Sudoku School

Learn techniques to solve any Sudoku puzzle

Guides

How to Play

How to play on SudokuStreak: pick a game type and difficulty, enter digits with the mouse or keyboard, use notes and hints, and see how Sparks, coins, streaks and jokers work.

How to Use Pencil Marks in Sudoku

Pencil marks — the small candidate numbers you note in a cell — are the foundation every advanced technique builds on. Learn how and when to use them, the tidy way.

How Many Clues Does a Sudoku Need?

What is the fewest given numbers a Sudoku can have and still be solvable? The answer is a proven 17 — here is the story, the maths, and what it means for difficulty.

Sudoku Techniques by Difficulty

Every Sudoku solving technique, ordered from beginner to expert, with a one-line summary and a link to each full guide. The map to the whole Sudoku School.

Sudoku Glossary

Candidate, unit, naked, hidden, cage, given — the language of Sudoku in one place, each term defined plainly and linked to the technique that uses it.

Sudoku Tips for Beginners

Twelve practical Sudoku tips for new players: where to scan first, when to use pencil marks, why you should never guess, and how to get unstuck — with examples.

How to Get Better at Sudoku

Stuck at the same Sudoku level? Climb the technique ladder deliberately: what to learn at each difficulty, how to practise it, and a concrete 30-day plan.

Is Sudoku Good for Your Brain? What Research Actually Says

Sudoku trains working memory, deduction and focus — and a 19,000-person study links regular number puzzles with sharper minds. The honest picture, caveats included.

Game Types

Each guide covers the rules, the solving techniques you need, and an interactive step-by-step tutorial.

Classic Sudoku

Rules, solving techniques & tutorial

Killer Sudoku

Cages, sums & combinations

X-Sudoku

Diagonals must hold 1–9

Center Dot Sudoku

Box centres form a tenth region

German Whispers Sudoku

Neighbours on the line differ by 5+

Dutch Whispers Sudoku

Neighbours on the line differ by 4+

Nabner Sudoku

No repeats, no consecutive digits

Samurai Sudoku

Five overlapping grids

Play by Difficulty

Guides to each difficulty level — what to expect and the techniques it takes.

Easy Sudoku 36 givens — the perfect place to start. → Medium Sudoku 30 givens — the everyday level. → Hard Sudoku 26 givens — where method takes over. → Expert Sudoku 22 givens — the advanced patterns become mandatory. → Evil Sudoku 18 givens — the hardest level we make. →

Solving Techniques

Every technique in depth, with worked examples you can follow on a real grid. Start at the top — each one builds on the last.

More Sudoku Variants

Rules and how-to guides for the wider world of Sudoku variants — the ones we don't generate here yet.

