What Is Fog of War Sudoku? Rules, Strategy and How to Play
Fog of war Sudoku is an ordinary Sudoku puzzle with most of the grid hidden. You start with a small clear patch in the middle, and every digit you get right lifts the fog around it, so the clues arrive while you solve instead of all at once.
The name comes from strategy games, where the fog of war is the part of the map you have not scouted yet. The Sudoku rules underneath are untouched. What the fog changes is how much of the evidence is on screen at the moment you have to decide something.
The rules that do not change
Every row, every column and every 3x3 box holds each of 1 to 9 exactly once. The puzzle has one solution, and it can be reached without guessing. If none of that is familiar yet, the rules and the step-by-step guide come first — all of it still applies with the fog on.
The rule the fog adds
A fogged cell shows nothing. You cannot read it and you cannot type into it. Put a correct digit anywhere and the fog lifts around it.
That is the whole mechanic, and it is about as far as the genre agrees. How far “around” reaches is left to whoever built the game. The boards on this page use Sudoku Fog’s answer — the cell you filled and the eight cells touching it, corners included — so read the specifics below as that game’s, not as rules of fog of war Sudoku.
What you can see at the start
Twenty-one cells out of eighty-one. Look at where they are: a row straight through the middle, a column straight down it, and the box where the two cross. That cross is Sudoku Fog’s opening, and it is a choice — another game would draw a different shape.
The constraint behind the choice is not optional, though. Fog only lifts next to a cell you have just filled, so whatever is clear at the start has to be able to reach the rest of the board by spreading. A patch tucked in one corner would leave the far side unreachable. Starting in the middle is the cheap way to satisfy that, and not the only way.
Grey does not mean empty. On this board 39 of the 60 hidden cells are clues, already printed on the grid, waiting for you to earn a look at them.
What one correct digit does
The centre box is fully clear, so it can be read like a box in any other Sudoku. It holds a 3, a 6, a 7 and an 8, and five of its cells are empty. Ask where its 2 goes.
Four of the five die immediately, and every one of those eliminations comes from the middle row or the middle column. The middle column carries a 2 near the top, which rules it out of the two empty cells sitting in that column. The middle row carries a 2 at its right-hand end, which rules it out of the two sitting in that row. One cell is left, and it takes the 2.
Twelve cells that were fog a moment ago, and eleven of them carry a clue. That is the exchange the fog sets up: you spend a deduction, and you get paid in information.
That is more than the eight cells the placement touches, and the reason belongs to Sudoku Fog rather than to the genre. On its easy and medium boards the reveal reaches one step further: when it uncovers a clue, it clears that clue’s neighbours as well. On hard and expert it stops at the eight.
What a wrong digit does
Nothing lifts. In Sudoku Fog a wrong digit also costs a life, and the board sits exactly where it was. In a plain Sudoku a wrong digit stays on the grid until some contradiction turns up and you trace it back; under fog you find out straight away, and you are told nothing else.
The practice board further down this page is gentler than the game. It refuses a wrong digit and leaves no mark, because it is here to demonstrate the mechanic rather than to test you.
It is still a fair puzzle
Worth stating plainly, because “most of the board is hidden” sounds like it ought to mean guessing. Before a Sudoku Fog board reaches you, the generator plays it out with a solver that may only use what a player could actually see, at that difficulty’s reveal depth, and throws the board away if that solver cannot finish it. It also checks the grid has exactly one solution. A board that needs a guess does not ship.
What the fog changes about solving
One thing, and it runs in a single direction. The fog can hide a clue that would have ruled a digit out. It can never invent a clue that is not there. So a candidate list you build under fog is too generous rather than too strict.
Two consequences are worth carrying into every board:
- A cell down to one candidate is safe. The true list sits somewhere inside the list you can see, and it cannot be empty, so one candidate means one answer. Naked singles need no adjustment here.
- “This digit can only go here” is not safe unless you can see the whole unit. If part of the row is under fog, the digit may already be sitting in it.
Those two, and the question of which cell to fill when several are available, are what the solving guide covers.
Try it
An easy board, opened the way the game opens one. The centre box is where the first deduction lives, and the middle row and column are the evidence for it.
Three ways this goes wrong
Reading a grey cell as an empty one. Most of them are clues. A row that looks half empty may be nearly full.
Trusting “only one place left” in a fogged unit. Count the grey cells in the unit first. If there are any, the digit you are placing may already be in there.
Filling the cell you are sure of without looking at what it opens. Two placements you have equally proved can uncover wildly different amounts of board, and one of them may uncover nothing at all.
Where to go next
How to solve fog of war Sudoku picks this up and works through which move to play first. Naked singles and hidden singles is the technique both articles lean on. Everything published so far is on the Learn index.
Sudoku Fog is on Android — What Sudoku Fog is, Get it on Google Play.