Sudoku FAQ

Sudoku is a number-placement puzzle solved with logic. Fill the 9 by 9 grid so that every row, column, and 3 by 3 box contains 1 through 9 once. There is no arithmetic; the digits could just as easily be nine different symbols. Read the Sudoku rules or try a puzzle.

Fill the empty cells without repeating a digit in any row, column, or 3 by 3 box. A good first move is to find an area with only two or three gaps, work out which digits are missing, and check where each one fits. Leave uncertain cells alone for now. Another placement may settle them later. The full Sudoku rules and an Easy Sudoku will get you started.

The goal is a completed grid in which every row, column, and 3 by 3 box contains 1 through 9 exactly once. Finishing quickly is a personal challenge, not part of the rules. A careful solution counts just as much as a fast one.

Classic Sudoku has three rules:

1. Each row must contain the digits 1 through 9 exactly once.
2. Each column must contain the digits 1 through 9 exactly once.
3. Each 3 by 3 box must contain the digits 1 through 9 exactly once.

The starting digits, often called givens or clues, cannot be changed. No calculation is required. For examples and a visual explanation, read the full Sudoku rules guide.

No. A classic Sudoku only checks rows, columns, and 3 by 3 boxes, so repeated digits on a diagonal are perfectly legal. Diagonal Sudoku is a separate variant that marks the diagonals as additional units. Unless the puzzle says otherwise, follow the classic rules.

Yes, but not in the same unit. Each digit appears nine times in a finished classic grid: once in every row, once in every column, and once in every box. Before placing a number, check those three areas. Matching digits elsewhere, including on the same diagonal, do not break the Sudoku rules.

A sound classic Sudoku should not require a blind guess. When two candidates seem equally plausible, leave that cell and inspect another row, column, or box. You may have missed a simpler elimination. Advanced solvers sometimes test an assumption and follow its consequences, but that is a controlled proof, not a coin toss. Try the stuck checklist first.

It can, although a conventional published Sudoku is expected to have one solution. Multiple solutions mean the clues leave at least one choice unresolved: two different completed grids still obey every rule. The grid may be playable, but it is underconstrained rather than a proper standard puzzle. A contradictory grid has the opposite problem and cannot be completed at all. See Are Sudoku puzzles always solvable?.

No. Any random set of starting digits can produce a contradiction or leave several possible solutions. A published Sudoku should have been checked for one valid answer. If your puzzle suddenly reaches a dead end, look for a duplicate in a row, column, or box before blaming the clues; a misplaced digit earlier in the solve is the more common cause.

A clue and a given are two names for a digit supplied with the puzzle. You do not change it. A candidate is different: it is one of the values that could still fit an empty cell after checking its row, column, and box. Candidates are usually shown as small notes and disappear as the solve narrows the options. Here is how to use pencil marks without cluttering the grid.

Use the same short routine each time:

1. Find a row, column, or box with only a few gaps.
2. Work out which digits are missing.
3. Check where those digits are blocked.
4. Place only the value you can prove.

Every new digit changes three units, so scan them again. When the obvious moves run out, add candidate notes and look for Naked Singles or Hidden Singles. An Easy Sudoku is a forgiving place to practise.

Stop looking at the same cell for a moment. Pick a row, column, or 3 by 3 box with only a few gaps and check which digits are missing. Still nothing? Choose one digit, such as 5, and trace where it can go in each box. A move elsewhere may unlock the spot that held you up. The guided hint can show the next available technique if the grid still will not budge.

Speed usually comes from seeing familiar moves sooner, not from tapping faster. Scan in a regular order, keep your notes accurate, and pause before entering a digit that only looks obvious. Practise at one difficulty long enough for its common patterns to become familiar. Your own history in Statistics is a fairer benchmark than somebody else's record. When progress levels off, add the next method from the technique learning path.

There is no normal time that every solver should hit. A puzzle that takes one person five minutes may keep another busy for half an hour, and both experiences are fine. Compare your times only across similar difficulty levels in Statistics. If a new level feels dramatically slower, learn its techniques before trying to race the clock.

Pencil marks are the small possible values written inside an empty cell. Start with 1 through 9, then cross out anything already used in the same row, column, or box. The digits left are its candidates. Update them as the grid changes; stale notes can hide the next move. On TheSudoku.com, use Notes mode while you play. Patterns such as Naked Singles, Pairs, and X-Wings then become much easier to spot.

Not for every puzzle. Try scanning first. If several empty cells keep competing for the same numbers, add notes only there and remove candidates as you place digits. Notes are a memory aid, not an extra Sudoku rule.

Select Hint while you are playing, and the site first checks the digits already on the board. It will flag a wrong entry instead of building advice on top of it. If the grid is sound, highlighted cells walk you through the simplest technique currently available; the last step either places a digit or removes candidates. Some techniques need a complete set of notes. The hint can add those automatically, but doing so adds three minutes to the timer. The Smart Hints overview lists the techniques it can currently explain.

Yes. TheSudoku.com lets you start a free puzzle without creating an account. Pick a difficulty and play with the keyboard or on-screen controls. Notes and guided hints are available if you need them. New to the game? Easy Sudoku is the gentlest starting point.

Yes. Open a puzzle, choose Print, and use the dialog supplied by your browser. From there you can pick a printer, adjust the available paper settings, or save a PDF if your browser offers that option. Check the preview first when you have already entered numbers or notes, since the printed page may include the puzzle's current state.

Difficulty comes from the solving path. A grid resolved with Singles belongs below one that needs Pairs, fish patterns, wings, or chains, even if the two puzzles start with a similar number of clues. The number of steps and how deeply they depend on one another matter too. Publishers use different rating systems, so treat the TheSudoku.com levels as a progression within this site rather than a universal scale.

A hard Sudoku makes you connect information from several parts of the board. Singles dry up, candidate notes matter, and one useful elimination may require a pattern spanning multiple rows or columns. That is where methods such as X-Wing, XY-Wing, and Swordfish enter the solve. A sparse-looking grid is not automatically hard; the logic it demands is what counts.

Not reliably. Fewer clues may make a grid look intimidating, but their arrangement can still expose an easy chain of Singles. A fuller grid can hide a much tougher candidate pattern. Ratings therefore look at the techniques and sequence of deductions needed to finish. The famous 17-clue minimum concerns whether a unique classic puzzle can exist, not how difficult that puzzle feels to a human solver.

Start with Last Free Cell, Last Remaining Number, Last Possible Cell, Naked Single, and Hidden Single. Each method finds a number or position with only one legal option. If one of the names is unfamiliar, open its guide and try the idea on your current Easy Sudoku. You should not need an advanced pattern at this level.

Try Medium Sudoku when Easy puzzles feel familiar, not when you hit a particular time. You should be able to explain most placements and recover when you get stuck. Medium adds eliminations such as Pairs, Pointing, and Claiming, so practise candidate notes first. If the new level feels busy, drop back for a few games and return later.

Medium keeps the Easy techniques and adds Naked Pairs, Hidden Pairs, Pointing, and Claiming. Do not expect every pattern to place a number immediately. Its job may be to erase one or two candidates. Once it does, scan for Singles again; that small cleanup is often what moves a Medium Sudoku forward.

Expect to use every earlier method, then add X-Wing, XY-Wing, and Swordfish. These patterns connect candidates across distant cells, so incomplete notes make them easy to miss. Learn one shape at a time and practise it until you can find it without being told where to look. That is better preparation for Hard Sudoku than memorising several definitions at once.

