Flood-It Solver
How to use the Flood-It solver
Choose the board size and number of colours, then copy your board: pick a colour and drag across the squares, or start from a random board. Press Solve. A short solution appears straight away as a row of coloured moves; step through it, or press Play to watch the flood spread. Meanwhile the solver keeps searching for a few seconds to prove whether any solution is shorter, and says what it found. Greg Puzzles is not affiliated with the makers of any Flood-It app.
The rules of Flood-It
The flood starts in the top-left corner. Each move picks a colour: the flooded area turns that colour, and every square of the colour touching it joins the flood. The board is won when it is all one colour, and the challenge is to get there within a set number of moves.
How to win at Flood-It
Grow towards the middle. Moves that push the flood into the centre of the board reach more of it later. Do not chase small gains. The colour that takes the most squares now is often not the best move; a colour that opens a path to a big patch beyond is. Finish colours off. Once a colour has no squares left outside the flood, it never needs choosing again, and every colour still on the board needs at least one move, which is one of the two counts the solver uses to prove that no shorter solution exists. The other is distance: the flood can only reach one patch further per move.
How it was checked
The solver was run on all 120 boards of the daily Flood-It. It flooded every one, each in no more moves than the daily page's own target, and 333 moves fewer across all 120. On small boards its proof of the fewest moves was checked against an exhaustive search.
More puzzles
There is a new daily Flood-It every day. The Lights Out solver works out another board-wide puzzle, and the 15 puzzle solver proves the fewest moves the same way. For puzzles you hold in your hands, see our brain teasers. There are more solvers on the puzzle tools page.
Puzzles you can hold
More puzzles from the shop.
Frequently Asked Questions
How do you win at Flood-It?
Push the flood towards the middle of the board, where it reaches the most squares later, and favour colours that open a path to a big patch beyond over the colour that takes the most squares right now. Finish each colour off so you never need to choose it again.
What are the rules of Flood-It?
The flood starts in the top-left corner. Each move picks a colour: the flooded area turns that colour and every square of it touching the flood joins in. You win when the whole board is one colour within the move limit.
What is the fewest number of moves in Flood-It?
It depends on the board. Every colour still on the board needs at least one more move, and the flood reaches only one patch further per move, so the farthest patch sets another minimum. The solver uses both to prove when a solution is as short as it can be.
Is Flood-It always solvable?
Yes, every board can be flooded eventually; the question is whether it can be done within the move limit, and that is what the solver works out.
Why can the solver not always prove the fewest moves?
Finding the shortest Flood-It solution is a famously hard problem for computers on big boards. The solver always finds a short solution, and on boards too big to settle in a few seconds it says how far from proven it is.