Play Sudoku Online
The classic 9×9 number puzzle with 6 difficulty levels from Easy to Extreme. Notes mode, undo & error checking — all free.
Created by Brian Hamilton
Select a cell and enter a number
How to play Sudoku
Fill the grid so that every row, every column and every 3×3 box holds the digits 1 to 9 exactly once.
- Grey digits are given and cannot be changed.
- Easy and Medium boards can always be finished using single candidates alone.
- Harder levels are graded by the technique they require, not by how many digits they start with.
- Every puzzle has exactly one solution.
Controls: tap a square and type or tap a digit. Notes lets you pencil in candidates, Undo steps back, and Check highlights mistakes.
Clue count is not difficulty
Almost every Sudoku app sells its difficulty levels on the number of starting digits. Fewer clues, harder puzzle. It is an intuitive idea and it is close to worthless, and we can show that from this site's own generator.
Until recently the six levels here were defined purely by clue count, the hardest of them promising 17 to 21 starting digits. We measured what the generator actually produced. The answer was uncomfortable.
| Level | Clues promised | Clues delivered | Solvable with singles alone |
|---|---|---|---|
| Expert | 24–26 | 24–26 | 15 of 30 |
| Master | 22–23 | 22–27 | 16 of 30 |
| Extreme | 17–21 | 22–27 | 9 of 30 |
Three separate difficulty levels producing the same board. Master and Extreme delivered an identical spread of clues, and half of all Expert boards could be finished using nothing but single candidates — the first technique anybody learns.
Why the 17-clue promise was impossible
The Extreme setting asked for 17 clues because 17 is a famous number. In 2012 Gary McGuire, Bastian Tugemann and Gilles Civario proved by exhaustive computer search that no Sudoku with 16 clues has a unique solution, making 17 the true floor.
But almost no puzzle reaches it. Generators build a full grid and then remove digits one at a time, keeping a removal only when the solution stays unique. We ran that process to exhaustion — removing until not one further digit could come out — ten times:
Every run stopped between 22 and 25 clues, averaging 23.9. A second pass over the grid never removed a single extra digit: one pass already reaches the floor. Random removal simply does not get near 17, and no amount of retrying changes that. The 17-clue grids exist, but they have to be hunted for specifically.
So the bottom three difficulty levels were all asking for something unreachable, and all quietly settling on the same answer: about 24 clues.
What actually makes a Sudoku hard
Not how many digits you start with — which techniques the grid forces you to use. A 24-clue board that falls to hidden singles is easier than a 30-clue board that needs an X-Wing, however the clue counts look.
The most useful technique to learn first is the hidden single, because beginners routinely walk past them. A naked single is a square with only one possibility left, and it is easy to spot. A hidden single is the reverse: the square may have four or five candidates of its own, but it is the only square in its box where some digit can go.
Scanning for naked singles means asking “what can go in this square?”. Scanning for hidden singles means asking “where can this digit go?” — a different sweep, and the one that unlocks most stalled grids.
The six levels, rebuilt
Difficulty here is now graded on the hardest technique a board actually requires. Every generated puzzle is run through a solver that applies techniques in order of difficulty, and a board is thrown away and regenerated unless it lands in the right band.
| Level | Clues | Hardest technique needed |
|---|---|---|
| Easy | 38–45 | Naked and hidden singles |
| Medium | 31–37 | Naked and hidden singles |
| Hard | 27–29 | Locked candidates, pairs |
| Expert | 24–26 | Locked candidates, pairs |
| Master | 23–25 | Beyond triples and X-Wing |
| Extreme | 22–23 | Beyond triples and X-Wing |
Easy and Medium differ in density, not technique — both fall to singles, but Medium makes you hunt longer. Hard and Expert both require you to reason about where a digit cannot go. Master and Extreme need something past the standard intermediate repertoire.
One honest caveat on those top two. Our grader tests singles, locked candidates, pairs, triples and X-Wing. A board it cannot crack needs something beyond that list — which for an experienced solver often means an XY-Wing, a Swordfish or a colouring argument rather than genuine guesswork. We can prove those boards resist the standard repertoire; we do not claim they are unsolvable by pure logic.
A worked board
Where solvers get stuck
Scanning squares instead of digits. Working out what fits in each empty square finds naked singles only. Pick a digit and sweep the whole grid asking where it can go in each box — that is a different search, and it is the one that breaks most deadlocks.
Filling in every pencil mark. Marking all candidates in all squares creates a wall of numbers that is slow to maintain and easy to get wrong. Mark only the squares you are actively reasoning about.
Guessing when stuck on Easy or Medium. On those settings a guess is never needed — every board is verified to fall to singles alone. If you are stuck, there is a hidden single somewhere you have not swept for.
Two things you can check about these puzzles
Every puzzle has exactly one solution. Digits are removed one at a time and a removal is kept only when a solver confirms a single solution still remains. Because no step that would create a second answer is ever accepted, ambiguity cannot appear. 120 boards across all six levels were re-checked against a separate counter written from the rules alone; all 120 were unique.
Difficulty is verified, not estimated. Each board is graded by the technique it demands, and regenerated if it grades wrong. On the twenty boards tested per level, all 120 landed in their intended band — where previously half of Expert was singles-only.
Sudoku, Number Place and the rest
Sudoku was invented by Howard Garns, an American architect, and published in 1979 as Number Place. It was picked up in Japan by Nikoli in 1984, renamed, and returned to the West as a craze in 2004. Nikoli added the two conventions that define the modern puzzle: symmetrical clue placement, and a cap on the number of givens.
If the plain grid starts to feel routine, the variants change one rule each. Jigsaw Sudoku replaces the 3×3 boxes with irregular regions, Killer Sudoku removes the starting digits and adds cage sums, and Suguru drops rows and columns entirely.
More number-placement puzzles
If you like filling grids with digits, try these: