Free online Sudoku puzzles at every difficulty, each with exactly one solution
Sudoku asks you to fill a nine-by-nine grid so that every row, every column and every three-by-three box contains each digit from 1 to 9 exactly once. It requires no arithmetic at all — the digits are just nine distinct symbols. Every puzzle here has exactly one solution and is solvable by logic alone.
Look at the given digits.
Each row, column and 3×3 box must contain 1 to 9 with no repeats.
Find cells where only one digit is possible.
Fill them in and repeat — each entry constrains its neighbours.
The puzzle is complete when every cell is filled.
Easy starts with 45 given digits, medium with 35 and hard with 28. Every puzzle is checked as it is generated so that it has exactly one solution.
Select a cell and enter a digit. A digit that does not match the solution flashes red and adds one to your error count. The puzzle is complete when every cell is correct, and your fastest completion time is saved in this browser.
Constraint propagation and systematic elimination. There is no guessing and no calculation; the whole game is applying one rule consistently and noticing what it forces. Another logic game to try next: Number Guess.
Sudoku is pure constraint logic: every row, column and 3 × 3 box must contain the digits 1 to 9 exactly once, and a well-made puzzle has exactly one solution that can be reached by deduction without guessing.
It also has real mathematics behind it. There are about 6.67 × 10²¹ completed grids, and in 2012 a computer search by Gary McGuire and colleagues proved that no puzzle with only 16 clues has a unique solution — 17 is the minimum.
Scan for naked singles first: a cell where only one digit remains possible. Easy puzzles are often entirely naked singles.
Then look for hidden singles: a digit that can only go in one cell within a row, column or box. This is the single most productive technique.
Use pencil marks once scanning stalls. Writing candidates in is what makes the harder techniques visible.
Never guess. Every puzzle here is solvable logically, so a guess means there is a deduction you have missed.
No. The digits are arbitrary symbols — the same puzzle works with nine letters or nine colours. No arithmetic is involved at any point.
Yes. A puzzle with more than one solution is considered improperly formed, and none are served here.
Not on these puzzles. If you feel you must, there is a technique you have not applied.
There is no good evidence it improves general memory. Large observational studies do link regular mental activity with better cognitive ageing, but that is a different and weaker claim. Play it because it is a good puzzle.
After singles, learn naked and hidden pairs. They unlock most of what people find hard about medium puzzles.