Sudoku looks like a math puzzle. It isn't. There's not a single sum, product, or calculation involved — the numbers 1 through 9 are just nine symbols you could swap for nine colors or nine animals and the puzzle would play identically. Sudoku is pure logic, which is exactly why it's one of the most satisfying puzzles ever invented. This guide takes you from "I've never solved one" to "I know how to make progress on any grid."
We build logic puzzles for a living, so we have a deep appreciation for how much depth Sudoku hides behind one tiny rule.
The one rule
A Sudoku grid is 9×9, divided into nine 3×3 boxes. Some cells start filled; your job is to fill the rest so that:
Every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once.
That's the entire game. No digit repeats in any row, column, or box. Everything you'll ever do in Sudoku is a consequence of that single constraint.
How to read the grid
Three overlapping regions govern every cell:
- Its row (9 cells across)
- Its column (9 cells down)
- Its 3×3 box
A digit you place must be legal in all three at once. So when you're deciding what goes in a cell, you're really asking: "Which digits are not already present in this cell's row, column, or box?" Whatever's left is a candidate.
A well-made Sudoku has exactly one solution, and it can always be reached by logic alone — you never have to guess. If you find yourself guessing, there's almost always a deduction you've missed.
Technique 1: Scanning for the obvious
The fastest way to start is "cross-hatching." Pick a digit — say 5 — and look at where 5s already appear. For each 3×3 box that's missing a 5, the existing 5s in nearby rows and columns "block" certain cells. Often only one cell in a box is left where a 5 can legally go. Place it.
Work through 1 to 9 this way and you'll fill a surprising number of cells before doing anything clever. On easy puzzles, scanning alone can nearly finish the grid.
Technique 2: The "last empty cell"
Whenever a row, column, or box has eight of its nine cells filled, the ninth is forced — it must be the one missing digit. These are free moves. Every time you place a number, glance at its row, column, and box to see if you just completed one of these "eight-of-nine" situations and unlocked another free cell. Solving Sudoku is largely a chain reaction of these.
Technique 3: Pencil marks (candidates)
When scanning stops producing moves, switch to pencil marks: in each empty cell, lightly note every digit that's still legal there. This turns an abstract puzzle into a visible one. Now you can spot:
- A cell with only one candidate → place it (a "naked single").
- A digit that can go in only one cell within a row, column, or box, even if that cell has other candidates → place it (a "hidden single").
Hidden singles are the single most important intermediate skill. A cell might show candidates 3, 7, 9 — but if it's the only cell in its box where a 7 could possibly go, then 7 it is, and the other candidates evaporate.
A worked mini-example
Imagine a box where the empty cells could hold {1,4}, {1,4}, and {4,8}. The first two cells both allow only 1 and 4. Between them, they will use up 1 and 4 entirely — so the third cell cannot be 4. That leaves 8. You just solved a cell without ever knowing which of the first two is 1 and which is 4. That's the flavor of all Sudoku logic: you don't always need to know everything to know something.
Common beginner mistakes
- Guessing. If you're tempted to guess, you've missed a deduction. Slow down and re-scan.
- Forgetting to update pencil marks. Every number you place eliminates candidates elsewhere. Cross them off immediately or your notes lie to you.
- Tunnel vision on one box. If a box stalls, switch to scanning a different digit across the whole grid. Progress in one region unlocks others.
- Ignoring columns. Beginners scan rows and boxes but forget columns carry the same constraint. Check all three regions for every cell.
Why Sudoku is worth the habit
Sudoku trains a very specific muscle: holding several constraints in mind at once and finding the cell where they collapse into a single answer. That's the same deductive feeling behind the word puzzles we build — our how to play Wordle guide describes the identical "eliminate until one option remains" logic, just with letters instead of digits.
Once scanning and singles feel automatic, you'll hit harder puzzles that need real strategy. That's where our guide to Sudoku techniques like naked pairs and X-Wing takes over. And if you want more puzzles in this family, our roundup of the best Sudoku and number puzzle games is a good place to start. Or, if numbers aren't your thing, the same deductive itch gets scratched by a daily word game like Pairdle.
Start with scanning, lean on the last-empty-cell freebies, and reach for pencil marks when you stall. That's a complete solving toolkit for your first dozen grids.