Slitherlink Puzzle Generator
Slitherlink asks for one closed loop drawn along the grid lines, where each number says how many of that cell's four sides the loop uses. It is a deceptively hard rule to satisfy, because the loop must be single and closed — no branches, no crossings, no second loop hiding in a corner. That last condition is global, which is what makes the puzzle interesting and what makes generating a fair one genuinely difficult.
What this generator does
Grows a random closed loop, reads the clue for every cell from it, then removes clues one at a time — re-solving after each removal and putting the clue back if the puzzle stops having exactly one answer. The published puzzle therefore has no redundant numbers, and its uniqueness has been checked rather than assumed.
How to use this tool
- Pick a grid size and generate a puzzle.
- Draw along the dots: a cell marked 3 has the loop on three of its sides, a 0 on none.
- Remember the loop must close on itself and be the only one on the grid.
- Reveal the answer when you want to check, or seed the puzzle to set it again later.
Understanding the controls
- Grid size
- Between 3 and 6. The solver's cost climbs steeply with size, and 6 is where it still finishes in well under a second.
- Show the answer
- Draws the solution loop over the clues. Hidden by default so the puzzle can be printed or shared unsolved.
- Seed
- Any word reproduces exactly the same puzzle and the same loop, so a puzzle can be set now and checked later.
Common use cases
- Printable loop puzzles for a puzzle page or a classroom starter
- Practising the deduction patterns that Slitherlink solvers rely on
- Demonstrating a constraint that cannot be checked locally
- Setting the same puzzle for several people from one seed
- Test instances for a Slitherlink solver
How this generator works
The loop is built as the boundary of a randomly grown blob of cells, with any addition that would create a hole rejected — a hole would make the boundary two loops rather than one. Solving searches over which cells are inside the loop rather than over which edges are drawn: a 6x6 grid has 84 edges and only 36 cells, and shading a cell lets the clue diagonally behind it be checked immediately, so wrong branches die within a row.
Randomness and fairness
The loop is drawn at random and the clues follow from it. A seed reproduces the same puzzle exactly and is therefore explicitly not cryptographically secure. Without a seed the browser's cryptographically secure generator is used.
For how randomness is produced across the whole site, see how Generate Random works.
Limitations and good to know
- Square grids only, from 3x3 to 6x6 — larger boards need a constraint solver rather than a search.
- Difficulty is not graded; a small puzzle with few clues can be harder than a larger one.
- Clues are minimal in the sense that none can be removed, which does not guarantee the intended solving path is pleasant.
- No step-by-step solving hints are given, only the finished loop.
- A puzzle is gone once you reload — print it or note the seed.
Privacy and your data
The puzzle is generated and solved entirely in your browser. Nothing about the grid or your seed is transmitted or kept.
Related generators
- Bridges Puzzle GeneratorHashiwokakero island puzzles where bridges never cross and every island ends up connected, with exactly one valid layout.
- Nonogram GeneratorPrintable nonograms whose clues are proved to have exactly one solution, and to be solvable without guessing.
- Hitori Puzzle GeneratorShade out the repeats without letting two shaded cells touch or the rest come apart, with a single answer proved by construction and by search.
- Shikaku Puzzle GeneratorCut the grid into rectangles, one number in each, that number being its area — with the single answer proved before you see it.