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Minesweeper Probability

Sooner or later a board leaves you without a certain move. A guess is still a decision: here is how to make the good one.

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Classic minesweeper is not always solvable by logic. Mines are placed at random, and some positions simply do not contain enough information. What separates a good player from a lucky one is what happens then: first making sure it really is a guess, and then taking the tile with the best odds.

First: is it really a guess?

Most "guesses" are deductions nobody looked for. Before you risk anything, read every number along the edge of the open area again, check the common patterns, and count the mines that are left: near the end of a board the counter in the corner often settles a position that the numbers alone cannot.

The odds around one number

If a number still needs m mines and has t hidden tiles around it that nothing else tells you about, each of those tiles is a mine with probability m ÷ t.

A lone 1 with eight hidden tiles: each one is a mine 1 time in 8, or 12.5%.
A lone 3: each tile around it is a mine 3 times in 8, or 37.5%.

Compare with the rest of the board

A tile that no number touches is not free of risk. Its odds are roughly the mines still unaccounted for divided by the hidden tiles they could be in. At the start of a board that is simply the mine density:

BoardMinesOdds for an untouched tile
Beginner1012.3%
Intermediate4015.6%
Expert9920.6%

Now the two diagrams mean something. On Expert, a tile next to that lone 1 (12.5%) is a much better guess than a random tile elsewhere (20.6%). A tile next to the lone 3 (37.5%) is much worse: leave it and open something far away instead. The rule in one line: guess where the number is small compared with the tiles around it, and stay away from big numbers.

The honest 50/50

Two tiles, one mine, and both numbers see exactly the same two tiles. Nothing on the board will ever tell them apart.

When two tiles share one mine and every number that touches one also touches the other, no later move changes the odds. Waiting gains nothing, so take it when you get there instead of leaving it for the end: if it goes wrong, you have lost the least time. If numbers you have not opened yet could still split the pair, it is not a true 50/50: open those first.

Not all safe tiles are worth the same

Two guesses with the same odds can be very different in value. A tile in the middle of unknown ground may be an empty tile that opens a whole area; a tile wedged between numbers will only show one more number. With equal risk, take the guess that can give you the most information.

When is a guess worth it in multiplayer?

In a solo run a mine ends the board, so you guess only when you must. In multiplayer minesweeper a mine costs two points and 2.5 seconds, which you can weigh against what the guess might win. In a 1v1 duel there are 388 safe tiles and territory pays up to 12 points, so one point is about 32 tiles.

Practise without the luck

The first click in the Drill Yard always opens an area, so no run starts with a coin flip. After that it is real minesweeper: play a few boards and, every time you are about to guess, say the odds out loud first. It becomes automatic surprisingly fast.

Questions

Is minesweeper luck or skill?

Mostly skill. Most positions can be solved by logic, and when a guess is forced, choosing the tile with the lowest chance of a mine is itself a skill. Luck decides single boards, not your average.

Is every minesweeper board solvable without guessing?

No. With mines placed at random, some boards contain positions that logic cannot resolve. "No-guess" variants generate special boards to avoid that; classic minesweeper does not.

How do I work out the chance a tile is a mine?

For tiles around a single number: the mines that number still needs divided by its hidden tiles. For tiles nothing touches: the mines left on the board divided by the hidden tiles they could be in.

Where should I guess in minesweeper?

Next to small numbers with many hidden tiles around them, or on an untouched tile when every numbered option is worse than the board's mine density. Avoid tiles next to big numbers.

Should I guess straight away or leave it until the end?

A true 50/50 that nothing can ever resolve: straight away. Any other guess: after you have opened everything that might still give information about it.

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