Do you need to be good at math to play Sudoku?

No. Sudoku uses digits only as symbols. You never add, subtract or multiply; you only check which digits are already in a row, column or box.

Is guessing allowed in Sudoku?

Nothing stops you, but a well-made puzzle never requires it: every digit can be worked out by logic. If you do guess, keep notes so you can undo the guess cleanly if it leads to a conflict.

Can a Sudoku have more than one solution?

A well-made puzzle has exactly one. Puzzles with very few clues can have more, so every puzzle on this site is checked by a solver as it is made and always has exactly one solution.

What is the fewest number of clues a Sudoku can have?

17. An exhaustive computer search published in 2012 showed that no standard 9×9 Sudoku with 16 or fewer clues has a single unique solution.

What are notes or pencil marks in Sudoku?

Small digits written in an empty cell to record which candidates are still possible there. They are the key to harder puzzles, because once candidates are written down, patterns such as pairs become visible.

Is there a time limit or a mistake limit?

Not on LetsPlayCards.Online. The timer and the mistake counter are there for your own tracking, and a puzzle never ends because of them.

Where does the name Sudoku come from?

The puzzle first appeared in American puzzle magazines in 1979 under the name Number Place. The Japanese publisher Nikoli popularized it in the 1980s and gave it the name Sudoku, short for a Japanese phrase meaning roughly "the digits must be single".

Sudoku rules

Everything a beginner needs: the three rules, the terms players use, a first solving routine and how our free online puzzles work.

The goal

Fill every empty cell of the 9×9 grid with a digit from 1 to 9 so that each row, each column and each of the nine 3×3 boxes contains every digit exactly once. The puzzle starts with some digits already in place, and you work out the rest by logic.

Sudoku rules for beginners

Sudoku has just three rules:

Rows: Each row contains the digits 1 to 9, with no repeats.
Columns: Each column contains the digits 1 to 9, with no repeats.
Boxes: Each 3×3 box contains the digits 1 to 9, with no repeats.

That is the whole game. There is no arithmetic: the digits could just as well be letters or colors, and adding them up never helps. Because every cell sits in one row, one column and one box at the same time, each digit you place rules out that digit for up to 20 other cells. A well-made Sudoku has exactly one solution, and you can reach it without guessing by following those eliminations.

The grid and key terms

Cell: One of the 81 small squares. Each holds a single digit.
Row: Nine cells running across. The grid has nine rows.
Column: Nine cells running down. The grid has nine columns.
Box: One of the nine 3×3 blocks outlined with thicker lines. Also called a block or region.
Givens: The digits printed at the start, also called clues. They cannot be changed.
Candidates: The digits that could still go in an empty cell. Writing them in small is called taking notes or pencil marking.

How to play Sudoku: a first walkthrough

New players often stare at the grid waiting for an answer to jump out. A simple routine works better:

1
Pick a digit and scan. Choose a digit that already appears several times, say 5. For each box that has no 5 yet, look along the rows and columns that pass through it. Any row or column that already holds a 5 rules out its cells in that box. This is called cross-hatching.
2
Place it when one cell is left. If cross-hatching leaves only one empty cell in the box where a 5 can go, that cell must be the 5. Fill it in and move on to the next box, then the next digit.
3
Check what’s missing. Look for rows, columns and boxes with only one or two empty cells. If a row is missing only a 3 and a 7, check which of its two empty cells can’t take the 3 because of its column or box.
4
Use notes when scanning runs dry. On harder puzzles you will reach a point where no digit can be placed directly. Then write the possible candidates into each empty cell and look for cells with only one candidate left. Every digit you place gives you new information, so go back to step 1 and repeat.

Worked example

The top-left box needs a 5. Rows 1 and 2 already have a 5 further along, so this box’s 5 must sit in row 3. Column 1 also has a 5 lower down, which leaves the row 3 cells in columns 2 and 3. If one of those is already filled, the 5 goes in the other. No guessing, only elimination.

Playing Sudoku on LetsPlayCards.Online

Difficulty levels

Choose a level above the grid. The level sets how many clues the puzzle starts with; fewer clues means more cells to work out and usually more advanced techniques.

Level	Clues	Cells to fill
Easy	45	36
Medium	35	46
Hard	28	53
Expert	22	59

Picking a different level starts a fresh puzzle, and New game gives you another puzzle at the same level. If you have already made a move in the current puzzle, the game asks first.

Controls

Action	Mouse or touch	Keyboard
Select a cell	Click or tap it	Arrow keys move the selection
Enter a digit	Number pad by the grid	1 to 9
Erase a cell	Erase button	Backspace, Delete or 0
Notes on or off	Notes button	N
Reveal one cell	Hint button	None

Helpful features

Highlighting: The selected cell’s row, column and box are shaded. Selecting a filled cell also highlights every other cell with the same digit, which makes cross-hatching quick.
Notes: With Notes on, pressing a digit adds or removes it as a small pencil mark in the selected cell. Placing a full digit clears that cell’s notes and removes the digit from the notes in its row, column and box.
Conflicts: If the same digit appears twice in a row, column or box, both cells are underlined in red.
Mistake counter: Goes up whenever you enter a digit that doesn’t match the puzzle’s solution. There is no limit; it is feedback, not a way to lose.
Hint: Select an empty or wrong cell and press Hint to fill in the correct digit. Otherwise Hint fills the empty cell with the fewest possible digits left. A hinted digit is locked and shown in gray with a folded corner, so you can tell it from the givens. Hints never count as mistakes.
Timer: Starts with each new puzzle and stops when you finish. Your best time for each level is kept on this device.

Good to know

Every puzzle is generated fresh by removing clues from a completed grid, and a clue only comes out if the puzzle still has exactly one solution afterwards. Even on Expert there is always one answer, so the mistake counter only counts digits that are really wrong.

How you win

The puzzle is solved when all 81 cells are filled and no row, column or box contains a repeated digit. Online the timer stops and the game shows your finishing time next to your best for that level.

Start on Easy, practice the scanning routine, and move up a level when it feels comfortable. When you get stuck, the Sudoku strategy guide covers the next techniques to learn.

Sudoku FAQ

Do you need to be good at math to play Sudoku?: No. Sudoku uses digits only as symbols. You never add, subtract or multiply; you only check which digits are already in a row, column or box.
Is guessing allowed in Sudoku?: Nothing stops you, but a well-made puzzle never requires it: every digit can be worked out by logic. If you do guess, keep notes so you can undo the guess cleanly if it leads to a conflict.
Can a Sudoku have more than one solution?: A well-made puzzle has exactly one. Puzzles with very few clues can have more, so every puzzle on this site is checked by a solver as it is made and always has exactly one solution.
What is the fewest number of clues a Sudoku can have?: 17. An exhaustive computer search published in 2012 showed that no standard 9×9 Sudoku with 16 or fewer clues has a single unique solution.
What are notes or pencil marks in Sudoku?: Small digits written in an empty cell to record which candidates are still possible there. They are the key to harder puzzles, because once candidates are written down, patterns such as pairs become visible.
Is there a time limit or a mistake limit?: Not on LetsPlayCards.Online. The timer and the mistake counter are there for your own tracking, and a puzzle never ends because of them.
Where does the name Sudoku come from?: The puzzle first appeared in American puzzle magazines in 1979 under the name Number Place. The Japanese publisher Nikoli popularized it in the 1980s and gave it the name Sudoku, short for a Japanese phrase meaning roughly "the digits must be single".

Ready to play?

Four levels, from 45 clues down to 22, and every puzzle has exactly one solution.

Play Sudoku