Light Up Solver, Step by Step
Tap a square to make it black, then tap again to give it a number from 0 to 4; once more turns it white. On a keyboard, type a number, B for black or Delete for white.
How to use the light up solver
Set the width and height, then copy the black squares and their numbers from your puzzle. Press Explain the next step: the solver places the bulbs that are certain, marks squares that cannot hold one with a dot, outlines what changed, and says why. Lit squares turn yellow. Show the answer places every bulb at once and checks the solution is the only one. The puzzle is also called akari.
The rules of light up
Place bulbs in white squares. A bulb lights its own square and every white square in its row and column, as far as the first black square or the edge. Every white square must be lit, and no bulb may shine on another. A number on a black square says exactly how many bulbs sit in the squares beside it, above, below, left and right. Black squares without a number can have any number of bulbs beside them.
How to solve light up, step by step
Start with the 0s and 4s. Nothing goes beside a 0, and a 4 needs every square around it. A 3 against an edge, or a 2 in a corner, is just as full. Finish satisfied numbers. Once a number has its bulbs, the rest of its neighbours are empty. Find the squares with one way to be lit. A dark square that only one remaining square can see needs a bulb there, which is often the square itself. When those run out, the solver tries a square both ways and keeps the one that does not leave a square dark or a number unmet.
How it was checked
The solver was run on all 150 puzzles of the daily light up. It found each stored answer and found it to be the only one, and every step it explained was checked against those answers: it never placed or ruled out a bulb wrongly. All 150 were explained from start to finish without a trial. It was also run on 40 larger grids up to 16 by 16, most of which do need trials.
More logic puzzles
There is a new daily light up every day. The minesweeper solver reasons about neighbours in a similar way, and the nurikabe solver explains its steps the same way. For a puzzle you hold in your hands, see our brain teasers. There are more solvers on the puzzle tools page.
Frequently Asked Questions
How do you solve a light up puzzle?
Start with the 0s, which have nothing beside them, and the 4s, which need a bulb on every side. Once a number has its bulbs, its other neighbours stay empty. Then look for a dark square that only one remaining square can light: the bulb goes there.
What are the rules of light up?
Place bulbs in white squares so that every white square is lit. A bulb lights its row and column up to the first black square, no bulb may shine on another, and a numbered black square has exactly that many bulbs beside it.
Is light up the same as akari?
Yes. Akari is the name used by Nikoli, the Japanese publisher behind many of these puzzles; Light Up is the usual English name.
Do black squares without numbers need bulbs beside them?
No. An unnumbered black square only blocks light; it can have any number of bulbs beside it, including none.
Does every light up puzzle have one solution?
A well-made one does. The solver checks, and says if the grid you entered allows more than one.