Slitherlink

Draw one closed loop along the dots. Each number says exactly how many of that square’s four sides the loop uses. Boards from 5×5 to 10×10, a no-mistake challenge and a daily loop — every puzzle generated here and proved to have exactly one solution.

Making a puzzle…

How to play

Tap or click between two neighbouring dots to draw a line. Tap again and it becomes a cross, meaning you have decided the loop does not use it; once more clears it. Right-click, or press X on a focused line, to go straight to a cross.

A number tells you exactly how many of the four lines around that square the loop uses. Squares without a number are unconstrained, and most squares have none.

At every dot the loop either turns, passes through, or is not there — so a dot has two lines or none, never one and never three. And the finished answer has to be a single closed loop: two separate rings obey every other rule and are still not an answer.

Rules and controls

Lines run between neighbouring dots, never diagonally.
A number is an exact count, not a minimum.
Every dot the loop visits has exactly two lines on it.
Lines never cross.
The answer is one single closed loop, never two.
Tab moves between lines, Enter or Space cycles the focused one, and X marks it as definitely empty.

Modes

Small 5×5 — The smallest board and the place to start.
Medium 7×7 — More room, and longer chains of reasoning.
Large 10×10 — A hundred cells and a long loop.
No mistakes — One wrong edge ends the attempt. Be certain before you commit.
Daily Loop — One board, the same for everybody, seeded by the UTC date.
Choose your own — Pick the board and how hard it should be.

Tips

A 0 is the best clue on the board. Cross all four of its sides straight away — every one of those crosses helps somewhere else.
A 3 in a corner of the grid forces its two outer sides, because the loop cannot leave the board.
Two 3s side by side always use the line between them, and the far side of each.
A 1 in a corner never uses either of its two outer sides.
Once a line reaches a dot and only one way out is left, it has to take it. That single idea does most of the work on most boards.
If closing a line would make a small ring while clues elsewhere are still unmet, that line is wrong — a premature loop can never become the answer.

Privacy and local play

Everything runs in your browser. Your current puzzle, your settings and your best times are kept on this device only, and the button under the board clears them without touching any other Zerdly tool. No account, no sign-in, and nothing about your play leaves the machine you are on.

Questions

My clues all check out but the puzzle will not finish. Why?: Almost certainly because you have two separate loops rather than one. Two rings can satisfy every number and every corner and still not be an answer — the rule is one single closed loop, and it is the one people trip over. Look for a small ring somewhere off on its own.
Is a number a minimum or an exact count?: Exact. A 2 means exactly two of that square’s four sides are used by the loop, never three. A square with no number is unconstrained, and most squares have no number.
What does the cross mean?: That you have decided the loop does not use that line. It is a real state the route stores, not a decoration — marking what the loop cannot use is how the puzzle is actually solved. Tapping cycles undecided, drawn, crossed and back; right-click or press X to go straight to a cross.
Do I have to cross out every empty line?: No. A line you simply leave undecided is not used by the loop either. Crosses are there to help you think, and the puzzle is judged on the lines you have drawn.
How does the no-mistake mode differ?: One wrong line ends the attempt, so it is a different game rather than a harder one: you have to be certain before you commit instead of trying something and taking it back. It keeps its own best times, because a time earned under that rule is not comparable with one earned under the other.
What do the difficulty names mean?: A band is measured by solving the board rather than attached to it by the generator. Gentle can be finished by working out what each clue and each corner forces, one step at a time. Steady needs a little looking ahead. Tricky has far fewer clues, and at some point you have to try a line and follow it through.
Does the hint ever guess?: No. It only offers a line that follows from what is already on the board by the same deductions a player could make, and it says why. When nothing follows in a single step it says so rather than showing a line it could not justify. If the board already contradicts itself it tells you that instead of reasoning from a wrong line.
Why does Check not tell me which lines are wrong?: Because that would be a hint wearing a different label, and the route already has one of those. Check tells you how many lines cannot be right, which is enough to know whether to look again without doing the looking for you.
Can I read the answer out of my browser storage?: No. The board is not stored at all — only which puzzle it was, so it is rebuilt each time — and what is stored beside it is an eight-character hash of the solution rather than the solution. The hash is there so a restored board can be confirmed to be the board that was saved. It is not a security measure and is not offered as one.
Is anything sent to a server?: No. The generator, the solver, the hints, the sounds and your saved puzzle all run in your browser. There is no account, and clearing this game’s data leaves every other Zerdly tool untouched.

Related interactives

Arithmetic Cages — a Latin square with arithmetic targets.
Kakuro — cross sums, the other arithmetic grid on the site.
Nonogram — picture logic, also with exactly one solution per board.
Codebreaker — deduction against a hidden code, with a hint that cannot cheat.