How to play Sudoku

One player, one grid of eighty one cells, and nine numbers to place in every row, every column and every box.

Players: 1
What you need: A grid of nine by nine, a pencil, and here a number pad
How long: Five minutes for an easy puzzle, up to an hour for an expert one

What Sudoku is

Sudoku is a puzzle for one. A grid of nine cells by nine comes with some numbers already in it, and you fill the rest so that every row, every column and every box of three by three holds each number from 1 to 9 exactly once.

There is no guessing in a well made puzzle. Every puzzle here has one answer and only one, and it can be reached by reasoning from the numbers you can see, one cell at a time.

An easy puzzle. The numbers printed in it are the givens, and they never change.

Rows, columns and boxes

The grid has nine rows across and nine columns down. The thicker lines cut it into nine boxes, each three cells by three.

Every cell belongs to one row, one column and one box, and the rule is the same for all three: no number twice. So a number already in a cell’s row, column or box cannot go in that cell.

That one rule is the whole game. Everything else, every way of finding a number, comes out of it.

A row, a column and a box. Each of the twenty seven holds the nine numbers once.

Playing it here

Tap a cell to pick it. Its row, its column and its box light up, and so does every other cell holding the same number.
Tap a number on the pad to write it in the cell. Tap the same number again to take it out.
Tap Pencil to write small notes instead, of the numbers a cell could still take. Tap it again to go back to writing.
Erase clears the picked cell. Hint puts one right number down and says on the rail why it goes there.

Each number on the pad shows how many of it are still to place, and goes quiet once all nine are down.

On a computer the keys work too: 1 to 9 write, Backspace erases, the arrows move the pick, and N takes the pencil out or puts it away. A Nepali keyboard’s १ to ९ work the same.

The only place for a number

Pick a box and a number it does not have yet, and ask where in the box that number could go. Each copy of it in a row or column crossing the box rules out the cells of that line.

When only one cell is left, the number goes there, whatever else that cell could hold. This is the first thing every solver looks for, and most of an easy puzzle is nothing else. It works for a row and a column too.

The 8 in the third row and the 8 in the fourth column leave one place for an 8 in this box.

The only number for a cell

Turn it round and start from a cell. Count the numbers already in its row, its column and its box together. If eight different numbers are there, the ninth is the one the cell must hold.

This is harder to see than it sounds, because it means reading three lines of the grid at once, which is why the hints call it the second way and not the first.

Its row holds 1, 2 and 3, its column 5, 6 and 7, and its box 8 and 9. Only 4 is left.

Pencil marks

When neither way finds a number, write down what each cell could still be. Those small numbers are pencil marks, and a cell with only one left is a cell you have solved.

Here a mark comes off by itself when the number it marks is written anywhere in the cell’s row, column or box, so the notes stay honest as you go.

Harder puzzles are solved in the marks rather than in the grid: two cells of a box that can only take the same two numbers, for one, keep those two out of every other cell of the box.

The numbers each empty cell of a box could still take, written small.

Mistakes

With mistakes shown, which is how a table starts, a wrong number turns red the moment it goes in and counts as a mistake. A right number stays black and is locked in.

With mistakes hidden nothing is checked until the grid is full, the way a puzzle on paper is played. If something is wrong then, the rail says so and you look for it.

Either way, two of the same number in one row, column or box are washed red, because anybody can see that without knowing the answer. A mistake limit of three ends the puzzle at the third.

Hints

A hint does what a good solver would do next: it finds the easiest number that can be placed and places it. The rail says which way found it, the only place in a box, row or column, or the only number for a cell.

If a number on the grid is wrong, the hint puts that right first, because reasoning from a wrong number leads nowhere. Hints are counted on the sheet at the end, and nothing else.

Easy to expert

A level is not how many numbers are given but what it takes to find the rest.

Easy: about forty numbers given, and the only place for a number is always there to be found.
Medium: about thirty given. Still nothing but the two ways above, with fewer clues to find them from.
Hard: somewhere the two ways run out and pencil marks are needed, at least once.
Expert: the marks have to be worked, with pairs, triples or the shape called an X-wing.

Every puzzle here can be solved by reasoning alone, at every level, which is also what lets a hint always say why.

Today’s puzzle

Each day has a puzzle at every level, and it is the same puzzle for everybody in Nepal that day. The day turns at midnight in Kathmandu, wherever you are.

Pick A new puzzle on the game page for one made just for you. Your best time at each level, with no hints, is kept on your account, so every phone you sign in on shows it.

Nepali numerals

The numbers can be printed in English numerals or in Nepali ones, on the grid, on the pad and on the clock. A Nepali page starts in Nepali numerals.

One row in English numerals.

The same row in Nepali numerals.

The last key on the pad swaps between the two, and so does the menu. The puzzle is the same whichever you read it in.

Settings

Set on the game page before you start.

Setting	Standard	What it changes
Level	Easy	How hard the puzzle is. See Easy to expert above.
Puzzle	Today’s puzzle	Today’s puzzle at the level, or a new one of your own.
Mistakes	Shown as you go	Whether a wrong number is marked at once or only when the grid is full.
Mistake limit	No limit	Three mistakes end the puzzle. Only counted while mistakes are shown.

The words

Cell: One of the eighty one squares, which holds one number.
Row, column, box: The twenty seven groups of nine cells that each hold 1 to 9 once.
Given: A number the puzzle comes with. It cannot be changed.
Pencil mark: A small note of a number a cell could still take.
Single: A number that has only one place left, or a cell that has only one number left.

Every rule on this page is the one the app plays by. Change any of them on the game's own page before you start.