A strategy for hard puzzles
Refresh your candidates after each placement. Look for two cells in one unit that can contain only the same two numbers.
What makes these puzzles hard
Every hard puzzle needs a naked or hidden pair or triple at some point. Singles and locked candidates alone will not finish it, so full pencil notes become essential.
Work in passes
Fill in every candidate first. Then make one pass for singles, one for locked candidates and one for pairs. Return to singles after any elimination, because a single removal can unlock several placements.
Use the tools at your own pace
Notes record possibilities. Undo reverses your last change, one move at a time. Fill notes lists candidates from your current entries; Redo restores an undone move. Restart replays the same starting puzzle. A hint explains a step before placing a number. Pause hides the grid and stops the timer, and your game is saved on this device so you can come back later.
Still too easy?
Expert Sudoku adds patterns that span several rows and columns, such as the X-wing. If hard puzzles feel like a struggle, medium Sudoku is a good place to build speed with locked candidates.
Ready for another puzzle?
Choose New puzzle to start again, or visit the daily challenge for a puzzle that stays the same throughout the day. Prefer paper? Make a printable Sudoku sheet at the same level.
A few helpful answers
Yes. The game works in your browser. Progress is saved on this device when local storage is available.
Turn Notes on, select an empty cell, then choose numbers to add or remove pencil marks. Turn Notes off to enter a final number.
Yes. Every puzzle has exactly one solution, and our step-by-step solver finished each one using logical techniques before it was published.
With Auto-check on, a number that does not fit the finished grid gets a red outline and is counted as a mistake. There is no game over. Turn Auto-check off in the settings if you prefer to find errors yourself.
Yes. Use the arrow keys to move, type 1 to 9 to enter a number, press Backspace or Delete to erase, N to switch Notes on or off and U to undo.