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What Is Sudoku?

At its heart, Sudoku is a logic puzzle. You get a 9×9 grid and need to fill every cell so each row, column, and 3×3 box contains the digits 1 through 9 exactly once. No addition, no multiplication. Just pure deduction.

The puzzle we know today was created by Howard Garns, an American architect, and first published as "Number Place" in Dell Pencil Puzzles & Word Games magazine in 1979. Nikoli Co. republished it in Japan in 1984, where editor Maki Kaji gave it the name "Sudoku" - short for "Sūji wa dokushin ni kagiru" ("the digits must remain single"). It remained a niche puzzle until 2004, when retired judge Wayne Gould pitched a computer-generated version to The Times of London. Within a year, Sudoku appeared in newspapers worldwide.

Today, Sudoku is published in over 2,000 newspapers across 80+ countries. Mathematicians Bertram Felgenhauer and Frazer Jarvis proved in 2006 that there are 6,670,903,752,021,072,936,960 valid 9×9 Sudoku grids - so you will never run out of puzzles. On Sudoku91, you can play 2,000+ unique puzzles for free, straight in your browser. No account, no download.

How to Play Sudoku

Three rules. That's all there is. But don't let the simplicity fool you; getting good takes real practice.

The grid

You're looking at an 81-cell grid, organized into nine 3×3 boxes (some people call them regions or blocks). A few cells already contain numbers. Those are the "givens" and they're locked in place.

The one rule

Every row gets 1 through 9. Every column gets 1 through 9. Every 3×3 box gets 1 through 9. That's it. No digit can appear twice in any of those groups.

How to solve

Look for cells with limited options. If a row already has 1, 3, 5, 7, 8, and 9, then the remaining cells can only hold 2, 4, or 6. Keep eliminating until each empty cell has one answer left.

To get started on Sudoku91: tap an empty cell, pick a digit. You can toggle Notes mode to pencil in candidates. Hit undo if you make a mistake. That's about it.

Read the full beginner's guide →

Sudoku Tips & Strategies

Whether you picked up Sudoku yesterday or you've been solving for years, a handful of solid strategies can speed things up and keep you from staring at the grid in frustration.

Scanning and crosshatching

Start simple. Look across each row and down each column to find where a number must go. Crosshatching takes this a step further: you check a 3×3 box by examining the rows and columns that pass through it, narrowing candidates until only one cell remains.

Pencil marks (Notes)

Why trust your memory when you don't have to? Toggle Notes mode and scribble candidate digits into empty cells. Every time you confirm a placement, erase that number from candidates sharing its row, column, and box. Past easy puzzles, this habit is non-negotiable.

Naked singles

One candidate left in a cell? Fill it in. Done. Check for more naked singles right away because they tend to chain together.

Hidden singles

This one trips up a lot of people. A cell might show three or four candidates, but if one of those numbers can't go anywhere else in that row, column, or box, it belongs there. Scan each group slowly. Hidden singles are the #1 thing intermediate solvers overlook.

Advanced techniques

Once you're comfortable with easy and medium puzzles, it's time to learn naked pairs, pointing pairs, and X-wing. All three work by eliminating candidates across multiple cells simultaneously. Hard and expert grids pretty much require them.

See all strategies from beginner to expert →

Sudoku Difficulty Levels

Sudoku91 offers five levels. The fewer givens on the board, the more advanced techniques you'll need.

Easy Sudoku

Between 36 and 45 cells are already filled in for you. Most of the puzzle solves itself through basic scanning - just look across rows and columns for cells that only have one option. Think of easy as your warm-up lap.

Medium Sudoku

Thirty to thirty-five givens. You'll actually need your pencil marks here. Hidden singles show up more often, and there are stretches where nothing jumps out immediately. It asks you to slow down and pay attention, but it's not trying to break you.

Hard Sudoku

Now we're talking. Twenty-five to twenty-nine givens, and you're looking at naked pairs, methodical crosshatching, the works. Skipping notes? Bad idea. These grids demand discipline.

Expert Sudoku

Twenty-two to twenty-seven givens. Pointing pairs show up. X-wing shows up. Chain logic might show up. The margin for sloppy thinking is basically zero - if you haven't been practising, these puzzles will let you know.

