How to play Sudoku

Everydoku has Sudoku in five sizes, from 4×4 to 16×16. The rule is the same at every size; bigger boards simply have more digits and more cells.

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There is only one rule

Fill the empty cells so that every row, every column and every box (the areas inside the thick lines) contains each digit exactly once. On a 9×9 board, each row, column and box holds 1 to 9 once.

The digits printed at the start (the givens) cannot be changed.
Every board has exactly one solution, so you never need to guess.
No arithmetic is involved. The digits are just distinct symbols.

Board sizes

Each size has its own box shape and set of digits. Numbers above 9 are written as A to G (A=10, B=11 … G=16) so that every cell holds a single character. Turn on “Numbers above nine” in settings to use 10–16 instead.

Size	Box	Digits	Difficulties
4×4	2×2	1–4	Tutorial
6×6	2 rows × 3 columns	1–6	Easy, Normal
9×9	3×3	1–9	Easy, Normal, Hard, Expert
12×12	3 rows × 4 columns	1–9, A–C	Normal, Hard
16×16	4×4	1–9, A–G	Normal, Hard, Master

If you are new, learn the rule on the 4×4 tutorial, move through 6×6 and settle into 9×9. 12×12 and 16×16 are long boards that take from twenty minutes to well over an hour.

How difficulty is set

The number of empty cells sets how long a board takes.
The techniques required set how hard it is. Easy boards need only the first technique below; Hard boards need the candidate-eliminating ones; Expert and Master boards go beyond all four.

Every board was generated in advance and checked by computer: it has a single solution and really needs the techniques its level promises. There are 5,000 boards per size and level (1,000 for 4×4).

Four basic techniques

These are also the steps the hint button explains. Press it once to see the reason, press it again to fill the cell.

1. Naked single

Look at the blue cell (row 1, column 3). Its row already has 1 and 2, and its column has 4. With 1, 2 and 4 ruled out, 3 is the only digit left for this cell.

Start from one cell and cross out every digit already in its row, column and box. When one digit remains, that is the answer. Easy boards can be finished with this alone.

2. Hidden single

Where can 1 go in row 1? The empty cells are in columns 1 and 2, but column 1 already has a 1 (orange). So the 1 of row 1 can only go in column 2.

This time, start from a digit: “where can 1 go in this row (or column, or box)?” If only one place is left, that is the answer, even when the cell itself still has several candidates. Normal boards rely on it.

3. Locked candidates

Suppose that inside one box, every cell that could hold a 5 lies on the same row. The box’s 5 must then be somewhere on that row, so 5 can be removed from the rest of that row outside the box. It does not fill a cell by itself, but it narrows the candidates until the next move appears. Hard boards need it.

4. Naked pair

If two cells in a row (or column, or box) have exactly the same two candidates, say {2, 7}, those two cells will take the 2 and the 7 between them. So 2 and 7 can be removed from every other cell in that row. Hard boards need this too.

Tools in Everydoku

Notes: with notes on, digits go into a cell as small candidates. Placing a digit clears it from the notes in the same row, column and box. On 12×12 and 16×16 the notes show as dots inside the cell, and the notes bar beside the board (below it on phones) lists them at full size.
Auto notes: 12×12 and 16×16 only. Fills the current candidates of the selected cell, so big boards stay manageable without filling the whole grid for you.
Hints: three per board. They explain the next possible move and why, and point out a wrong digit first if there is one.
Pause: the board is covered while paused, so the clock cannot be stopped to study the grid.
Resume: every move is saved on your device. Close the tab and come back to the same board.
Keyboard: on a computer, use the number keys (the numeric keypad too), the arrow keys, Backspace, and Space to toggle notes.

Daily puzzle and friend challenges

The daily puzzle is one 9×9 board that opens every day at midnight UTC; everyone gets the same board on the same day. Your streak and a monthly calendar show the days you solved it.

After solving, Challenge a friend sends the same board and your time as one link. Your friend’s game shows your time next to the clock.

Tips for beginners

Start with rows or boxes that are nearly full. The fewer digits are missing, the faster they settle.
Pick one digit and scan the whole board for it. The small number under each key on the pad shows how many of that digit are still missing.
When stuck, use notes. Written candidates make pairs and locked candidates easy to spot.
Still stuck? Press hint once and read the reason. Next time you will find it yourself.

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