How to play Sudoku

Fill the grid with numbers so that no number repeats in any row, column or box.

The rules, step by step

Some squares already have numbers. These are the clues, and they cannot be changed.
Your job is to fill every empty square. In a 9 by 9 grid you use the numbers 1 to 9. In a 6 by 6 grid, 1 to 6. In a 4 by 4 grid, 1 to 4.
Each number can appear only once in every row, once in every column, and once in every box. A box is a smaller block marked by thicker lines.
Tap an empty square, then tap a number on the pad (or type it on a keyboard).
You never need to guess. There is always a square where only one number fits. Find it, fill it, and look for the next.
If you put in a wrong number, the square turns red straight away, so you can fix it at once. If you would rather not be told, untick the box called Show mistakes as I go.
Press Check to see if any numbers are wrong. Press Hint if you are stuck, and one square will be filled in for you.

Look at the top row. It already has a 1 and a 4, so the two empty squares must be 2 and 3.

Now look at the second empty square in that row. Below it, in the same column, there is already a 3. So it cannot be a 3. It must be 2.

That leaves 3 for the last square in the top row. One square solved, and the next one gets easier.

Start with the row, column or box that has the most numbers in it. There are fewer choices there.
Look for a number that appears many times already. Where can its last copies go?
The 6 by 6 grid is a good place to begin, and the 4 by 4 is good for learning the idea.
There is no timer. Take as long as you like.