Jigsaw Sudoku The 3×3 boxes are replaced by nine irregular nine-cell regions; row, column and region rules otherwise unchanged. → Hyper Sudoku Adds four shaded 3×3 windows that must each also hold 1–9, on top of the normal boxes. → Thermo Sudoku Digits along a thermometer must strictly increase from the bulb to the tip. → Arrow Sudoku The digit in a circle equals the sum of the digits along the arrow leaving it. → Sandwich Sudoku A clue outside the grid gives the sum of the digits sandwiched between the 1 and the 9 in that row or column. → Anti-Knight Sudoku Classic rules plus a chess constraint: identical digits may not sit a knight's move apart. → Miracle Sudoku Classic plus anti-knight, anti-king and non-consecutive constraints — famously solvable from just two given digits. → Sudoku 4×4 A 4×4 grid with 2×2 boxes and digits 1–4 — the gentlest introduction, popular with children and beginners. → Sudoku 6×6 A 6×6 grid with 2×3 boxes and digits 1–6 — a common stepping stone between 4×4 and the full 9×9. → Sudoku 16×16 A 16×16 grid with 4×4 boxes using digits 1–16 or hex characters 0–F — the most common large format. → Kropki Sudoku Dots between cells reveal relationships: a white dot means the two digits are consecutive, a black dot means one is double the other. → XV Sudoku Marks between cells give sums: an X means the two digits add to 10, a V means they add to 5. → Odd-Even Sudoku Shaded cells restrict parity: grey squares must hold even digits, circles must hold odd ones. → Consecutive Sudoku Bars between cells mark every pair of consecutive digits; where there is no bar, the neighbours are not consecutive. → Non-Consecutive Sudoku A single global rule: no two orthogonally adjacent cells may contain consecutive digits. → Greater Than Sudoku Inequality signs between cells in each box show which digit is larger — often the only clues given. → Wordoku A normal 9×9 Sudoku using nine different letters instead of digits — often with a hidden word in the finished grid. → Color Sudoku Classic Sudoku with nine colours in place of the digits — the same puzzle, made friendlier for young solvers. → Little Killer Sudoku Arrows outside the grid give the sum of the digits along the diagonal they point to — and on those diagonals, digits may repeat. → Renban Sudoku Marked lines must contain a set of consecutive digits, in any order — a run like 4-5-6-7 scrambled along the line. → Extra Regions Sudoku Additional nine-cell regions, of any shape, must also contain the digits 1–9 without repeating — on top of the usual rows, columns and boxes. → Anti-King Sudoku Classic rules plus a chess rule: no two cells a king's move apart — including diagonally — may hold the same digit. → Disjoint Groups Sudoku Every cell that sits in the same position within its 3×3 box forms a ninth group that must also hold 1–9. → Asterisk Sudoku A ninth region of nine marked cells arranged in an asterisk must also contain the digits 1–9. → Girandola Sudoku A ninth region of nine marked cells in a pinwheel — the four corners, the centre, and four cells set out from the centre along each side — must also contain the digits 1–9. → Palindrome Sudoku Grey lines are palindromes: the digits read the same forwards and backwards along each line. → Between Lines Sudoku A line strung between two circled cells may contain only digits that fall numerically between the two endpoint values. → Region Sum Lines Sudoku Blue lines cross box borders; the digits within each box the line passes through sum to the same total. → Zipper Lines Sudoku On a zipper line, cells equidistant from the centre add up to the value in the centre cell. → Entropic Lines Sudoku Any three consecutive cells on an entropic line must include one low (1–3), one middle (4–6) and one high (7–9) digit. → Quadruple Sudoku A small clue printed where four cells meet lists digits that must appear among those four cells. → Clone Sudoku Two or more shaded regions are clones — identical in shape and filled with the same digits in the same arrangement. → Fortress Sudoku Digits on shaded fortress cells must be strictly greater than each of their orthogonal neighbours. → X-Sums Sudoku An outside clue gives the sum of the first N cells in its row or column, where N is the digit nearest the clue. → Outside Sudoku Clues outside the grid list digits that must appear in the first three cells of that row or column, nearest the clue. → Numbered Rooms Sudoku An outside clue names the digit sitting in the Nth cell of its row or column, where N is the digit in the cell nearest the clue. → Next to Nine Sudoku The digits printed outside a row or column are exactly the digits directly adjacent to the 9 in that line, listed in increasing order. → Descriptive Pairs Sudoku A pair of digits X and Y outside a row or column means the Xth cell holds Y, or the Yth cell holds X — at least one of the two must be true. → 159 Sudoku Columns 1, 5 and 9 are index columns: in each row the digit in column 1 gives the column that holds that row's 1, and likewise column 5 indexes the 5s and column 9 the 9s. → Friendly Cells Sudoku Each specially marked cell must hold a digit