Play Slitherlink Online

How to Play Slitherlink

Modes: Easy · Medium · Hard · Daily

Slitherlink is played on a dotted grid. Draw a single closed loop along the line segments between the dots.

Every numbered cell tells you exactly how many of its four sides the loop uses: a 3 means three sides are part of the loop.

Pick the Line tool to draw loop edges, or the Cross tool to mark edges that are definitely not part of the loop.

The loop must never touch itself, and the whole thing must be one single unbroken loop.

Rules

A numbered cell shows how many of its four sides the loop passes along.

The loop is a single closed shape; it never branches or crosses itself.

No vertex of the grid can have more than two loop edges.

Empty cells give no information about their sides.

Scoring

History & Origins of Slitherlink

Slitherlink is a loop-drawing puzzle that first appeared in Japanese puzzle magazines in the late 1980s and quickly became one of the best-loved Nikoli-style formats. The grid is a field of dots and numbers, and the goal sounds simple: trace one unbroken closed loop along the dotted lines so that every number tells the truth about the loop edges around it.

Loop puzzles reward a different kind of thinking from filling in cells. Because the loop can never branch, touch itself or contradict a numbered square, solvers must weigh local counts against one global shape, often working both ends of a corridor toward a meeting point. Closing the final gap of a loop is one of the most satisfying moments in pencil logic.

ThinkyPlay draws easy loops on a 5×5 dot field, rising to a 7×7 Medium grid and a sprawling 9×9 Hard one. Proving uniqueness for big loops is expensive, so the harder boards come from a pre-verified pool, and Daily Slitherlink serves the same 7×7 board to players everywhere.

Common Mistakes to Avoid

• Joining a third line into a single grid point: each dot may carry at most two loop edges, and the game marks the offending lines red immediately.
• Skipping the cells marked 0, which forbid all four of their sides and usually unlock whole chains of forced edges once applied.
• Treating the Cross tool as an eraser, when a cross records a deliberate decision that an edge is definitely not part of the loop and is the key to tracking deductions.
• Finishing with extra edges or a disconnected shape: the puzzle demands one unbroken closed loop, so a dangling end or a second loop is never accepted.

Tips & Strategy

Look for cells marked 0 — none of their four sides are on the loop.

A 3 cell almost always needs its three outer sides; place them and mark the rest.

When two lines meet, the corner between them is forced: only the opposite sides can continue.

Mark excluded edges with Cross to avoid mistakes and expose forced lines.

Use the Hint button when stuck; it draws one correct segment.

Frequently Asked Questions

Is Slitherlink free to play?

Yes — Slitherlink on ThinkyPlay is completely free, with no download and no sign-up.

What does the number in a cell mean?

It is how many of the cell’s four sides belong to the loop. A blank cell tells you nothing.

Can the loop touch itself?

No — a vertex may only be crossed by two loop edges, and the finished shape must be one single loop.

What are the tools for?

The Line tool draws loop edges; the Cross tool marks edges you know are not part of the loop, which makes solving much easier.

Why does a line I draw turn red?

Red means a conflict: either three loop edges now meet at one grid point, or a numbered cell already has more of its four sides filled than its clue allows. Undo it and look for the safer edge instead.

How do Easy, Medium and Hard differ?

Easy is a 5×5 dotted grid, Medium is 7×7 and Hard is 9×9, so bigger boards wrap longer loops around more numbered cells. Medium and Hard draw from a pre-verified pool of puzzles whose single solution was checked in advance.

Must I use the Loop size you suggest, or can I draw it as small as I like?

There is exactly one correct loop, decided by the clues, and the game accepts only that solution. The loop often traces the border of a large connected region, but you should let the numbers guide its exact route.