How to play the 15 puzzle
The 15 puzzle is the classic sliding puzzle: fifteen numbered tiles in a 4×4 frame, with one cell left empty. The tiles start out mixed up. You slide them around, one cell at a time, until they read 1 to 15 in order. In China it is called 数字华容道; in Japan, 15パズル.
The goal
Put the tiles in order from left to right and top to bottom, like reading a page: 1, 2, 3, 4 on the top row, 5 to 8 on the second, and so on. The empty cell ends up at the bottom right. As soon as the last tile is in place the puzzle is solved and the clock stops.
Tiles already in their right place have a light blue background, so you can see your progress at a glance.
How tiles move
- Tap a tile next to the empty cell and it slides into the gap.
- Tap a tile further along the same row or column as the empty cell, and every tile between them slides together. Each tile that moves counts as one move.
- Swipe on the board: the tile beside the empty cell moves the way your finger goes.
- Keyboard: the arrow keys or W A S D slide a tile into the empty cell from that side. H shows a hint, U undoes a move.
A tile can never jump over another tile or move diagonally.
Moves and time
The move counter goes up by one for every tile that slides. The clock starts with your first move and pauses while the page is hidden. When you finish you see your moves, your time, and your best so far for that size.
On 3×3 and 4×4 boards you also see the fewest moves possible for that exact board, worked out by a computer search. Most players need two or three times as many; matching it is a real achievement.
Undo, hints and restart
- Undo takes back the last slide, up to 50 in a row. The tiles go back, but the move still counts.
- Hint marks the next tile to move with a blue ring. You get three per game. Following hints always leads to a solved board, though not always by the shortest path.
- Restart puts the board back where it started, with the moves and the clock at zero.
- New game deals a fresh board. If you have already started, it asks first.
Your game is saved in your browser as you play, so you can close the tab and carry on later.
Board sizes
| Size | Name | Tiles | Fewest moves (hardest board) |
|---|---|---|---|
| 3×3 | 8 puzzle | 8 | 31 |
| 4×4 | 15 puzzle | 15 | 80 |
| 5×5 | 24 puzzle | 24 | not known |
| 6×6 | 35 puzzle | 35 | not known |
| 7×7 | 48 puzzle | 48 | not known |
Start with 3×3 to learn how tiles move around each other, then move up to the classic 4×4.
Why some boards can’t be solved
If you take a solved 15 puzzle and swap two tiles, for example 14 and 15, no amount of sliding will ever fix it. Exactly half of all possible arrangements are like this. In the 1880s a famous prize was offered for solving that “14–15” board, safe in the knowledge that nobody could.
The reason is a counting rule called parity: every slide changes the order of the tiles in a way that keeps one simple count (even or odd) the same. Every board on Number Slide is checked against this rule before you see it, so every board can be solved.
The daily challenge
Every day there is one new 4×4 board, the same for everyone. Each daily board takes between 44 and 56 moves at best, so every day is about as hard. When you solve it you get one to three stars:
- ★★★ at most one and a half times the fewest moves possible,
- ★★ at most two and a half times,
- ★ for any finish.
The calendar lets you go back and play the days you missed. Solving today’s board keeps your daily streak going.
Ready to get better? Read how to solve the 15 puzzle, step by step.
Curious where the puzzle came from? Read its history.