equal to its own row number, its column number, or its box number. → Frame Sudoku Each clue in the frame around the grid gives the sum of the three cells nearest to it in that row or column. → Slow Thermometer Sudoku A thermometer whose digits must never decrease from bulb to tip — they only need to stay level or rise. → Fog of War Sudoku The grid starts hidden under fog that lifts around every correct digit — and because the fog also hides the clues, Fog of War is always played on top of another variant rather than on its own. → Sudoku 8×8 An 8×8 grid with 2×4 boxes using the digits 1–8 — a compact step up from the 6×6 board. → Sudoku 10×10 A 10×10 grid with 2×5 boxes using the digits 1–10 (often 0–9). → Sudoku 12×12 A 12×12 grid with 3×4 boxes using the digits 1–12. → Sudoku 15×15 A 15×15 grid with 3×5 boxes using the digits 1–15. → Sudoku 25×25 A 25×25 grid with 5×5 boxes using 25 symbols — one of the largest square formats in common use. → Double Arrow Sudoku A line with a circle at each end; the digits on the line between the circles sum to the total of the two circled digits. → Lockout Lines Sudoku The two diamond endpoints of a line must be far apart, and no digit on the line may fall between them. → Modular Lines Sudoku Every three consecutive cells on a modular line hold digits from three different remainder groups modulo 3. → Counting Circles Sudoku Each circled cell's digit equals the number of circles across the grid that contain that same digit. → Anti-Diagonal Sudoku Each of the two main diagonals may contain only three different digits. → Twodoku Two 9×9 Sudoku grids that overlap in one shared 3×3 box, each solved under normal rules at once. → Argyle Sudoku A set of diagonal lines crosses the grid in an argyle pattern; no digit may repeat along any marked diagonal. → Greater Than Killer Sudoku Killer cages with inequality signs inside them showing the order of the digits. → Killer Sudoku Pro Killer cages whose clue can use any of the four operations — addition, subtraction, multiplication or division — not just sums. → Anti-Queen Sudoku Chosen digits — most often the 9s — may not lie a queen's move apart along the grid's diagonals. → Windmill Sudoku Five 9×9 grids arranged as a pinwheel — a central grid overlapping four others at its corners — each a full Sudoku. → Battenburg Sudoku Marked 2×2 areas must show a chequerboard of odd and even digits, like the squares of a Battenberg cake. → Rossini Sudoku An arrow outside a row or column shows whether its first three cells, counting inward, rise or fall in order. → Toroidal Sudoku The grid wraps at its edges like a torus, so jigsaw regions can run off one side and continue on the other. → Skyscraper Sudoku Clues outside the grid count how many digits are "visible" along that row or column, reading each digit as a building height — a taller digit hides every smaller one behind it. → Chaos Construction The nine regions are not drawn in — you deduce their boundaries as you solve, so that each region is nine orthogonally-connected cells and every row, column and region still holds 1–9 once. → Butterfly Sudoku Four 9×9 grids packed into a 12×12 square so each overlaps its neighbours — the four central boxes belong to all four grids at once, and every grid is a full Sudoku. → Sohei Sudoku Four 9×9 grids set as a ring — top, bottom, left and right — each sharing one corner box with each neighbour, around a hollow centre that belongs to no grid. → Tridoku A big triangle of 81 small triangular cells grouped into nine triangular nonets — each nonet, each of the three outer edges and an inner triangle must hold 1–9, and touching cells may never match. → Honeycomb Sudoku A family of hexagonal-grid Sudoku puzzles: digits fill hexagonal cells so none repeats along any straight line in the three honeycomb directions. Board size and digit range (commonly 1–7) vary by design — there is no single standard grid. → Tight Fit Sudoku Some cells are split by a diagonal slash and hold two digits — the smaller written above the larger; both digits count against the row, column, region and diagonal neighbours. → Parity Lines Sudoku Along a marked line the parity alternates — odd, even, odd, even — so neighbouring cells on the line are always one odd and one even. → Sujiken Half a Sudoku, cut along the diagonal: 45 square cells in a right triangle where no digit repeats in any row, column, diagonal, or bold triangular region. → Japanese Sums Sudoku Ordinary Sudoku where some cells are shaded out; an outside clue lists the sums of the unshaded blocks in that row or column, in order. → Global Entropy Sudoku The digits split into three groups — low (1–3), middle (4–6), high (7–9) — and every 2×2 area of the grid must contain all three groups. → Pencilmark Sudoku There are no given digits — instead every cell shows a small list of candidates, and you choose one digit per cell to build a valid Sudoku. → Battleship Sudoku A hybrid of Sudoku and Battleships: place the digits and a fleet of ships together, where ships never touch — even diagonally — and outside numbers count the ship segments in each row and column. →

