Hard Sudoku· CardHearth
How to play, tips & FAQ
Hard Sudoku for confident solvers
Fewer clues, longer deduction chains. Hard boards demand more than scanning — but like every Sudoku here, they're guaranteed solvable by logic, so there's always a real next move to find. Fill each row, column and 3×3 box with 1–9.
Techniques that unlock hard boards
Beyond singles, watch for locked candidates: when a digit's only spots in a box all sit in one row or column, it can't appear elsewhere in that line — eliminate it and new singles appear. Naked pairs (two squares in a unit sharing the same two candidates) clear those digits from the rest of the unit. Disciplined pencil marks make both jump out.
Frequently asked questions
What makes a Sudoku "hard"?
Hard puzzles start with fewer given clues (around 29) and need techniques beyond simple scanning — notably locked candidates, where a digit confined to one line within a box lets you eliminate it elsewhere. Every hard puzzle here is still guaranteed solvable by logic alone.
Do hard puzzles ever require guessing?
No. CardHearth verifies every generated board is solvable by deduction within the hard technique set, so a logical next step always exists — you never have to guess and backtrack.
Any tips for hard Sudoku?
Keep pencil marks meticulous. Hunt for locked candidates and naked pairs to chip away at possibilities, and work box by box rather than rushing a single line.
Too tough, or not tough enough? Drop to standard or Easy, or climb to Expert.