Evil Sudoku

Barely any starting cells. You'll need swordfish, advanced chains, and occasionally a technique you had to look up five minutes ago. This level exists for the people who email us saying expert is 'too easy.'

More Sudoku Games

Looking for a twist? These variants add new rules to the classic Sudoku formula.

Mini Sudoku 6×6

A 6×6 grid with 2×3 boxes. Quicker solves, fewer digits. Popular with kids and Sudoku newcomers. Also great for a five-minute break.

Killer Sudoku

Colored cages with target sums replace the usual givens. You'll need addition on top of standard Sudoku logic. Sometimes called Sum Sudoku.

Diagonal Sudoku (Sudoku X)

Same rules as classic Sudoku, but both main diagonals also need 1 through 9 with no repeats. The extra constraint makes it surprisingly tricky.

Samurai Sudoku

Five 9×9 grids overlapping at the corners. Solving one grid affects the others. Also known as Gattai-5. Expect to spend a while on these.

Killer Sudoku Combinations

A reference table of all valid cage combinations. Bookmark it. You'll want it open any time you're solving a Killer puzzle.

Sudoku Solver

Stuck on a puzzle? Enter your grid and get a step-by-step solution with technique explanations.

Frequently Asked Questions

How do you play Sudoku?
Place the digits 1 through 9 in every row, column, and 3×3 box of the grid. Each digit appears exactly once per group. Some cells already have numbers (those are your starting clues), and you figure out the rest using logic. Every puzzle on Sudoku91 has one valid solution, so guessing isn't needed.
What do the numbers mean in Sudoku?
They're purely symbols. You could replace them with letters or colours and the puzzle would work the same way. No adding, no subtracting - the only thing that matters is where you put each one.
Is Sudoku good for your brain?
Yes. A 2019 study from the University of Exeter, published in the International Journal of Geriatric Psychiatry, found that adults who regularly solve number puzzles like Sudoku have cognitive function equivalent to people up to 10 years younger. The benefits are strongest for attention, reasoning speed, and short-term memory.
Does Sudoku make you smarter?
Smarter as in higher IQ? Probably not. But pattern recognition, working memory, deductive reasoning - those get noticeably better the more puzzles you solve. Whether that counts as 'smarter' depends on your definition.
What does Sudoku mean?
It comes from "Sūji wa dokushin ni kagiru" - a Japanese phrase meaning "the digits must remain single." The puzzle was created by Howard Garns and first published in Dell Pencil Puzzles & Word Games (USA) in 1979. Nikoli Co. in Japan picked it up in 1984, and editor Maki Kaji coined the shortened name "Sudoku."
How do you solve a hard Sudoku?
Basic scanning won't be enough. Write pencil marks in every empty cell, then hunt for naked pairs (two cells in a group sharing the same two candidates), pointing pairs, and X-wing patterns. Systematic elimination always beats trial and error.
Can you play Sudoku for free online?
Yes. Sudoku91 has unlimited puzzles across five difficulty levels, completely free. No account, no app download, nothing. Just open the site and start solving.
What is the best strategy for Sudoku?
Scan for easy wins first - any row, column, or box where a digit can only go in one spot. After that, pencil in candidates and start eliminating. Naked singles, then hidden singles, then (for tougher grids) naked pairs and X-wing. The one thing you should never do is guess.
What is the best Sudoku app without ads?
Shameless plug: Sudoku91. No ads at all - not a banner, not a pop-up, nothing. Dark mode, auto-save, notes, undo. Five levels from Easy to Evil, and it runs in your browser so there's nothing to install.
What is the 3 rule in Sudoku?
Three things have to be true about every digit you place: it appears once in that row, once in that column, once in that 3×3 box. Get any of those wrong and the whole puzzle breaks.
Is Sudoku completely free on Sudoku91?
Yep. Every puzzle, every difficulty, zero cost. No ads either. We don't even ask for an email.
Can I play Sudoku for free in Spanish?
Sí! The whole site - menus, instructions, all the page content - is available in Spanish. Phone, tablet, desktop, whatever you've got. No account needed.
What is the best website to play Sudoku online for free?
We're obviously going to say Sudoku91 - 2,000 puzzles, five levels, dark mode, auto-save, English and Spanish. No ads, no account. But hey, try a few puzzles and make up your own mind.