Sudoku vs Other Puzzles

How Sudoku compares to Kakuro, Futoshiki and other logic puzzles — and which skills carry across.

Sudoku vs Kakuro A crossword-shaped grid where you fill white cells with 1–9 so each across or down run adds up to its clue, with no digit repeated inside a run. → Sudoku vs Futoshiki A small Latin-square grid where inequality signs between cells tell you which neighbour is larger. → Sudoku vs Str8ts A 9×9 grid split by black cells into compartments, each of which must contain a "straight" — a set of consecutive numbers in any order. → Sudoku vs Calcudoku A Latin-square grid divided into cages, each printed with a target and an operation the cage's numbers must produce. → Sudoku vs Suguru A grid divided into cages of different sizes; each cage of n cells holds 1–n, and no two identical numbers may touch — even diagonally. → Sudoku vs Nonogram A grid where number clues on each row and column tell you the lengths of filled runs, revealing a hidden picture. → Sudoku vs Magic Square A grid filled with distinct numbers so every row, column and both diagonals add up to the same total. → Sudoku vs Strimko A Latin-square grid where, on top of the rows and columns, curved "streams" of cells must each contain every number once. → Sudoku vs Sujiko A 3×3 grid filled with 1–9 where four circled numbers give the sum of the four cells around each inner corner. → Sudoku vs Hitori A grid of numbers where you shade cells so that no number repeats in any row or column among the cells left unshaded. → Sudoku vs Star Battle Place a fixed number of stars in every row, column and region so that no two stars ever touch. → Sudoku vs Binairo Fill a grid with 0s and 1s so each row and column is balanced, never has three in a row, and no two rows or columns match. → Sudoku vs Slitherlink Draw a single continuous loop along the grid lines so that each numbered cell has exactly that many of its four edges used. → Sudoku vs Sudoku Cube A Rubik's-Cube-style toy whose stickers are digits — twist the layers until every face shows 1–9 exactly once. → Sudoku vs Shingoki Draw one closed loop through every circle; white circles are passed straight, black ones force a turn, and each number is the total length of the two straight runs from that circle. → Sudoku vs Pipes Rotate each tile so all the pipe ends link into one connected network with no loose ends and no closed loops. → Sudoku vs Masyu Draw a single loop through the cell centres that passes through every pearl, obeying a straight-or-turn rule for white and black pearls. → Sudoku vs Nurikabe Shade cells into a connected "sea" so that each numbered cell forms an island of exactly that many unshaded cells. → Sudoku vs Hashiwokakero Join numbered islands with bridges so each island has exactly its number of bridges and everything is connected. → Sudoku vs Fillomino Divide the grid into regions so that each number sits in a region of exactly that many cells, and no two same-size regions touch. → Sudoku vs Shikaku Cut the grid into rectangles so that each contains exactly one number, equal to that rectangle's area. → Sudoku vs Akari Place light bulbs so every white cell is lit, no bulb lights another, and numbered black cells have exactly that many bulbs beside them. → Sudoku vs Inshi no Heya A Latin-square grid split into rooms, each printed with the product its cells must multiply to — the puzzle that inspired KenKen. → Sudoku vs Suko A 3×3 grid of 1–9 where four circled sums cover overlapping quads of cells and coloured groups carry their own totals. → Sudoku vs Ripple Effect A grid split into rooms; each room of size m holds 1–m, and equal digits in a line must be spaced at least that many cells apart. → Sudoku vs Tenner Grid A ten-column grid where each row holds 0–9 once, column totals are fixed, and touching cells must differ. → Sudoku vs Easy as ABC Place a few letters and blanks so each letter appears once per row and column, with outside clues naming the first letter seen from each edge. → Sudoku vs Kakurasu Shade cells so that the weights of the shaded cells add up to the clue at the end of every row and column. → Sudoku vs Numbrix Fill the grid with a single run of consecutive numbers that snakes through it using only horizontal and vertical steps. → Sudoku vs Hidato Fill the grid with a consecutive run of numbers so that each number touches the next, diagonals included. → Sudoku vs Numberlink Connect each pair of matching numbers with a path so the paths never cross and, in the classic form, fill every cell. → Sudoku vs Tents