Sudoku has one rule: fill the 9×9 grid so every row, every column, and every 3×3 box contains each digit from 1 to 9 exactly once. That's it. No arithmetic. No letters. No vocabulary. Just pure logical deduction.
Yet most beginners stare at a Medium puzzle and feel completely stuck after five minutes. The problem isn't intelligence — it's technique. Sudoku has a specific set of solving moves, and once you know them in order, puzzles that seemed impenetrable become straightforward. Here they all are, from simplest to most powerful.
The 30-Second Rules Recap
Before techniques: the structure. A standard Sudoku grid is 9×9 = 81 cells. It's divided into:
- →9 rows — running left to right
- →9 columns — running top to bottom
- →9 boxes — the 3×3 sub-grids (top-left, top-center, top-right, etc.)
Each of these 27 "units" must contain digits 1 through 9 without repetition. The pre-filled digits are your clues — they're fixed and can't be changed. Your job is to fill in the rest through logic alone. Every well-made puzzle has exactly one valid solution.
Technique 1: Naked Singles
Solves: Easy puzzles
A Naked Single is a cell where only one digit is possible — all other digits have been eliminated by the constraints of its row, column, and box.
How to find them: pick an empty cell and check its row, column, and box. Write down (or mentally track) which digits already appear. Whatever digit is missing from all three units and isn't already elsewhere — that's your answer.
Example:
A cell's row already contains: 1, 2, 4, 5, 6, 7, 8
Its column already contains: 3
Its box already contains: 9
Remaining possibility: only one digit not covered. That's the answer.
For Easy puzzles, scanning for Naked Singles repeatedly — going row by row, column by column, then box by box — will often solve the entire puzzle. Every time you fill in a digit, new Naked Singles emerge. Keep scanning until no more appear, then move to the next technique.
Pro tip: use pencil marks. Most digital Sudoku apps let you note candidate digits in each cell. Filling in candidates for the whole grid — even before solving — makes Naked Singles trivially visible as single-candidate cells.
Technique 2: Hidden Singles
Solves: Medium puzzles (with Naked Singles)
A Hidden Single is a cell that's the only possible location for a particular digit within a unit (row, column, or box) — even though the cell itself might appear to have multiple candidates.
The key shift here: instead of asking "which digit goes in this cell?", you ask "where can this digit go in this unit?" When only one cell in a row can contain, say, a 7 — then that cell must be 7, regardless of what other candidates it seems to have.
Example:
You're scanning the top-right 3×3 box for the digit 4. Six cells in the box are already filled. Of the three empty cells, two have 4 blocked by their respective rows or columns. The third cell has no such block.
That cell must be 4 — it's the only place 4 can go in that box, even if the cell also appears to be a candidate for 2 and 7.
Scan each unit systematically: for digits 1-9, find which cells in each row/column/box are valid placements. Whenever only one valid cell exists for a digit in any unit, fill it in immediately. This is the workhorse technique for Medium-level puzzles.
Technique 3: Pointing Pairs (and Triples)
Solves: Hard puzzles
Pointing Pairs appear when a digit's only valid positions in a 3×3 box are all within the same row or column. This creates a constraint that extends beyond the box.
The logic: if digit 5 can only go in two cells within a box, and both cells are in row 3, then 5 cannot appear anywhere else in row 3 outside that box. Why? Because 5 must go somewhere in that box — and wherever it lands, it occupies row 3. Any other 5 in row 3 would conflict.
Example:
In the middle-left box, digit 3 can only appear in two cells: both in row 5.
Conclusion: remove 3 as a candidate from all other empty cells in row 5 (outside this box).
This might now create new Naked Singles or Hidden Singles elsewhere in row 5.
Pointing Pairs don't directly fill in a cell — they eliminate candidates from other cells, which then enables other techniques to proceed. This "candidate elimination" thinking is the core of Hard-level Sudoku solving.
Technique 4: Box-Line Reduction
Solves: Hard puzzles
Box-Line Reduction is the inverse of Pointing Pairs. Here, if a digit can only appear in cells within a single box along a particular row or column, that digit can be eliminated from other cells in that same box.
Example:
In column 7, digit 8 can only appear in three cells — and all three are within the top-right box.
Conclusion: digit 8 cannot go in any other cell in the top-right box (besides these three column-7 cells), because the column constraint forces 8 to land in column 7 within that box.
Remove 8 as a candidate from all other cells in the top-right box.
