How to Play Sudoku: Rules, Strategies & Tips for Beginners
Sudoku takes one minute to learn and a lifetime to master. This guide covers the rules, walks through a real example you can explore step by step, and teaches the solving techniques that take you from easy puzzles to expert grids.
What is Sudoku?
Sudoku is a logic puzzle played with the numbers 1 to 9. You start with a partly filled grid and complete it so that no number repeats in any row, column or 3×3 box. Despite the numbers, Sudoku involves no math at all: the digits are just symbols, and the puzzle would work the same with letters or colors.
The modern puzzle first appeared in 1979 in Dell Magazines in the United States under the name "Number Place". It became popular in Japan in the 1980s, where it got the name Sudoku (roughly "single number"), and spread worldwide in the mid-2000s. Today it is one of the most popular logic puzzles in the world. Want the full story? Read What is Sudoku?
Parts of the Sudoku grid
Knowing these terms makes every Sudoku strategy easier to follow:
- Cell: one of the 81 squares in the grid. Each cell holds a single digit.
- Row: a horizontal line of 9 cells. There are 9 rows.
- Column: a vertical line of 9 cells. There are 9 columns.
- Box (also called a block or region): one of the nine 3×3 squares outlined in bold.
- Unit: any row, column or box. Every unit must end up containing 1 to 9 exactly once.
- Givens (or clues): the numbers printed at the start. They can't be changed.
- Candidates (or pencil marks): the digits that could still go in an empty cell.
Sudoku rules
The rules of Sudoku fit in three lines. Fill every empty cell with a digit from 1 to 9 so that:
- Every row contains each digit from 1 to 9 exactly once.
- Every column contains each digit from 1 to 9 exactly once.
- Every 3×3 box contains each digit from 1 to 9 exactly once.
That's all. A proper Sudoku puzzle has exactly one solution, which means every number can be found by logic. You never need to guess. The Sudoku rules page goes further, with what makes a puzzle valid and the rules of 4×4, 6×6 and Killer Sudoku.
How to solve a Sudoku step by step
This method works for every Sudoku, from easy to expert. Move to the next step only when the previous one stops giving you new numbers.
- Look at the givens. Notice which digits already appear most often. A digit that is placed 7 or 8 times is the easiest to finish.
- Scan for easy placements. Pick one digit and check each box. Rows and columns that already contain that digit block cells in the box. If only one cell is left, the digit goes there.
- Find single candidates. Pick an empty cell and rule out every digit already in its row, column and box. If only one digit remains, fill it in.
- Write pencil notes. When scanning dries up, note every possible candidate in each empty cell. This is the key habit that separates medium players from hard and expert solvers.
- Eliminate candidates. Use the techniques below (naked pairs, pointing pairs, box-line reduction) to remove candidates until a cell has only one left.
- Repeat. Every new digit removes candidates from its row, column and box. Check those cells first, because new singles usually appear there.
For a longer routine, with advice on pencil notes and what to do when you are stuck, see Sudoku strategies.
Worked example: finding your first number
Here is a classic easy Sudoku. Look at the highlighted cell in the very center of the grid (row 5, column 5). Which digit belongs there? Check its row, column and 3×3 box one at a time to find out.
What can the center cell not be?
- 1, still possible
- 2, still possible
- 3, still possible
- 4, still possible
- 5, still possible
- 6, still possible
- 7, still possible
- 8, still possible
- 9, still possible
Which digit goes in the highlighted cell (row 5, column 5)? Right now any of the nine could. Check what already sits in its row, column and 3×3 box to rule digits out. Press Next step, or pick a check yourself.
Check what the center cell can't be:
- Its row already has 4, 8, 3 and 1.
- Its column already has 7, 9, 6, 2, 1 and 8.
- Its box already has 6, 8, 3 and 2.
Together that rules out 1, 2, 3, 4, 6, 7, 8 and 9. The only digit left is 5, so the center cell must be 5. (The row and column alone already leave only 5, but you still check the box: a 5 there would have blocked it.) This technique is called a naked single, and it solves a large part of every easy Sudoku.
Your turn: look for cells like this one on a real grid. Start an Easy Sudoku and hunt for any empty cell that has only one possible digit.
Sudoku strategies: beginner to advanced
Try the techniques in this order, cheapest first. Each has a small diagram, and Sudoku techniques explained goes deeper with more examples.
Beginner Sudoku strategies
Cross-hatching (scanning)
Choose a digit and trace its existing rows and columns through a box. The cells those lines don't cover are the only places the digit can go. If just one is left, place it.
Use it when: you start a puzzle, and for any digit that already appears many times. More on scanning
Naked single
A cell where every digit except one is already used in its row, column or box, as in the worked example above. In pencil marks, it is a cell with a single candidate left.
Use it when: you have just placed a digit. It removes that candidate from the 20 other cells that share its row, column or box, so new singles tend to appear there. More on singles
Intermediate Sudoku strategies
Naked pair
Two cells in the same unit that both contain only the same two candidates, for example {3, 7}. Those two digits must go in those two cells, so you can remove 3 and 7 from every other cell in that unit. The same logic works with three cells and three digits (a naked triple).
