Fill a 9×9 grid so that every row, every column and every 3×3 box holds the digits 1 to 9 exactly once. Each puzzle here is generated in your browser and checked for a single solution before it is handed to you, so there is never a square you have to guess. Difficulty is the number of clues left on the board — 44 for easy, 36 for medium, 30 for hard, 25 for expert. Your board is saved as you play, so closing the tab does not cost you the puzzle.
When the same digit appears twice in a row, column or box, both cells turn red and the clash counter goes up. That is a rule collision in the board you are looking at, not a comparison against the answer.
The small number under each key on the pad is how many of that digit are still missing. At zero the digit is fully placed.
Best times are kept per difficulty. Twenty minutes on expert and four minutes on easy are not comparable numbers.
The board, the clock and your best times live in this browser only (localStorage) and are never sent to a server.
Sudoku has exactly three constraints: each row holds 1–9 once, each column holds 1–9 once, and each 3×3 box holds 1–9 once. No addition, no subtraction, no outside knowledge. Swap the digits for nine colours and the puzzle is unchanged.
That is why sudoku travels. The grid is the same in every language, which is also why this page serves one board to readers in ten of them.
The move that solves puzzles is elimination, not insertion. Look at a cell, cross off every digit already present in its row, its column and its box, and see what survives. When one candidate survives, the cell is settled. Solving is that operation, repeated in a good order.
The first is the naked single. Pick a cell, strike out everything already visible in its row, column and box, and if a single digit remains, write it in. Easy boards are solvable end to end with nothing else.
The second is the hidden single, and it flips the question around. Instead of asking what fits in a cell, ask where a digit fits in a box. If 7 can only go in one square of a box, that square is a 7 even when three other candidates still look possible there.
In practice you alternate. Take the digit that appears most often on the board, sweep the nine boxes asking where it can still go, and you will usually open three or four cells in a single pass — far faster than staring at one stubborn square.
From hard upward, no cell is immediately down to one candidate. The first tool for that is the naked pair. If two cells in a row are both exactly {3, 8}, then no other cell in that row can be 3 or 8. You still do not know which is which, and you do not need to.
The second tool is pointing. If, inside one box, every remaining spot for a 4 sits on the same row, then 4 can be crossed off that row everywhere outside the box. The box has not decided where its 4 goes, but the row has decided that its 4 belongs to that box.
The two run in opposite directions: a pair says these cells own these digits, pointing says this digit owns this line. Expert boards that feel frozen usually thaw when you try both directions on the same region.
If neither works, re-read your notes before reaching for a harder technique. Half of all stuck positions are notes that were never updated after an earlier cell was confirmed.
On this page difficulty is the clue count: 44, 36, 30, 25. Fewer clues means the opening is slower and the region you have to scan before anything is certain is wider.
Clue count is not the whole story. Twenty-five clues bunched into three boxes make an easy puzzle; the same twenty-five spread evenly across all nine make a hard one. Human difficulty tracks the level of technique a board demands, and that correlates only loosely with how many digits you can see.
There is a hard floor on how far clues can be removed. Sixteen clues can never define a unique sudoku — that was settled in 2012 by an exhaustive computer search, leaving 17 as the proven minimum. This page stops at 25 because generation time climbs steeply below that while the felt difference stays small.
For scale: the number of complete grids that satisfy the rules is 6,670,903,752,021,072,936,960. Repeating a board is not something to worry about.
Notes hold the candidates for a cell. Above medium they stop being optional, and the reason is bookkeeping rather than memory: candidates shrink every time another cell is confirmed, and without notes you recompute that shrinking from scratch on every pass.
The discipline is not to write everything. A grid where every empty cell carries five pencil marks is unreadable. Note the cells that are down to two or three candidates, and each time you confirm a digit, erase it from the notes in that row, column and box.
Here, with Notes on, tapping a digit toggles it in the selected cell. Confirming a real digit in that cell wipes its notes, and erasing a cell clears both the digit and the pencil marks.
Puzzles are generated in your browser, one per game. A complete grid is filled first by randomised backtracking, then cells are removed in random order, and after each removal the solver counts how many solutions remain. The cell goes back if a second one appears.
The counter stops at two. Counting all solutions is hopeless on a sparse grid — there can be millions — and two is all a uniqueness test needs. The search always branches on the cell with the fewest candidates, which is what keeps expert generation in the milliseconds rather than the minutes.
Symmetric removal, the pattern that makes newspaper grids look tidy, is deliberately not used. It looks better and it costs consistency: uniqueness breaks before the clue target is reached, so the same difficulty label ends up covering very different boards.
Hints are limited to three. With unlimited hints the fastest route through a board is to press the button 56 times, and the clock stops meaning anything. The cell a hint opens is the one with the fewest candidates, which is usually the cell you were about to solve anyway.
The board in progress, the clock and the per-difficulty best times stay in this browser. Nothing is uploaded, which is also why there is no global leaderboard — and why clearing site data clears your records.
Fill the 9×9 grid with the digits 1 to 9 so that no digit repeats within a row, within a column, or within a 3×3 box. That is the whole rule set — there is no arithmetic and nothing to guess.
Not here. While a puzzle is built, every removed cell is tested by counting solutions, and the cell is put back the moment a second solution appears. Every board you get therefore has exactly one answer.
The number of clues you start with: 44, 36, 30 and 25. Fewer clues means fewer cells you can settle immediately, so you have to scan more rows and boxes before a single square becomes certain.
As soon as a cell has two or three candidates you cannot decide between. From hard upward, keeping candidates in your head means recomputing them every time another cell is filled, and that is where most mistakes come from.
Yes. Tap a cell, tap a digit, and the board scales to the screen. Nothing is installed, no account is needed, and the puzzle is saved so you can leave mid-solve and come back to it.
What this tool bases its numbers on, and how far those numbers go.
A complete grid is built by randomised backtracking. Cells are then removed in random order and a removal is kept only while the solver still finds exactly one solution; solution counting stops as soon as a second is found.Expert leaves 25 clues, so you solve 81 − 25 = 56 cells. A board can end up with slightly more than the target when uniqueness breaks before the count is reached, because that cell is restored.