Sliding Puzzle - Free Online 15 Puzzle - Free Online

About the Sliding Puzzle

The sliding puzzle — also called the 15 puzzle — is one of the oldest and most famous mechanical puzzles ever made. Numbered tiles sit in a grid with a single empty space; you slide tiles into that gap, one at a time, until the numbers run in order from the top-left with the gap in the corner. It sounds simple, but planning the moves so you do not undo your own progress is a real test of spatial reasoning and patience. Play the classic 4×4, warm up on a 3×3, or take on the 5×5 for a proper challenge — every shuffle is guaranteed solvable.

How to play

Pick a size. The tiles are shuffled into a solvable scramble.
Tap any tile next to the empty space to slide it into the gap.
Work the numbers into reading order, 1 first, gap in the corner.
Solve it in as few moves as you can — your best is saved.

What it measures

This test focuses on your spatial reasoning, planning and working memory. The 3×3 can be solved in well under a minute with practice; the classic 4×4 takes real planning, and efficient solvers finish it in around 150 moves or fewer.

Why play it

Trains spatial planning — you must think several slides ahead
A pure logic challenge with no reading or arithmetic required
Three sizes scale from a quick warm-up to a serious puzzle

Did you know?

The 15 puzzle sparked a craze across America and Europe in the 1880s, with newspapers offering cash prizes for a specific unsolvable arrangement.
Exactly half of all possible tile arrangements are solvable — the other half can never be completed, which is why good versions only ever deal solvable scrambles.
Mathematician Sam Loyd famously popularised the puzzle, though he did not invent it.
The same sliding mechanic underlies countless later puzzles, from picture-slide toys to app-store hits.

What counts as a good score?

No published norms exist for this format, so the ranges below are practical skill tiers rather than measured population data.

Level	Moves to solve a 15-puzzle	What it means
Learning	200+ moves	Solving by trial and error rather than by a layer method. Very common before learning the technique.
Comfortable	120–200 moves	Using a row-by-row method but still fixing mistakes as you go.
Strong	80–120 moves	Clean layer solving with efficient corner cases — near the worst-case optimum for a hard scramble.
Expert	Under 80 moves	Below the hardest-case bound, meaning genuinely efficient routing on an easier scramble.

The hardest possible 15-puzzle position requires exactly 80 moves in the single-tile metric, proved exhaustively in 2011. Tiers below compare your move count to that known bound.

How to improve your sliding puzzle score

1.Solve top row, then left column

Complete the top row, then the leftmost column of what remains. This shrinks the puzzle to a smaller one each time and never disturbs solved work.

2.Learn the last-two trick

The final two tiles of a row cannot be placed directly. Position them together one row down, then rotate them in as a pair — this is the step people get stuck on.

3.Finish with a 2×2

Reduce to a 2×2 corner and simply rotate it. Everything before that is setup for this.

4.Plan the blank's route

You are really moving the empty square. Thinking in terms of where the blank needs to travel is far more efficient than thinking about tiles.

5.Never break a solved row

If you find yourself disturbing completed work, the method has gone wrong. Back up rather than improvising.

Half of all scrambles are impossible

The 15-puzzle has 16! arrangements, but only half are reachable from the solved state. Whether a position is solvable is determined by a parity invariant — the permutation parity of the tiles combined with the blank's row distance from home — proved by William Woolsey Johnson and William Story in 1879.

This is why Sam Loyd's famous late-1800s prize for solving a position with the 14 and 15 swapped was safe money: that arrangement is provably unreachable. Loyd claimed to have invented the puzzle, which he had not — it was created by Noyes Chapman around 1874 — but the unsolvable-challenge stunt is genuinely his.

The worst case is exactly 80 moves

God's number for the 15-puzzle — the maximum moves needed from any solvable position under optimal play — is 80 in the single-tile metric, established by exhaustive computation in 2011. In the multi-tile metric, where sliding several tiles at once counts as one move, it is 43. Finding the shortest solution for a general n-puzzle is NP-hard, which is why human layer methods trade optimality for something you can actually execute.

Frequently Asked

How do you solve a sliding puzzle?

Work in stages: solve the top row and left column first and "lock" them, then reduce the remaining puzzle to a smaller grid and repeat. The trick is placing tiles without disturbing the ones you have already finished, which is what the corner-rotation technique is for.

Is every sliding puzzle solvable?

Not every arrangement is — only half of them can be solved. Our shuffles are generated by making random legal moves from the finished state, so every puzzle you are dealt is guaranteed solvable.

What is the 15 puzzle?

It is the classic 4×4 sliding puzzle with 15 numbered tiles and one empty space — the most common size. Our game also offers a 3×3 (the 8 puzzle) and a tougher 5×5.

Why can I not solve this puzzle no matter what I do?

If tiles were physically removed and reinserted, you may be in an unreachable position — exactly half of all arrangements are unsolvable. Digital versions including this one only generate solvable scrambles, so a genuine scramble here always has a solution.

What is the fewest moves needed?

It depends on the scramble, but no solvable 15-puzzle position ever needs more than 80 single-tile moves. That bound was proved by exhaustive search in 2011.

Sliding Puzzle - Free Online 15 Puzzle