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Sudoku is not arithmetic; it is fitting things into place. Nothing is ever added or subtracted, and the digits are simply nine symbols that differ from each other. Swap them for letters and the game works the same. Being good at maths and being good at sudoku are separate things — what this asks for is an eye that misses nothing.
No sign-up, no install. The grid above is already yours, and closing the tab leaves you where you were.
What sudoku is
A 9×9 grid filled with the digits 1 to 9. The modern form first appeared in a US puzzle magazine in 1979 under the name Number Place, and took its current name when the Japanese publisher Nikoli ran it again in 1984 — the name means roughly “the digits stay single”. It spread worldwide after a British newspaper began running it in 2004.
Every puzzle must have exactly one solution. With two, it isn't sudoku any more, just blank-filling. Every puzzle built here is checked after each clue is removed to confirm the solution is still unique, and the removal is undone the moment a second one appears.
How it is solved
There are three rules, and all three are the same sentence wearing different faces: nothing may repeat.
- no digit appears twice in the same row
- no digit appears twice in the same column
- no digit appears twice inside the same 3×3 box, the blocks marked off by the heavy lines
The digits that were there from the start cannot be changed; fill the rest without breaking a rule and you're done. It isn't a game of guessing — every cell is settled by logic alone.
Two things to try
One. Start where the grid is already full. Pick a line or box with more than five cells filled and the remaining candidates are few. Count what could go in each empty cell and one with a single option turns up quickly. Filling it thins the neighbouring lines and boxes, and the next cell opens.
Two. Pick one digit and sweep the whole grid with it. Look only at 7, say. If a horizontal band of three boxes already has a 7 in two of them, the third must hold one too — and its home narrows to the row that doesn't have a 7 yet. Following the digit like this is often faster than following the cells.
That is as far as unaided solving goes. Above it sit thirteen more methods with names — pairs, intersections, X-Wing. Each has its own page with an example.
The hint doesn't give the answer away
A hint that ends at “this one is a 7” takes the fun with it and teaches nothing. This sudoku searches the techniques people actually use, easiest first, and tells you why that digit belongs there — something like “in row 3, this is the only cell a 7 can go in”. It lights up the cells the reasoning rests on. Whether to place it is then yours to decide.
Some hints erase candidates instead. “Every place a 4 could go in this box lies on one line, so 4 comes off that line outside the box” — that kind. It is progress every bit as real as filling a cell, and few places bother to say it. There is a page listing the thirteen techniques a line each, and a page per technique working it through with an example.
What was erased, the next hint remembers. A technique that erases candidates achieves almost nothing on its own — the road opens when the next one stands on what the last erased. That is why following hints alone finishes 31 of 40 Hard puzzles and 20 of 40 Expert ones.
Things built to save your hands
- A wrong digit is flagged as you enter it. Mistakes are counted up to three, and at three the grid stops. “Try again” clears only the wrong cells and lets you carry on. If you'd rather not play that way, turn it off in settings and use Check yourself.
- There are six settings. Highlight row, column and box; highlight the same number; highlight duplicates; count mistakes; quick entry (pick a digit first, then just tap cells); auto notes. Leave one on and it stays on next time.
- Placing a digit clears the notes for you. You never hand-erase that digit from twenty cells of the same row, column and box.
- With auto notes on, candidates are recomputed after every move. Candidates you erased with a hint, or switched off by hand, do not come back — they're subtracted from each recomputation.
- The score climbs while you play. A difficulty multiplier times each correctly filled cell, minus mistakes and hints. The formula is written under the board — it is not a number to compare with anyone, only something this browser keeps.
- Matching digits light up together. Select a cell and every cell holding the same digit brightens, along with its row, column and box.
- You can see how many of each digit are left. A digit with all nine placed fades out.
- Undo goes all the way back. Even switching a single note on is undone.
- Pressing the same digit again erases it. The fastest undo when you mistype.
- Pausing covers the board. A pause that stops the clock while the answers stay visible isn't a pause. There is also Restart, which replays the same puzzle from the beginning.
- A new daily puzzle arrives each day. The date is the puzzle number, so everyone solves the same grid. Your browser builds it on the spot rather than asking a server, so there is nothing to log into.
- Best times per difficulty and your streak are kept. The streak counts the daily puzzle only — a number you can raise by tapping one Easy grid a day says nothing.
- Every puzzle has a number. Pass the number along and someone else opens the same grid. “Copy link” builds an address that carries it, and Print puts it on paper.
- Close the tab and pick up where you were. Your position and your notes are still there.
- It opens with no connection. One visit and the browser keeps the puzzles, the guides and the rules. It works underground.
The difficulty tells you what the puzzle needs
What does Hard actually mean? Every time a puzzle is built here it is solved by the hint engine, and graded by the hardest technique that was needed along the way. That is why the screen says things like “Medium · needs hidden singles” — the label is simply what this puzzle asks of you.
One promise follows from that. Up to Very hard, following the hints alone will finish the puzzle, because the grade means “solvable within the thirteen techniques”. Expert is either an advanced technique or a puzzle the thirteen can't finish. When that happens the hint says so and gives you one cell.
It is graded neither by the usual count of remaining clues nor by the number of times the solver backtracked, which is what was tried first. Both disagreed with what people actually feel — of 60 puzzles that backtracked more than 500 times, 23 fell to hidden singles alone.
Every puzzle has exactly one solution. After each clue is removed the uniqueness is rechecked, and the removal is undone the moment a second solution appears.