Pitch one tent beside each tree so no two tents touch and the row and column clues count the tents correctly. → Sudoku vs Magnets Fill a domino-tiled board with magnets and blanks so like poles never touch and the plus/minus counts around the edge come out right. → Sudoku vs Dominosa Divide a grid of numbers into a full set of dominoes so that every domino combination appears exactly once. → Sudoku vs Doppelblock Place two black cells and the numbers 1 upward in each row and column, so the clue equals the sum of the numbers trapped between the two black cells. → Sudoku vs Mathrax A Latin-square grid with arithmetic clues at the inner intersections that must hold for both diagonal pairs of the surrounding four cells. → Sudoku vs Aquarium Fill aquariums with water so that within each one the water settles to a level, and the edge clues count the water in every row and column. → Sudoku vs Tapa Shade cells into one connected wall so each clue matches the run-lengths of shaded cells around it, with no solid 2×2 block. → Sudoku vs Yajilin Draw one loop through the empty cells and shade the rest so that arrow clues correctly count the shaded cells they point at. → Sudoku vs Heyawake Shade cells so numbered rooms hold exactly that many shaded cells, shaded cells never touch, and no white line crosses more than one room border. → Sudoku vs LITS Shade one tetromino in every region so all shaded cells connect, no 2×2 is filled, and touching tetrominoes are never the same shape. → Sudoku vs Slant Draw a diagonal in every cell so that each numbered vertex has exactly that many diagonals meeting it, and no closed loops form. → Sudoku vs Spiral Galaxies Divide the grid into regions, each with 180° rotational symmetry about a given centre dot. → Sudoku vs Fill-a-Pix Shade cells so each number equals how many of the cells in its 3×3 neighbourhood are shaded, revealing a picture. → Sudoku vs Skyscrapers A Latin-square grid of building heights where each row and column holds 1–N once, and clues around the edges tell you how many buildings are visible from that side — a taller tower hides every shorter one behind it. → Sudoku vs Minesweeper A grid hiding mines, where every revealed number tells you exactly how many mines sit in the (up to eight) cells touching it — you deduce which hidden cells are safe and which are mined. → Sudoku vs Battleships Place a known fleet of ships in the grid so each row and column holds the stated number of ship segments, no two ships touch — not even diagonally — and the given hints are respected. → Sudoku vs Crossword A word grid where you fill the white cells with letters so every numbered Across and Down entry spells a word that matches its clue, and crossing words agree on their shared letters. → Sudoku vs Latin Square An N×N grid filled so each symbol appears exactly once in every row and every column — the pure mathematical skeleton that Sudoku is built on. → Sudoku vs Word Search A grid packed with letters hiding a list of words to find — each word runs in a straight line horizontally, vertically or diagonally, in any direction. It is a spotting task, not a deduction. → Sudoku vs Rubik's Cube A 3D twisty puzzle with six coloured faces; you rotate its layers to return every face to a single colour, using learned move sequences and spatial reasoning. → Sudoku vs Yin-Yang Shade every cell one of two colours so that each colour forms a single connected group and no 2×2 block is entirely one colour. → Sudoku vs Jigsaw Puzzle The physical puzzle of assembling hundreds of irregularly-cut pieces into a complete picture, guided by piece shape and image rather than by logic. → Sudoku vs 15 Puzzle A 4×4 tray of fifteen numbered tiles and one empty space; you slide tiles into the gap to put them in order — a spatial sequencing puzzle, not a deduction. → Sudoku vs Scrabble A competitive word game: players place lettered tiles to build interlocking words on a board of bonus squares, scoring points by vocabulary and tactics. → Sudoku vs Tangram An ancient Chinese dissection puzzle: arrange seven flat shapes to fill a given silhouette exactly, with no overlaps or gaps. → Sudoku vs Wordle A daily word game: find a hidden five-letter word in six guesses, with colour feedback narrowing the possibilities each try. → Sudoku vs Mastermind A code-breaking game: deduce a hidden sequence of coloured pegs from feedback that scores each guess for exact and near matches. → Sudoku vs 2048 A sliding-tile game: swipe to merge equal powers of two, chasing a 2048 tile as the board fills. →