To solve Light Up, start from the numbers, then follow the light. A 0 has no bulbs beside it, and a number with only just enough room has a bulb in every square left. Every bulb you place rules out its whole row and column. After that, look for dark squares that only one place can still light, and for squares where a bulb would block too much. Seven techniques cover every puzzle in the game.
New to Light Up? Read the Light Up rules first. This page assumes you know the three: every white square lit, no bulb shining on another, and each number counting the bulbs beside it.
1. Lit squares
A bulb lights its row and column until a black square stops it. No other bulb can stand in that light, so mark those squares with dots.
2. A satisfied number
A number that already has all its bulbs has no more beside it. A 0 has all its bulbs from the start, so it’s the place to begin.
3. A number that needs every square
Turn it round: a number with just enough room left for the bulbs it still needs gets a bulb in every one of those squares. A 4 always does. So does a 3 on an edge, a 2 in a corner, or any number boxed in by black squares and dots.
4. One way to light
A dark square with only one place left that could light it, itself or along its row or column, gets lit from there. Corners boxed in by black squares are the easiest to spot: they can only light themselves.
5. Number corners
A bulb lights two neighbouring squares at once when it stands diagonally between them. So a square diagonal to a number can’t have a bulb if that would leave the number short of room. The classic case is a 3 in the open: it needs three of its four squares, and a bulb on any diagonal would block two.
6. Leaving a square dark
Every dark square needs some way to be lit. A square whose bulb would rule out every way left to light another square gets no bulb.
7. What if
When nothing else works, pick a square and suppose it has a bulb, or doesn’t. Follow what that forces with the simple techniques. If a rule breaks, it’s the other way. The what-if page follows one through a real puzzle.
How often each technique is needed
We solved every one of the game’s 150 levels by following the hints, which always use the simplest technique that works. This is how many times each technique was used, on average, per level:
| Technique | Easy 7×7 | Medium 10×10 | Hard 12×12 |
|---|---|---|---|
| Lit squares | 6.9 | 12.6 | 16.5 |
| A satisfied number | 2.5 | 4.8 | 9.3 |
| A number that needs every square | 3.5 | 6.6 | 8.2 |
| One way to light | 3.5 | 6.4 | 10.0 |
| Number corners | 0.9 | 1.5 | 6.6 |
| Leaving a square dark | 0 | 0.3 | 0.7 |
| What if | 0 | 0 | 6.9 |
The first four do most of the work on every track. Number corners is needed on 19 of the 50 Easy levels, 27 Medium and 49 Hard; leaving a square dark on 13 Medium and 27 Hard; and what-if on every Hard level and nowhere else.
A 10×10, solved in stages
This is Medium level 44 from the game.
Stage 1: the numbers, and their light
Three 0s rule out the squares around them. The 4 in row 5, column 3 gets a bulb on every side. Then five 1s each have only one square left beside them, so each gets its bulb, and every bulb’s light rules out its row and column. That’s the first 18 steps.
Stage 2: dark corners
Five dark squares, most of them boxed in by black squares and dots, now have only one way left to be lit. Each gets its bulb.
Stage 3: number corners
The 1 in row 2, column 6 still needs its bulb, from one of two squares: above it or to its left. A bulb in row 1, column 5, diagonal to the 1, would light both of them, leaving the 1 with nowhere to go. So no bulb there.
Stage 4: leaving a square dark
Row 3, column 7 can now only be lit from two places: row 1, column 7, or row 3, column 8. A bulb in row 1, column 8 would light both, leaving it dark. So no bulb there.
Two more dark squares with one way to be lit finish it. In all, the level takes 30 steps: 10 from lit squares, 4 from satisfied numbers, 6 from numbers that need every square, 8 with one way to light, 1 number corner and 1 square that would be left dark.
When you get stuck
- Dot every square a bulb lights. Most of the work is ruling squares out.
- Check every number for the bulbs it still needs and the room it has left.
- Look for dark squares in tight spots, against black squares and dots: they often have only one way to be lit.
- Check the diagonals of numbers that need most of their squares.
- Don’t guess. Every puzzle here can be finished by logic.