How to Play Sudoku

Learning how to play sudoku takes about five minutes. This guide explains the rules, walks through a real puzzle one move at a time, and shows how to use the buttons on our board.

The Goal of the Game

A Sudoku puzzle is a square grid of 81 small squares, arranged in 9 rows and 9 columns. Thick lines divide it into nine boxes of 3×3 squares. When the game starts, some squares already contain numbers; these are called the givens. You fill in the rest. When every square holds a number and all the rules are satisfied, the puzzle is solved.

Each puzzle has exactly one correct answer. That is what makes Sudoku a game of logic rather than luck: with enough care, you can always work out the next number from the ones you already have.

The Basic Rules of Sudoku

There are only three rules, and they all say the same thing in a different direction:

  1. Each row must contain the numbers 1 to 9, each exactly once.
  2. Each column must contain the numbers 1 to 9, each exactly once.
  3. Each 3×3 box must contain the numbers 1 to 9, each exactly once.

That is all. There are no special squares, no scoring and no time limit unless you want one. People sometimes ask for the sudoku rules in more detail, but the three lines above really are the whole game.

The three kinds of units: a row (here row 2), a column (column 6) and a box (the bottom-left box). Each one must end up holding 1 to 9 exactly once.

A Few Useful Words

  • Square or cell: one of the 81 small spaces.
  • Row, column and box: the three kinds of groups of nine squares. Players often call them units.
  • Given: a number printed at the start. Givens never change.
  • Candidate: a number that could still go in an empty square. Candidates are what you write as pencil notes.

A Worked Example: Solve Sudoku Step by Step

The best way to learn how to solve sudoku is to watch a few moves on a real grid. Here is an Easy puzzle with 40 givens. We will find three numbers using three different ways of looking at the board.

The starting grid. Rows are numbered from top to bottom and columns from left to right.

Step 1: A row with two gaps

Look at row 7. It already has 6, 7, 3, 8, 9, 2 and 4, so only 1 and 5 are missing, in column 4 and column 9. Which goes where? Column 9 already has a 5 at the very bottom (yellow), so the 5 cannot go in column 9. That square must be 1, and the other gap in row 7 takes the 5.

Row 7 needs 1 and 5. The 5 in column 9 decides it: 1 goes on the right and 5 goes in column 4.

Step 2: Scanning a box for one number

Now take the number 8 and the top-left box, which has no 8 yet. Look at the 8s around it. The 8 in row 2 rules out the rest of row 2; the 8 in row 3 rules out all of row 3; the 8 in column 1 and the 8 in column 2 rule out the top squares of those columns. Only one square survives: row 1, column 3. That is where the 8 goes. This is called scanning, and it is the move you will make most often.

The four 8s in yellow block every empty square of the top-left box (red) except one. The 8 goes in the green square.

Step 3: The only number that fits

Sometimes it is easier to look at one square and ask which numbers could go there. Take row 5, column 8. Its row already holds 1, 7, 2 and 5. Its column holds 9, 5, 4, 6 and 8. Its box adds 4, 9 and 2. Put together, that is every number except 3, so the square must be 3.

Row 5, column 8 and the middle-right box between them contain every number except 3.

Keep repeating these three ways of looking and the whole grid will fall into place. Each number you write gives you new information for the next one. That, in short, is how to solve sudoku puzzles at every level: find the square or the number with the fewest options, prove it, and move on.

When You Get Stuck: Pencil Notes

On harder grids you will reach a point where no number can be placed by scanning alone. That is the time for notes. In each empty square of a busy row or box, write the small numbers that could still go there. Then look for a square with only one note left, or a number that appears in only one square of a row, column or box. When you place a number, cross it out of the notes in the same row, column and box. Our board does that part for you automatically.

How to Use the Board on Shelf of Games

  • Choose a square by tapping or clicking it. Its row, column and box light up, and the same numbers elsewhere on the board are highlighted.
  • Enter a number with the number pad. Each button shows how many of that number are still missing; when all nine are placed, the button fades out.
  • Notes switches pencil mode on and off. In pencil mode the pad writes small candidates instead of answers.
  • Erase clears the selected square. Undo takes back your last move, as many times as you like.
  • Hint fills in the correct number for the selected square, or for a random empty square if none is selected.
  • Show mistakes marks numbers that clash with the same number in their row, column or box.
  • Print gives you a clean paper copy with the starting numbers only.

On a computer you can also use the keyboard: the arrow keys move around the board, 1 to 9 enter numbers, Backspace or Delete clears a square, N switches notes, and Ctrl+Z (Cmd+Z on a Mac) undoes a move.

Common Beginner Mistakes

  • Guessing. A guess that looks right can quietly break the puzzle twenty moves later. If you are not sure, leave the square empty and look elsewhere.
  • Forgetting the box. New players check the row and the column but miss the 3×3 box. Every number must pass all three tests.
  • Staring at one spot. If a corner will not give way, move to another part of the grid. Progress elsewhere often unlocks it.
  • Not updating notes. Old candidates lead to wrong conclusions. On paper, cross them out as soon as a number is placed.

Sudoku Rules for Beginners: A Quick Summary

If you only remember one thing about how to play sudoku for beginners, make it this: every row, every column and every box gets the numbers 1 to 9 once each. Start with the Easy level, scan one number at a time across the grid, and use notes when scanning runs out. After a few games the patterns will start to jump out at you.

Play Now

Ready to try it? Start with a gentle Easy grid, or go straight to the Medium level.

Play Easy Sudoku Play Medium Sudoku

Beginner Questions

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

Not at all. Sudoku uses numbers only as symbols; you could play it with nine colors or nine letters. There is no adding or multiplying, only logic.

Is guessing allowed?

You can guess, but you never need to. Every puzzle on Shelf of Games has exactly one solution that can be found by reasoning. Guessing usually leads to mistakes that are hard to undo on paper.

Where should a beginner start?

Start with Easy Sudoku. It gives you 40 numbers to begin with, and every puzzle can be solved with the scanning method from this guide.

How can I get faster at solving?

Practice scanning one number across the whole grid before moving on to the next, and learn a few techniques from our Sudoku strategies page. Speed comes naturally after a few dozen puzzles.