Fog Sudoku: rules, strategy and how to start
Fog hides the board behind darkness and gives you back only what you prove. It turns sudoku into a game of information, not just logic.
Why the rule changes everything
Because the visible region grows with correct answers, the order in which you solve matters. A correct digit near the edge of the fog reveals more than one in the middle of the lit area.
The visible givens are your only anchor at the start. Every deduction chain must begin inside the lit area — an obvious naked single hidden in the fog is worthless until you reach it.
How to start a board
- Solve strictly inside the lit region, starting with the Givens nearest the fog boundary.
- When you place a correct digit, immediately re-scan the newly revealed cells — they usually produce another.
- Prefer frontier moves: a correct digit that touches three hidden cells is worth more than one that reveals nothing.
Common mistakes
- Guessing. A wrong digit reveals nothing and wastes the move.
- Ignoring half-revealed rows: a row can be almost complete while a single cell is still dark.
- Forgetting that the fog is only a view — the underlying puzzle is ordinary sudoku.
Frequently asked questions
Is fog sudoku harder?
It is not harder, it is slower. The logic is identical; you simply cannot see the parts of the board you have not earned yet.
Do mistakes reveal anything?
No. Only correct digits light up the neighbourhood.
Every board on this site is generated with a uniqueness check, so the puzzle in front of you has exactly one solution and every digit can be proved rather than guessed.
Play Fog Sudoku ↗