There is no single advanced trick that unlocks every Expert or Master grid. A puzzle may combine Quads, XYZ-Wing, Jellyfish, Skyscraper, 2-String Kite, Turbot Crane, Medusa methods, Nishio, or forcing chains with everything learned earlier. Work through Expert Sudoku before Master Sudoku. When a hint names an unfamiliar method, follow its highlighted logic on that grid instead of trying to memorise the whole advanced list in advance.

Build outward from the smallest patterns:

1. Last Free Cell, Last Remaining Number, and Last Possible Cell.
2. Naked Single and Hidden Single.
3. Naked and Hidden Pairs, Triples, and Quads.
4. Pointing and Claiming.
5. X-Wing, XY-Wing, Swordfish, and Skyscraper.
6. Larger fish, wings, loops, and forcing chains.

Do not wait until you have memorised every name. Add a new group when the previous one feels recognisable in a live puzzle. The Sudoku techniques guide has a reference page for each method. A separate comparison shows how classic patterns fit into modern solving systems.

Look at a cell whose notes have dwindled to one digit. That last candidate is a Naked Single and can be placed immediately. The answer is "naked" because it is visible in the cell itself; you do not need to compare the candidate with other cells in the unit. After placing it, update the cells that share its row, column, or box. See the Naked Single example.

A Hidden Single belongs to a row, column, or box rather than to one candidate list. Scan a unit for a particular digit. If every position but one is blocked, that digit must go in the remaining cell, even when the cell still shows other notes. This is why it can hide from a cell-by-cell scan. The Hidden Single guide shows both ways of looking at it.

Suppose two cells in one row both contain only 2 and 7. Those digits must occupy the two cells in some order, so no other cell in that row can be 2 or 7. That is a Naked Pair. The pair does not tell you which cell gets which digit, but the eliminations may expose a Single elsewhere. Try the same reasoning in a column or box with the Naked Pairs guide.

Turn the Naked Pair search around. If two digits appear nowhere except the same two cells of a row, column, or box, they form a Hidden Pair. Remove every other note from those two cells; the paired digits have reserved them. The pair was "hidden" because both cells may have started with several candidates. A worked Hidden Pairs example makes the contrast clearer.

A candidate becomes locked when all of its positions are trapped where one box crosses one row or column. Because the digit must appear somewhere in that overlap, matching candidates outside it can be erased. Direction gives the two versions their names: Pointing looks out from a box, while Claiming looks into one.

They use the same box-line overlap in opposite directions. Pointing starts inside a box: if all its candidates for 6 lie on one row, remove 6 from the rest of that row. Claiming starts with the row: if every possible 6 in that row lies inside one box, remove 6 from the other cells of the box.

Track one candidate across two rows. If it appears in exactly the same two columns in both rows, the four cells form an X-Wing. Whichever diagonal pair turns out to be true, those two columns already receive the digit from the pattern. Remove that candidate from their other cells. Rows and columns can be reversed. The rectangle is easier to recognise in the X-Wing diagram than in a full grid.

A Swordfish is the three-line relative of an X-Wing. Pick one digit. When its candidates in three rows are confined to the same three columns, those columns must receive the digit from within the pattern. Other occurrences of that candidate in the three columns can go. The cells need not form a neat rectangle, which is why the Swordfish diagrams help.

An XY-Wing links three two-candidate cells. The pivot, XY, sees an XZ wing and a YZ wing. If the pivot is X, the YZ wing becomes Z; if the pivot is Y, the XZ wing becomes Z. Either way, one wing contains Z. Any cell that sees both wings can therefore lose Z. Check the exact visibility rules in the XY-Wing guide.

A Skyscraper uses two strong links for the same candidate. One end of each link lines up, while the other two ends sit in different columns or rows like uneven rooftops. At least one roof must be true, so a cell that sees both roofs cannot contain that digit. Filter the board to one candidate before looking for the shape, then compare it with the Skyscraper walkthrough.

Sudoku is the shortened form of a much longer Japanese title, "Suuji wa dokushin ni kagiru." Nikoli translates the idea as numbers being limited to a single, or unmarried, state. "Su" means number and "doku" means single. The compact name stuck, even outside Japan. In play, that idea becomes the no-repeat condition in the rules.

