Sudoku Fog · TriTail Studio

Q: Do I need to install anything to try Sudoku Fog?

No. There is a demo board that runs in any modern browser, with no account and no install. The full game is a free download for Android.

What is Sudoku Fog?

Standard Sudoku hands you the whole puzzle at the start. Sudoku Fog does not. It wraps a standard Sudoku engine — every row, column, and 3×3 box still needs 1–9 without repeats — in a fog of war. Most cells start covered, and each correct placement lifts the fog around it, so the grid arrives a piece at a time.

That changes what solving feels like. You are not scanning a finished board for a pattern; you are working with the part you can currently see, and every placement can reveal more of it.

An Expert board early on, most cells still under fog — Early on, most of the grid is still covered.

The same board later, with a large area of fog cleared — Correct placements lift the fog, opening the board ahead.

How it works

Start in the fog. Most of the board is hidden when the puzzle opens. The rest of the grid is yours to uncover.
Solve to reveal. Place a correct number and the fog around it clears. A wrong one costs a heart.
One reveal leads to another. Clearing one area usually opens up the next, so a single placement can cascade.

Variants, solved under the fog

A variant adds one extra rule on top of classic Sudoku. Sudoku Fog’s variants run under the same fog as classic mode — so a constraint can be sitting in a cell you haven’t uncovered yet, and part of the work is reasoning about clues you cannot see.

Variant	The rule
King’s Move	Cells a king’s move apart — diagonals included — can’t hold the same digit.
Odd/Even	A circle marks a cell that must hold an odd digit; a square marks one that must hold an even digit. Unmarked cells can hold either.
X-Sudoku	Both main diagonals must also contain 1–9 with no repeats.
XV	An X between two cells means they sum to 10; a V means they sum to 5. No letter between two cells means neither.
Consecutive	A bar between two cells means their digits differ by exactly 1. Two adjacent cells with no bar between them never differ by 1.
Kropki	A white dot means the two cells are consecutive; a black dot means one is double the other, and a 1–2 pair carries a black dot. No dot between two cells means neither.
Arrow	The digit in a circle equals the sum of the digits along its arrow. The circle itself is not part of that sum.
Thermometer	Digits increase strictly along each thermometer, from the bulb to the tip.
Renban	Digits along a line form a run of consecutive numbers, in any order.
Anti-Knight	Cells a knight’s move apart can’t hold the same digit.

A Thermometer board: pale blue thermometer bars drawn across the grid, several of them running over cells still hidden by fog — Thermometer — the bars run over fogged cells too.

A Kropki board: white and black dots sitting between cells, including between covered ones — Kropki — the dots are readable before the cells are.

An Arrow board: arrows drawn from circled cells across the grid, most of the cells they cross still under fog — Arrow — a sum whose parts you cannot see yet.

Mixed variants

Some boards carry two rule sets at once. There are four hand-picked pairings — Thermometer × Arrow, Arrow × XV, Renban × Kropki and Thermometer × Kropki — and each opens up once you hold both halves. The two rules feed each other: the order a thermometer forces along its line is often the only thing that pins down what the arrow beside it can add up to.

An Arrow × XV board: arrows and X and V markings on the same grid, most cells still fogged — Arrow × XV — two rule sets on one grid.

A Kropki × Thermometer board: thermometer bars and black and white dots sharing the same grid under fog — Kropki × Thermometer — dots and bars, both readable through fog.

A free player starts with King’s Move and Odd/Even, plus classic fog which uses no variant rule at all. The other eight open with variant unlock points, earned three ways: a win on Hard or Expert, and finishing the day’s daily challenge, are worth a point each and are capped at one a day apiece; streak milestones pay a bonus on top, and that bonus is not capped. Mixed variants need no unlock of their own — holding both halves is enough. Pro has every variant and every mix from the start, with no unlocking.

Modes and challenge

Hardcore mode. Mistakes aren’t flagged automatically — no safety net. You fill cells freely, then tap the flashlight to verify, and each wrong cell costs a life. It has its own leaderboard.

A daily challenge. One board a day, the same one for every player, and completing it earns a food shard. Weekdays are classic fog; on Saturday and Sunday the daily carries one of the variants. Finishing it without a single mistake pays a bonus.

Three quests a day. Small objectives — finish a Medium or harder game, reveal forty fog cells, take a Hard board with no hints — each paying a shard.

Four difficulties, separated by how many clues you start with.

The difficulty picker: Easy, Medium, Hard and Expert with their starting clue counts — Easy through Expert, by starting clue count.

The lobby, showing the daily challenge, quests, and entries for the collection, statistics, achievements and leaderboard — Everything runs from one lobby.

A world to uncover

Hidden cells sometimes hold dishes from a global cuisine collection: ten countries, ten dishes each, from Taiwan and Japan through to France, Germany and Brazil. You don’t know what’s under the fog until it lifts, so every placement carries a second question alongside the logic. Shards buy the dishes; finishing all ten in a country lifts the fog to the countries beside it, so the map opens along a different route depending on where you started.

The Culinary Exploration map, with several destinations still shrouded in fog — Destinations stay shrouded until a country beside them is complete.

Fair play and pricing

Free

No account required to start

King's Move and Odd/Even from the start; the other eight earned through gameplay
All four difficulties, Hardcore mode
Hints, smart notes and revives on a daily allowance
The daily challenge, quests and every leaderboard
The full cuisine collection and Cloud Sync

Pro

One-time purchase · No subscription

Every variant and every mix from the start — no unlocking
More hints, and smart notes without a daily limit
No ads forever
New features unlocked as soon as they ship

Pro buys convenience, not standing. No variant costs money — a free player can earn every one of them. What Pro changes is when you get them, not whether. The daily allowance refills each day, and finishing the daily challenge adds a smart note and a revive on top. And paying does not buy a place on the leaderboard: using a hint takes that game out of the rankings for everyone, Pro included.

Everything else

AchievementsMilestones for mistake-free games, Expert wins and long-term play.
LeaderboardsSix global boards: best times, daily, Hardcore, career, streak and a hall of fame.
Daily streakKept across devices, with its own board.
StatisticsGames played, win rate, mistakes and times.
Cloud SyncCollection, streak and unlocked variants live with your account, not one phone.

What it is not

Sudoku Fog is built for players who enjoy steady, satisfying logic rather than hunting for advanced whole-board techniques like X-Wing or Swordfish. Those techniques need most of the grid visible at once, and the fog is designed to take that away. If chasing large patterns across a full board is what you enjoy most, this is probably not the Sudoku for you.

Common questions

What is Sudoku Fog? Classic Sudoku with the board hidden under fog. Every correct number you place clears the mist around it, so the grid is revealed as you solve.

Is it free? Free to download and play. Pro is an optional one-time purchase, not a subscription, and every variant can be earned without paying.

Do I need to install anything to try it? No. There is a demo board on this page that runs in any modern browser, with no account and no install. The full game is a free download for Android.

How is it different from normal Sudoku? The rules are identical. What changes is what you can see: you are deducing from a partial board and uncovering the rest as you go, instead of reading a finished grid.

Is there an iOS version? Not yet. Sudoku Fog is on Android only.

Where to play

Sudoku Fog is on Google Play, for Android. There is also a playable demo board right here on this site — one puzzle, in the browser, no account and no install.

I built this because after years of solving Sudoku I missed the feeling of finding something I hadn’t already seen. Play one puzzle; five minutes is enough to know whether it works for you.