Reversi Best Move Calculator
Set up any Reversi or Othello position and this solver will find the best move, rank every legal alternative, and break the position down into the four things that actually decide it — corners, mobility, frontier discs and, eventually, disc count.
Built by Brian Hamilton
Place some discs, choose whose turn it is, then press Find best move.
Why the player with more discs is usually losing
Almost everyone learns Reversi by flipping as many discs as possible each turn. It is the most natural thing to do and it is close to the worst possible strategy. Every disc you own is a disc your opponent might flip back, and a big block of them in the middle of the board is not an asset — it is a target.
Here is a real game played out by this site’s own engine. Black takes the largest available capture every single turn; White uses the search. After 22 moves:
Run that position through the calculator and it scores Black at roughly −113. The breakdown is the interesting part: −80 of it is the single corner, −13 is mobility, −10 is the frontier, and the disc count — the thing Black has been optimising for the whole game — contributes exactly zero.
Why zero? The evaluation ignores disc count entirely until there are more than 52 discs on the board. Before that point, being ahead on discs mostly means being ahead on flippable discs. Only in the last few moves, when there is no longer time to flip them back, does the count start to mean anything.
The four squares that decide the game
A disc in a corner can never be flipped. There is no line through it that has another square on the far side, so once it is yours it is yours for the rest of the game — and it anchors safe discs along both edges leading away from it.
That makes the squares next to the corners the most dangerous on the board. The X-squares (b2, g2, b7, g7) sit diagonally inside each corner; the C-squares sit beside it along each edge. Playing one while the corner is still empty usually hands the corner straight to your opponent.
The danger is conditional, not permanent. Once a corner is occupied, the squares beside it are ordinary — the calculator switches their penalty to a bonus the moment the corner is taken, because they now build a safe edge rather than giving one away.
What playing an X-square actually costs
Black’s best options here — d2 and b5 — score
around −13, and which of them comes top shuffles a little as the search goes deeper.
b2 does not shuffle. It comes last of the seven at every depth from
2 to 8, scoring between −113 and −120, which puts it a consistent
hundred points or more below the best move. Almost all of that gap is the corner changing
hands.
That stability is the useful part. The difference between the top two or three moves is often within the engine’s margin of error and will move about if you search longer. A gap of a hundred points will not.
Reading the score
The number the calculator gives you is scaled so that it maps directly onto what the engine weighs. Positive is good for the side to move.
| Factor | Max swing | What it measures |
|---|---|---|
| Corners | ±80 | Share of the four corners held. The single biggest term. |
| Mobility | ±60 | Share of the legal moves available. Running out of moves is how games are lost. |
| Position | ±50 or so | The weighted square map — corners high, X- and C-squares negative until their corner falls. |
| Frontier | ±40 | Share of discs touching an empty square, and therefore still flippable. Fewer is better. |
| Disc count | ±30 | Who has more discs — but only once more than 52 are on the board. |
A difference of five or ten points between two moves is noise. A difference of eighty means a corner is changing hands. When the calculator ranks a move a hundred points below the rest, it has almost always found that the move gives up a corner.
Mobility, and why passing is a disaster
You can only play where you would flip something. That means your legal moves depend entirely on where your opponent’s discs are — and if you have none, you must pass and let them move again.
This is why flipping greedily backfires so reliably. Every disc you flip is a disc that is no longer your opponent’s, and your moves are generated by their discs. Taking a huge capture can leave you with three legal moves while they have fifteen, and from there they simply choose the moment to hand you the corners.
Strong play usually means flipping as few discs as possible while keeping your options open. The calculator will often recommend a move that flips exactly one disc, and the ranked list shows the flip count beside each option so you can see it doing it.
How this calculator works
It is the same engine that plays you on the Reversi game page, given more time and asked to show its working rather than just move.
- Search: minimax with alpha–beta pruning, assuming best play from both sides.
- Passes are handled properly. If a side has no legal move the turn passes back without consuming the position, and the game ends only when neither side can move — not when the board fills up.
- Move ordering: high-value squares are searched first, which prunes more of the tree and buys extra depth for the same time.
- Iterative deepening: it searches one ply, then two, then three, keeping the last completed result, so when the clock runs out you get the deepest finished answer rather than a partial one. Fast, Standard and Deep set the budget to roughly 0.8, 2.5 and 6 seconds.
- Evaluation: the five factors in the table above, combined into the single number shown with each move.
Where it is weak
- No opening book. Named openings mean nothing to it; from the standard start it is reasoning from scratch like everyone else.
- It does not solve the endgame exactly. A real Othello engine switches to a perfect solve in the last twenty or so moves. This one keeps using the same heuristic search, so in a tight endgame it can be beaten by a program that counts to the end.
- “Best” means best under those five factors, to the depth it reached. The depth is displayed with every result for exactly that reason.
- Stability is approximated, not computed. It knows corners cannot be flipped and that frontier discs are vulnerable, but it does not work out the full set of permanently safe discs.
Reversi, Othello, and the rules used here
The two names are not quite interchangeable. Reversi is the older Victorian game, in which the first four discs are placed by the players rather than fixed. Othello is the 1971 standardisation by Goro Hasegawa, which fixes the opening four discs diagonally in the centre and is what almost everyone now plays.
This calculator uses the Othello rules: the fixed diagonal start, Black moves first, you must flip at least one disc for a move to be legal, a player with no legal move passes, and the game ends when neither player can move. Whoever has more discs at that point wins.
Squares are named the way Othello players write them — columns a to
h left to right, rows 1 to 8 top to bottom, so the
top-left corner is a1 and Black’s four opening moves are
d3, c4, f5 and e6.
Play a full game
The analyser works on single positions. To play one out, the free Reversi game uses this same engine at three difficulty levels or as pass-and-play. There is also a checkers best move calculator built the same way, and if you like games in this family try Gomoku, Checkers and Nine Men’s Morris, or browse all the free online puzzle games.