The name is Japanese, but the modern puzzle has an international history. Howard Garns is generally credited with the Number Place puzzle published in an American magazine in 1979. Nikoli brought it to Japan in 1984, shortened its Japanese title to Sudoku, and developed a distinctive publishing style around it. So calling Sudoku simply Japanese misses an important part of the story.

Howard Garns is generally credited with creating modern Sudoku. The American architect also constructed puzzles, and Number Place appeared without a byline in a Dell magazine in 1979. Puzzle historian Will Shortz later identified Garns as its author. Two other names matter to the story: Nikoli introduced and named Sudoku in Japan, while Wayne Gould helped take it into newspapers around the world.

Modern Sudoku first appeared as Number Place in an American puzzle magazine in 1979. Its next home was Japan, where Nikoli introduced it in 1984 and eventually shortened the title to Sudoku. The puzzle became a worldwide newspaper fixture only later, after Wayne Gould placed computer-generated grids with British papers. In short: American origin, Japanese name and development, then international success.

Japan discovered Sudoku in the mid-1980s, but the worldwide boom came in 2005. The turning point was The Times in London, which began publishing Wayne Gould's puzzles in November 2004. Rival newspapers soon added their own grids, followed by books and digital versions. A puzzle that needs almost no translation could travel quickly, and it did.

It looks mathematical, but ordinary Sudoku is a logic game. You do not add, subtract, multiply, or divide anything. Replace 1 through 9 with nine colours and the puzzle would work the same way, provided every symbol remained distinct. Mathematicians do study Sudoku as a constraint problem, but you need deduction, not calculation, to solve the grid in front of you. The longer explanation of why Sudoku is not a math puzzle uses a few everyday examples.

The exact count is 6,670,903,752,021,072,936,960 completed classic 9 by 9 grids. Bertram Felgenhauer and Frazer Jarvis published that result in 2005. Group together grids that differ only through equivalent relabelling or symmetry, and Ed Russell and Jarvis found 5,472,730,538 essentially different grids. One easy detail to miss: these numbers count completed solutions, not all the partially filled puzzles that can be made from them.

Seventeen is the minimum number of clues in any known valid classic 9 by 9 Sudoku with one solution. More strongly, an exhaustive search by Gary McGuire, Bastian Tugemann, and Gilles Civario proved that a 16-clue example cannot exist. But 17 is a floor, not a recipe. Put those clues in the wrong places and the grid may have several solutions. It also tells you nothing certain about solving difficulty.

Sudoku gives your attention and logical reasoning something active to do, which is a perfectly good reason to play. Frequent number-puzzle use has been associated with better results on some cognitive tests, but the study could not show that puzzles caused those results. What practice clearly improves is your ability to solve Sudoku. Broader claims about brain health need much stronger evidence, so treat the game as an absorbing hobby rather than therapy.

While solving, you briefly hold candidates and grid relationships in mind. That feels like memory work, and regular play may make you better at handling the same information in later puzzles. It has not been shown to create a broad, lasting improvement in memory outside the game. Pencil marks are not cheating if you lose track of candidates; they free your attention for the actual logic.

No solid evidence shows that Sudoku raises IQ. Play often and you will probably become a better Sudoku solver: scanning gets quicker, candidate patterns become familiar, and fewer moves surprise you. Those are real skills, but gains on one practised task do not automatically become higher general intelligence. Our review of IQ and Sudoku performance looks more closely at that distinction.

No. Sudoku has not been shown to prevent dementia, and it should never be sold as a treatment. Mentally engaging hobbies can be part of an enjoyable, active life, but an association between puzzle use and test performance is not proof of protection from disease. If you are worried about changes in memory or thinking, speak with a qualified healthcare professional. A game cannot diagnose the cause.