Pocket Games
Sudoku
Sudoku is not a game about numbers: it is a game about positions, and the digits are just nine symbols that happen to be easy to write. You never add anything up. Here are the rules, the techniques that close almost any grid, and how the Pocket Games version works.
The rules, in three lines
The grid is 9×9, split into nine 3×3 boxes. Some cells are filled in already: those are the givens, and they never change. You fill in every other cell so that each row, each column and each box contains the digits 1 to 9 exactly once.
Everything else follows from that. If a 7 already sits in a row, no other 7 can go in that row. If a box has one empty cell left and the only unused digit is a 4, that cell is a 4. A well-made grid has exactly one solution, and you can always reason your way to it: guessing is never required.
How it works on Pocket Games
Tap a cell and pick a digit from the pad below the grid. Digits that have already been used nine times disappear from the pad, so you stop looking for them.
- Three difficulties. Easy starts with 35 cells filled in, Medium with 28, Hard with 26. It is not just about quantity: it changes which techniques you need. Easy falls to singles alone, Medium also needs locked candidates, Hard adds naked pairs — all explained below.
- Pencil marks. The pencil button switches to notes mode: the digits you write stay small in the corner of the cell, exactly like the candidates you would pencil into a newspaper grid. It is the difference between solving and trying to hold everything in your head.
- One hint per game. Just the one: it reveals a single cell. It is there to unstick you, not to solve the puzzle for you.
- The clock runs, and your best time for each difficulty is saved. Sign in with a Google account and you will find it on your other device, along with the number of grids you have completed.
- The game saves itself. Close it halfway through and you come back to the exact same position, pencil marks included.
- On a computer you can use the keyboard: arrow keys move the selection, 1 to 9 write, Backspace clears.
The techniques that solve almost everything
The naked single. Look at an empty cell and mentally cross out every digit that already appears in its row, its column or its box. If exactly one digit survives, that is your answer. It is the slowest technique to apply everywhere, but it is the one that finishes easy grids.
The hidden single. Same idea, reversed: instead of starting from the cell, start from the digit. Take a box and ask where the 5 can go. The rows and columns crossing that box may already contain 5s, which rules out almost every cell; if only one is left, the 5 goes there — even though other digits could, in principle, also fit that cell. This is the technique that unlocks the most positions, and the first one worth drilling.
Locked candidates. If every possible spot for the 3 inside a box sits on the same row, then that box’s 3 will be somewhere on that row. You still do not know where, but you know enough to remove the 3 from every cell of that row in the other two boxes. This is the first real step up: you deduce something without filling in a single cell.
The naked pair. Two cells in the same row (or column, or box) that can only hold the same two digits, say 2 and 7. You do not know which goes where, but those two digits are reserved: 2 and 7 can be crossed off every other cell in that row. You need it from Hard upwards, and without pencil marks it is essentially invisible.
Mistakes that cost you time
- Guessing. A well-formed grid never requires it. A digit placed on a hunch surfaces twenty moves later, when you can no longer tell which cells depend on it.
- Pencilling candidates everywhere. Filling every empty cell with notes gives you an unreadable grid. Mark them only where you are actually reasoning.
- Only scanning rows. Hard grids almost always break open by reasoning box by box, which is the direction your eye checks least.
- Not updating your notes. Every digit you write invalidates candidates somewhere else. A stale pencil mark is worse than no pencil mark at all.
Frequently asked questions
Do I need to be good at maths?
No. The nine symbols could be letters or colours and the game would be identical: nothing is ever added or multiplied.
How long does a Sudoku take?
An easy grid takes five to ten minutes, a hard one can take twenty or more. The clock is there for you to beat your own record: other people’s times mean nothing, because the grids are different.
Does every Sudoku have a single solution?
The ones generated here do, which is why you can always finish by deduction. A grid with two solutions is, technically, a broken puzzle.
Can I play offline?
Yes. Grids are generated on your device, so the game works with no connection at all; you only need one if you want your records synced.
More games to try
See the full list of games, or open Pocket Games in your browser: they are free and you don’t need an account.