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📖 How to Play Variant Sudoku
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Variant Sudoku — fill every row, column and box with digits 1–9. Eight variants (Classic, Diagonal X, Jigsaw, Windoku, Anti-Knight, Even/Odd, Thermo, Killer), 7 difficulty levels from Beginner to Cyborg,…
Sudoku is the most loved logic puzzle in the world for a reason: simple rules, infinite depth. The board is a 9×9 grid divided into 81 cells. You're given a handful of digits to start; your job is to fill in the rest so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. Every Sudoku has one and only one valid solution.
The Diagonal variant adds two more constraints: the main diagonals (top-left to bottom-right, and top-right to bottom-left) must also each contain 1–9 once. Cells on the diagonals have additional peers and the puzzle solves with fewer givens. The diagonal cells are shaded gold on the board.
Jigsaw drops the 3×3 boxes entirely and replaces them with nine irregular regions, each in a distinct pastel colour. The row and column rules are unchanged, but the 1–9 constraint now applies to the coloured regions instead of boxes. Jigsaw puzzles look familiar but solve differently — your usual box-scanning techniques need adaptation.
Beginner puzzles often resolve through naked singles — a cell where 8 digits are already accounted for in its peers. Medium puzzles need hidden singles: a digit that can only fit in one cell of a particular row, column, or region. Harder puzzles need pencil-mark techniques: naked pairs/triples, pointing pairs, and at the extreme end X-Wing and swordfish patterns. Toggle ✏️ Notes mode to pencil candidates in any cell.
Tap any cell to select it. The selected cell turns yellow, all its peers (same row, column and region) tint blue, and every cell with the same digit highlights darker blue — perfect for spotting where else a digit can go. Tap a digit on the numpad to fill, tap the same digit again to clear, or hit ⌫ to erase. Given cells (the puzzle's starters) are bolded and can't be edited. Long-press any cell on mobile to clear it.
Difficulty controls the number of given cells. Difficulty is the technique a board actually requires, measured by the solver — not how many cells are blank. A board with fewer givens is not automatically harder. Beginner boards fall to naked singles alone. Medium needs at least one advanced deduction. Cyborg ☠ demands the full toolkit — X-Wings, XY-Wings and swordfish. Every board is proven to have exactly one solution and to be solvable without ever guessing. Daily challenges run at Medium so everyone can chase the streak.
Play Variant Sudoku free in your browser — no download, no sign-up. Eight variants (Classic, Diagonal X, Jigsaw, Windoku, Anti-Knight, Even/Odd, Thermo and Killer), up to seven difficulty tiers each, a seeded daily challenge that's the same for every player worldwide, and shareable challenge links so you can pass your best time to friends and see if they can beat it. Each variant offers the tiers it can genuinely deliver — Killer is a harder puzzle at every setting, so it offers fewer — and the list updates when you change variant.
Sudoku was popularised in Japan by Maki Kaji of Nikoli in the 1980s — though the format traces back to 18th-century Latin Squares and a 1979 Dell Magazine puzzle called "Number Place" by Howard Garns. The Diagonal X variant was added by Nikoli in the early 2000s; Jigsaw (sometimes called Squiggly Sudoku) emerged around the same time. This implementation uses a dig-from-solved generator with a uniqueness check at every step — every puzzle is guaranteed to have exactly one solution.
Sudoku is the puzzle that pulled me deeper into this whole world. When I was eleven, a national competition made the news back in India — the winner's time was printed in the paper along with the puzzle itself, and my father handed me the page and a stopwatch. The winning time was 10:54. I finished in 8 minutes 32 seconds, and I never really stopped chasing that feeling. It made me more curious about every grid puzzle I met afterward, and years later it's a large part of why this site exists at all.
So when I built the variant edition, I had strong opinions about what makes a variant fair. Here's the thing I learned tuning it: a variant's whole difficulty lives in its added rule. The base game is the same 9×9 you know. What changes everything is the extra constraint — and the hardest variant to make fair is the one whose added rule most easily forces the board. Our Diagonal mode is exactly that case: requiring both main diagonals to hold 1 through 9 injects so much extra deduction that a normally-clued board collapses into triviality. The answer is to tune the clue density down. Fewer givens keeps real deduction in play. It genuinely helps you solve, because the diagonals hand you inferences classic Sudoku never could — but it also trips players who don't adjust their expectations to a sparser board.
Which brings me to the mistake I see constantly. Players carry plain-Sudoku scanning into a variant and simply ignore the added rule. In Diagonal mode they scan rows, columns, and boxes beautifully — and never once check what the two diagonals force. In Jigsaw mode they keep looking for 3×3 boxes that no longer exist, instead of reading the irregular regions the puzzle actually uses. The variant's extra constraint isn't decoration. It is precisely where the puzzle's intended solving path runs. Skip it and you're solving a harder puzzle than the one in front of you, with fewer tools.
My practical advice: before placing a single digit, spend thirty seconds reading only the variant layer. In Diagonal, look at where the two diagonals cross the given digits — the centre cell sits on both, and it's frequently the most constrained cell on the board. In Jigsaw, trace each region's shape with your eyes until the geography feels familiar; the long snaking regions force digits in ways square boxes never do.
Start with Diagonal if you're coming from classic Sudoku — it adds pressure without changing the map. Save Jigsaw for when you want the map itself to fight back. Both dailies reset at midnight, same puzzle for everyone. My father's stopwatch rule still applies: time yourself, and only race your own number.