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 — STEP BY STEP
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, and I'll walk you through the first few moves one at a time so you can see the exact reasoning.
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.
Step 1 — 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 — 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. Now we just keep asking the same question.
Step 2 — same question, a busier box: where does the 2 go top-right?
A 2 in the top row knocks out the box's top squares; a 2 sitting in two of its 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 just sit there; you're only ever placing into the square nothing else can reach.
Here's the bit that makes Sudoku speed up as you go. Watch what happens when a number you place becomes the next blocker.
Step 3 — drop a 9 into the bottom-right box.
Same move, nothing fancy: the 9s already on the board pin this one to a single square. Notice where it lands — that fresh 9 is now part of the board, and a fresh 9 means a fresh blocking line for every box that shares its row or column.
Step 4 — and that new 9 instantly forces another.
Look at the right-middle box. One of the pink 9s doing the blocking is the very 9 we placed a moment ago — it sits in this box's column, so a 9 can't go anywhere down that column here. Add the other 9s already in range and, again, one square is left. Every number you place becomes a new line that boxes in the next one. That's why the back half of a Sudoku goes far quicker than the start.
Step 5 — and on it rolls: a 7 in the centre box.
Same question, same answer-by-elimination. You're not memorising anything — you're just repeating one move: pick a number, pick a box, see where it's forced.
That's the entire loop. 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; what you just placed will have opened something new. Keep going and the grid quietly fills itself in:
Over halfway — the same move, over and over.
Done. Every row, column and box has 1 to 9. 👻
WHEN SCANNING STALLS — YOUR OTHER MOVES
Cross-hatching solves most easy and medium puzzles on its own. But every so often you'll scan a box and no number is pinned yet. That's not a dead end — it just means it's time for the next move down. Here's the rest of the toolkit, in the order you'll reach for it.
1. Flip the question — "what goes here?"
Cross-hatching asks "where does this number go?" When that dries up, flip it and aim at a single empty square instead. Run through 1 to 9 and knock out any number that's already in that square's row, its column, or its box. Sometimes eight of the nine get knocked out and a single number survives — that's your answer. (Solvers call the cross-hatch a hidden single and this one a naked single; you can just call them "where does it go" and "what goes here.")
2. Pencil in the maybes.
When neither question cracks a square outright, jot the candidates — the numbers that could still go in each empty square — small in the corner. GridJoy has a notes mode that does exactly this. Seeing the maybes written down turns a fuzzy "hmm" into something you can actually scan, and most stuck moments dissolve the second you can see them.
3. Spot the little patterns.
Once the maybes are pencilled in, two easy patterns clear most of the rest. If a number's only possible squares in a box all sit in one row or column, it can't go anywhere else along that line — rub it out elsewhere (a pointing pair). And if two squares in a unit share the exact same two candidates, those two numbers belong to those two squares, so you can wipe them from every other square in that unit (a naked pair). You don't need these for a gentle puzzle — but they're the next rung when you want harder ones.
That's the whole ladder: scan for where a number's forced, flip it to ask what fits a square, pencil the maybes, then lean on a pattern or two. You never guess — every single step has a reason behind it.
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 ever, and a ghost who's weirdly proud of you when you finish.