Hard Sudoku
Tough 9×9 puzzles for when singles and pairs are no longer enough. Each one has a single solution, and a logical solver has proved you can reach it without guessing.
What makes these Sudoku hard
Graded by technique, not by clue count
A puzzle is Tough when the hardest step a logical solver needs is an X-wing, an XY-wing, a swordfish or a triple. Puzzles that fall to singles, pairs or pointing pairs are kept for the Gentle and Moderate levels, so a Tough puzzle always asks for at least one advanced step.
Fewer givens
Tough puzzles start with 22 to 30 digits, removed in pairs that mirror each other through the centre of the grid. There are 240 of them in the bank, and each game shuffles one (relabelled digits, swapped rows and columns, sometimes a flip) and checks it again before you see it.
X-wing and swordfish
If a digit fits in only two cells of one row and only two cells of another, and those four cells share two columns, the digit must sit in two opposite corners of that rectangle. It can be removed from every other cell of those two columns. A swordfish is the same idea with three rows and three columns.
XY-wing and triples
An XY-wing is a cell with two candidates, say 3 and 7, that sees one cell with 3 and 5 and another with 7 and 5. Whichever digit the first cell takes, one of the others becomes 5, so 5 can go from any cell that sees both. A triple is three cells in a row, column or box that share three candidates between them: those digits can leave the rest of that unit.
Notes make it possible
At this level, write candidates in every empty cell (turn on Notes or press N) before you look for patterns. Placing a digit removes it from the notes in its row, column and box for you. Stuck? Hint shows the next logical step and names the technique it used.
Questions
Is a hard Sudoku harder because it has fewer clues?
Not by itself. Some puzzles with few clues are easy and some with many are hard. Here difficulty is set by the hardest technique a logical solver needs, and Tough puzzles always need one of the advanced ones.
Do I ever have to guess?
No. Every Tough puzzle has exactly one solution and was solved by logic alone at its level before it was added to the bank. If you find yourself guessing, there is a step to find: try the hint.
Is the daily Sudoku hard?
No. The daily Sudoku is a Moderate puzzle, the same for everyone, so more people can finish it. Hard Sudoku is practice you can play at any time, as many as you like.
Can I switch to an easier level?
Yes. Use the level button above the board to pick Gentle, Moderate, the Mini 6×6 or the daily. Each level keeps its own unfinished game.