The rules
The tray holds numbered tiles and a single gap. Slide a tile into the gap by tapping it; tap a tile further along the same row or column and the whole line between it and the gap moves together. You can also swipe or use the arrow keys.
You win when the numbers read in order, left to right and top to bottom, with the gap in the bottom-right corner.
Sizes
Easy is the 3×3 board with eight tiles. Medium is the 4×4 board after a short shuffle, so the tiles are not far from home. Hard is a fully random 4×4 position and expert a random 5×5 board with 24 tiles.
Why every shuffle here can be solved
Exactly half of all possible arrangements of a sliding puzzle can be solved and the other half cannot, no matter how clever you are. Whether a position can be solved depends on its parity: the number of pairs of tiles that are in the wrong order, together with the row of the gap on even-sized boards. Every board on this site is checked for parity before you see it.
How to solve it
Work in layers. Finish the top row, then the left column, and you are left with a smaller version of the same puzzle. Repeat until only a 3×3 corner remains, then solve that corner as one piece.
The last two tiles of a row are the tricky part. Bring them in together: place the last tile under its target first, then the second-to-last tile, then rotate them up into the row. Our sliding strategy guide walks through the move sequence.
On the 3×3 board the notes for each numbered puzzle give the shortest possible solution length, so you can see how close you came.
Controls
Tap a tile in line with the gap, swipe on the board, or use the arrow keys. Undo and redo step through your moves. The hint highlights the next tile to move on a path that solves the board.
Questions
What is the fewest number of moves?
It depends on the shuffle. For 3×3 boards we compute the exact optimum, and for short 4×4 shuffles too. For the larger random boards we show a lower bound: no solution can be shorter.
Does a move count once if I slide three tiles at once?
The move counter counts each tap or swipe as one move, even when several tiles travel together.
Why does the hint not take the shortest route?
On 4×4 and 5×5 boards it follows the row-by-row method, which always works and is easy to follow, but is longer than the optimum.
Can I get an unsolvable board?
No. Every position is checked before it is shown.