in the grid.
- Type a digit from 1 to 9, or use pencil marks to note the candidates you are still considering.
- Make sure no digit repeats within any row, column or three by three box.
- Fill every cell to finish. The timer tracks how long the solve took.
Tips & Strategy
- Scan for "naked singles" first — cells where only one digit can legally go. Easy puzzles are often solvable using nothing else.
- Use pencil marks properly on harder grids. Writing candidates into cells is what makes patterns like pairs and hidden singles visible.
- Never guess. Every valid puzzle is solvable by logic, so a guess usually means backtracking through a corrupted grid later.
Frequently Asked Questions
Not at all. There is no arithmetic in Sudoku — the digits are just nine distinct symbols and the puzzle is pure logic.
A properly constructed Sudoku has exactly one solution reachable by logic alone. That uniqueness is what makes deduction, rather than guessing, the correct approach.
They let you note which digits are still possible in a cell. On medium and hard grids they are essential, because most advanced techniques work by spotting patterns among candidates.
Re-scan each box for digits that can only go in one place, then look for pairs of cells in a unit sharing the same two candidates. Being stuck almost always means a deduction was missed, not that the puzzle is unfair.