Use it when: your pencil notes are filled in and plain singles have run out. More on pairs
Pointing pair
If a digit's candidates inside a box all sit in one row or column, the digit must be placed in that row or column within the box. Remove it from the rest of that row or column outside the box.
Use it when: you check a box one digit at a time and that digit's remaining spots all line up. More on pointing pairs
Also at this level:
- Hidden pair. Two digits that appear as candidates in only the same two cells of a unit. Those cells must hold those digits, so you can erase all other candidates from them.
- Box-line reduction. The reverse of a pointing pair. If a digit's candidates in a row or column all fall inside one box, remove that digit from the other cells of that box.
Advanced Sudoku strategies
X-Wing
If a digit can go in exactly two cells in each of two rows, and those cells share the same two columns, the digit must occupy opposite corners of that rectangle. Remove it from every other cell in those two columns. The same pattern works with columns and rows swapped.
Use it when: singles, pairs and pointing pairs have all stopped working. Scan one digit at a time for two rows (or columns) where it has only two places. More on X-Wing
Also at this level:
- Swordfish. The X-Wing idea extended to three rows and three columns.
- XY-Wing. Three cells with two candidates each, linked in a chain, that let you eliminate a shared candidate from any cell that sees both ends of the chain.
Tip: Most easy and medium Sudoku puzzles need nothing beyond hidden and naked singles. Learn those two first, and add pairs when you move up to hard puzzles. Practice the singles on Easy and Medium Sudoku.
Common beginner mistakes
- Guessing. A guess that looks fine can break the grid 20 moves later. If you're stuck, add pencil notes instead.
- Forgetting the box. Beginners check rows and columns but miss a digit already in the same 3×3 box.
- Not updating notes. After placing a digit, erase it from the candidates in its row, column and box, or your notes will mislead you.
- Focusing on one area. If a region stalls, move to another part of the grid. Progress elsewhere often unlocks the stuck area.
Still stuck? Type your grid into the Sudoku solver to see the full solution and check your work.
Which Sudoku difficulty should you play?
The number of starting clues is only a rough guide. What really makes a Sudoku hard is the logic it demands: whether the next number falls to a simple scan, or only after a pair, a pointing pair or an X-Wing. A puzzle with 30 clues that needs an X-Wing is harder than one with 26 clues that falls to naked singles.
| Level | Given numbers (approx.) | Techniques that may come up |
|---|---|---|
| Easy Sudoku | around 45 | Scanning and naked singles |
| Medium Sudoku | around 35 | Hidden singles, some pencil notes |
| Hard Sudoku | around 29 | Full pencil notes, pairs, pointing pairs |
| Expert Sudoku | around 24 (as few as 23) | Box-line reduction, X-Wing and chains |
On intosudoku the level you pick sets how many clues the puzzle starts with, and every puzzle is checked to have exactly one solution. Puzzles are not graded by technique, so read the last column as typical rather than guaranteed. In a test of 1,000 generated puzzles per level, every Easy puzzle fell to naked singles alone and nearly every Medium puzzle fell to naked and hidden singles. About 1 in 5 Hard puzzles and 3 in 5 Expert puzzles needed more than that, such as pairs, pointing pairs or an X-Wing.
Start with easy Sudoku until you can finish without hints, then step up one level at a time. Fewer givens don't only mean more cells to fill: they mean fewer clues, so each deduction takes more work.
How to play Sudoku on intosudoku
intosudoku lets you practice everything in this guide for free, right in your browser:
- Choose a difficulty (Easy, Medium, Hard or Expert) and press Start Puzzle.
- Select a cell and tap a number on the number pad, or type 1–9 on your keyboard. Use the arrow keys to move around the grid.
- Turn on Notes to enter pencil marks instead of answers.
- Undo any move with the Undo button or Ctrl+Z, and Erase a cell with Backspace.
- Use a hint when you're stuck. You get three per game, and each reveals one correct cell.
- Turn on mistake counting for a challenge: the game ends after three wrong entries.
Your game saves automatically, and your best times and streaks appear on your Sudoku stats page. Want the same puzzle as everyone else? Try the Daily Sudoku. Prefer pencil and paper? Print puzzles from the printable Sudoku generator.
Sudoku FAQ
Do you need to guess to solve a Sudoku?
No. A well-made Sudoku has exactly one solution and can be solved with logic alone. If you feel you need to guess, there is a technique you haven't spotted yet.
Do you need to be good at math to play Sudoku?
No. Sudoku uses numbers only as symbols. You never add or multiply anything.
What is the minimum number of clues in a Sudoku?
17. In 2012, mathematicians proved by exhaustive computer search that no Sudoku with only 16 givens has a unique solution.
How long does it take to solve a Sudoku?
Beginners often need 10–20 minutes for an easy Sudoku. With practice, easy grids take under 5 minutes, while expert puzzles can still take 30 minutes or more.
Where did Sudoku come from?
It was first published in 1979 in Dell Magazines as "Number Place", became popular in Japan in the 1980s as Sudoku, and spread worldwide in the mid-2000s.
Have another question? See the Sudoku FAQ on our homepage, or contact us.
Play Free Sudoku Online Now
Start with an easy Sudoku, use pencil notes to practice the techniques above, and work your way up to expert. No download, no sign-up.