THE WHOLE RULEBOOK (IT'S TINY)
You've got a 9×9 grid, split into nine 3×3 boxes. Some squares come filled in — those are your clues. Your job is to fill in the rest so that:
- every row has the numbers 1 to 9, each once, no repeats;
- every column has 1 to 9, each once, no repeats;
- every 3×3 box has 1 to 9, each once, no repeats.
That's it. That's the entire game. The numbers never get added or multiplied — they're just nine symbols that each have to show up once per row, column, and box. If you can spot a repeat, you can play Sudoku.
There's always exactly one right answer, and you never have to guess. If you're guessing, you've just not spotted the clue yet — it's in there.
LET'S ACTUALLY SOLVE ONE
Here's a real puzzle. Thirty-six numbers given, the rest to work out. Don't panic at the whole grid — we only ever look at one square at a time.
The starting grid — 36 clues, the rest is yours to find.
The move that actually solves Sudoku isn't "fill the gaps" — it's scanning. You pick a number and a box and ask one question: where can this number go in here? Then you rule out everywhere it can't go, and quite often only one square is left. Let's do it for real.
Where does a 9 go in the top-left box? Look at the pink 9s already on the board. There's one in the first row and one in the second row — a number can't repeat in a row, so a 9 can't go anywhere along those two (shaded) rows. There's also a 9 down the first column and one down the second column — so those are out too. Cross off every square those four lines hit and the whole box is ruled out except the bottom-right corner. The 9 has to go there. You didn't count a full line — you watched a number get boxed in.
That's cross-hatching, and it's the whole game. Here it is again with a busier box — same question, where does the 2 go?
A 2 in the top row knocks out the box's top squares; a 2 in each of those two columns knocks out the rest. Every empty square is crossed off but one — so that's where the 2 lives. The clues already in the box (the 6, 8 and 9) just sit there; you're only ever placing into the square nothing else can reach.
So the loop is: pick a number, pick a box, see if its rows and columns pin it to one square. Sometimes they don't — move to the next number or box and come back. Each digit you place adds another line that boxes in the next one, so it speeds up as you go. Keep scanning and the grid quietly fills itself in:
Over halfway — same move, over and over.
Done. Every row, column and box has 1 to 9. 👻
THE OTHER WAY ROUND (FOR WHEN SCANNING STALLS)
Cross-hatching asks "where does this number go?" When that dries up, flip it and ask the opposite about a single square: "what can possibly go here?"
Pick an empty square and run through 1 to 9. Knock out any number that's already in its row, its column, or its box. Sometimes eight of the nine get knocked out and a single number survives — and that's your answer for that square. Solvers call the first move a hidden single and this one a naked single; you can just call them "where does it go" and "what goes here."
Those two between them will carry you through most easy and medium puzzles. Harder grids add pencil-marking and a few pattern tricks, but you can pick those up once these feel like second nature. Walk before you run.
THAT'S IT — GO DO ONE
Spot what's missing, spot where a number's forced to go, repeat until it's full. The satisfying bit is real: that quiet little click when a square you've been staring at suddenly has only one answer left. There's a fresh one waiting below — no words, no ads in the middle of your puzzle, and a ghost who's weirdly proud of you when you finish.