How to solve Light Up: lighting every cell
5 min read ·
Light Up (Akari in Japanese) asks you to light a dark grid with bulbs. Each bulb shines along its row and column until a black cell stops it. Every white cell must end up lit, but no bulb may shine on another, and numbered black cells say exactly how many bulbs touch them.
The numbers give you the first bulbs; the "only one way to light this cell" rule gives you the rest. Here's how they fit together.
The rules
- Place bulbs on white cells. A bulb lights its whole row and column until a black cell blocks it.
- Every white cell must be lit.
- No bulb may light another bulb.
- A numbered black cell has exactly that many bulbs directly next to it (above, below, left, right).
Start with the numbers
- A 4 has bulbs on all four sides. A 3 against a wall or a black cell, with exactly three open sides, takes bulbs on all three.
- A 0 has no bulbs next to it: dot all its neighbours.
- Whenever a number has as many open neighbours as it needs bulbs, fill them all; when it already has enough bulbs, dot the rest.
Dot the cells that can't hold a bulb
Every cell lit by a bulb can't hold another bulb (it would light the first one). Every neighbour of a satisfied number can't either. Mark those with a dot. Dots shrink the board fast and make the next step visible.
Cells only one spot can light
Look at each unlit white cell and ask: which cells could put light on it? The candidates are the cell itself and the cells in its row and column up to the nearest black cells, minus every cell you've dotted. If only one candidate is left, the bulb goes there.
This rule is especially strong for dotted cells themselves. A dotted cell can't light itself, so if exactly one cell in its line can still hold a bulb, that cell gets it.
A solving routine
- Place bulbs around 4s and fully constrained numbers; dot around 0s.
- Dot every cell lit by a bulb and every neighbour of a satisfied number.
- For each unlit cell, find the cells that could light it; place a bulb if only one is left.
- Re-check every number after each new bulb.
Practise
On Puzzimo, tap a cell to mark it with a dot • (tap again to clear), double-tap to place a bulb 💡, and drag to mark many cells at once. Lit cells glow as you play. A bulb where the solution has none counts as a mistake: it's shown and turned into a dot, and the third ends the round. Boards come in 7×7, 10×10 and 12×12, with a 7×7 daily challenge.
Frequently asked questions
Is Light Up the same as Akari?
Yes. Akari is the original Japanese name; Light Up and Akari use exactly the same rules.
Do unnumbered black cells need bulbs?
No. A black cell without a number has no rule about bulbs next to it; it only blocks light.
What do the dots mean?
They're your notes for cells that can't hold a bulb, because they're already lit or next to a satisfied number. They don't light anything, but they make the remaining choices clear.