Sudoku
Sudoku is a 9 by 9 number puzzle where every grid is built to have exactly one solution. Fill numbers, conflicts turn red instantly. Free, no sign-up.
In Sudoku, fill the grid so that every row, every column and every 3 by 3 box contains the digits one to nine exactly once. Tap an empty square, then tap a number along the bottom to place it.
about 18.7 KB
Works offline after the first visit. No install, no sign-up.
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How to play Sudoku
Placing and removing numbers
Tap an empty square to select it, then tap a digit to place it. Tapping the same digit again removes it, so there is no separate eraser to find.
Every puzzle has exactly one solution
A full grid is built first, then squares are taken away one at a time — and after each removal the puzzle is re-solved to count how many answers remain. A square is only left blank if the puzzle still has exactly one solution, so a grid with two possible answers can never reach you.
The digits you were given cannot be changed
The starting numbers are fixed. Tapping one selects it so matching numbers light up, but it cannot be overwritten, so the puzzle cannot be edited into an unsolvable state.
Conflicts show up at once
As soon as a digit repeats in a row, a column or a box, both squares turn red. That catches a wrong entry the moment it happens instead of twenty minutes later.
Selecting a square helps you scan
Selecting any square shades its row and column, and selecting a filled square lights up every other square holding the same digit, so it is easy to see where that digit still has to go.
Sudoku Tips and Strategy
Scan for naked singles
A naked single is a cell with only one digit left in it, because the other eight are already used in its row, column or box. Entering those first cleans up three lines at once and is faster than hunting for clever patterns. Re-scan after every entry, since each one changes what is legal nearby.
Hidden singles open stuck grids
If a row, column or box already holds eight digits, the ninth cell must take the one missing digit, even if nothing else points at it. This needs no chain of reasoning, which is why it is often what a hard grid is waiting for. Search along lines rather than cell by cell; scanning the boxes diagonally is quickest.
Eliminate along the box
Once singles dry up, place a digit in a box and check where it can still go inside it. Usually one row or one column is left over, and putting the digit there removes it from that whole line. Two cells sharing a row and column but holding different digits are an XY-Wing: one of them must take a third.
Sudoku — Frequently Asked Questions
Can a puzzle have more than one solution?
No. Squares are only removed while the puzzle still has exactly one solution, and that is checked after every single removal rather than assumed.
Is there a hint button?
There is no hint button. The highlighting is the help: wrong entries turn red on their own and matching digits light up when you select a square.
What counts as finishing?
All 81 squares filled with nothing in conflict. A grid that is full but wrong does not count as finished.
Does it need installing or an account?
No. It runs in the browser on a phone, tablet or computer, works offline after the first visit, and your best result stays on this device.
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