Hard Sudoku
Hard Sudoku requires locked-candidate deductions in addition to singles. Every grid has one solution.
▶ Play Hard SudokuHow to play Sudoku
Fill the 9×9 grid so every row, every column, and every 3×3 box contains the digits 1–9 exactly once. Tap a cell, then a number — or use Notes to pencil in candidates. There’s no guessing: every QZBrain puzzle is solvable by logic.
Frequently asked questions
What techniques do hard Sudoku need?
Hard puzzles require “locked candidates” (pointing/claiming) — spotting that a number in a box is confined to one row or column — on top of singles.
Can I select another puzzle?
Yes. Return to the game controls to change the difficulty, theme, or grid size.
Is there a daily mode?
Sudoku, Word Search, and Nonogram each include a daily puzzle and practice options.
QZBrain Focus includes adaptive memory, attention, number, and word games. Puzzle results are tracked separately. You can also read articles on brain training.