Both Pointing Pairs and Box-Line Reduction are fundamentally about intersection logic — how rows/columns and boxes interact. If you practice spotting these patterns on Hard puzzles, they'll start to jump out naturally.
Technique 5: Naked Pairs (and Triples)
Solves: Hard / Expert puzzles
A Naked Pair is when two cells in the same unit both contain exactly the same two candidate digits — and no others. Because these two digits must occupy these two cells (in some order), they can be eliminated from all other cells in that unit.
Example:
In row 4, two cells each have exactly two candidates: {3, 7}.
Since 3 and 7 must fill those two cells (one in each), no other cell in row 4 can be 3 or 7.
Eliminate 3 and 7 from all other cells in row 4.
This often creates Naked Singles or Hidden Singles in the remaining cells.
The same logic extends to Naked Triples: three cells in a unit that between them contain only three candidate digits. These three digits must fill those three cells in some order, so eliminate them from the rest of the unit.
Naked Pairs can be surprisingly powerful. A single pair can unlock a cascade of placements across an entire puzzle. If you're stuck on a Hard puzzle and have exhausted the first four techniques, Naked Pairs is almost always what you need.
Common Mistakes (and How to Avoid Them)
Guessing when stuck
Guessing might seem harmless but it creates a web of errors that's extremely difficult to untangle. If a guessed digit turns out to be wrong ten cells later, you have no idea which cell introduced the error. Always deduce — never guess. If you're stuck, it means there's a technique you haven't applied yet.
Not using pencil marks
Trying to hold all possible candidates in your head is a memory exercise, not Sudoku solving. Fill in candidates from the start on any puzzle above Easy difficulty. You'll see patterns (pairs, singles) that are invisible without written candidates.
Forgetting to update candidates after placing a digit
Every time you fill in a cell, that digit must be removed from all candidate lists in the same row, column, and box. Skipping this step means your candidate lists become stale and you miss Naked Singles that should have appeared.
Only scanning rows and columns but not boxes
Beginners often check rows and columns intuitively but forget the 3×3 boxes. Many Hidden Singles are only visible when you scan box by box. Build a habit: scan rows, then columns, then boxes, for each digit.
Jumping to Hard puzzles before mastering Easy
Easy Sudoku uses exactly two techniques (Naked Singles + scanning). If you can't solve Easy puzzles quickly and comfortably, you don't have the foundation to benefit from learning Pointing Pairs on Hard. Build the foundation first.
Your Progression Path: Easy → Expert
Easy
Master naked singles by scanning rows, columns, and boxes systematically. Can you solve an Easy puzzle in under 10 minutes without errors? Then you're ready for Medium.
Medium
Add Hidden Singles. When Naked Singles run out, switch to asking "where can digit X go in this unit?" rather than "what goes in this cell?". Medium puzzles are fully solvable with these two techniques.
Hard
Learn Pointing Pairs, Box-Line Reduction, and Naked Pairs. Hard puzzles won't yield to the simpler techniques alone — you need candidate elimination. Fill in full pencil marks and look for intersection constraints.
Expert
Advanced patterns: Naked Triples, X-Wings, Swordfish, XY-Chains. These are techniques that took competitive Sudoku players years to codify. Take your time — there's no rush. Expert puzzles are genuinely hard.
Practice These Techniques Today
PuzzleBoxs Sudoku has four difficulty levels — Easy, Medium, Hard, and Expert — with a fresh grid every day. No ads, no account, pencil marks built in. Start with Easy and work your way up.
Play Sudoku Free →Frequently Asked Questions
What is the easiest Sudoku technique to learn first?
Naked Singles — cells with only one possible digit. Scan each empty cell and eliminate digits already in its row, column, and box. This single technique solves most Easy puzzles.
Should I ever guess in Sudoku?
No. Every well-constructed Sudoku has exactly one solution reachable through logic alone. Guessing leads to cascading errors that are very hard to undo.
What is the difference between Easy and Hard Sudoku?
Easy needs only Naked Singles and Hidden Singles. Medium adds Pointing Pairs and Box-Line Reduction. Hard requires Naked Pairs and similar elimination techniques. Expert needs advanced patterns like X-Wings.
What are pencil marks in Sudoku?
Small numbers you write in a cell to track which digits are still possible there. Essential for any puzzle beyond Easy difficulty — don't try to solve Medium or Hard without them.
How long does it take to get good at Sudoku?
Most beginners can solve Easy puzzles within a week and Medium within a month with regular practice. Hard typically takes 2-3 months. Master one difficulty before moving to the next.