Sudoku Strategies: From Beginner to Advanced (How I Went From Stuck to Solving Hard Puzzles)
I want to be upfront about something before we get into this.
When I first started playing Sudoku seriously, I was convinced I just was not smart enough for the hard puzzles. I could do easy ones fine. Medium took effort but I got there eventually. But hard? I would fill in maybe a third of the grid, hit a wall, start guessing, and inevitably end up with two 7s in the same column and no idea how I got there.
The problem was not intelligence. It was technique. I was playing hard Sudoku with easy Sudoku strategies — essentially trying to climb a mountain with a stepladder.
This post covers everything I wish someone had told me earlier: the strategies in the order you should learn them, why each one works, and when to use each. If you want to test yourself after reading, our free Sudoku game lets you pick your difficulty level and play immediately.
First, the One Rule That Changes Everything
Before any strategy: never guess.
I know that sounds obvious. But the single biggest mistake intermediate Sudoku players make is guessing when they get stuck — writing a number in because it feels right, then continuing. Sometimes you get lucky. Usually you do not, and you spend ten minutes solving a puzzle that was wrong from move eight.
A properly formed Sudoku puzzle has exactly one solution, and that solution is always reachable through pure logic. If you are stuck, it means you have not yet found the logical step that unlocks the next cell — not that guessing is the answer. When you hit a wall, the right move is to look harder, not to guess.
With that established, here is how to look harder.
Level 1: The Strategies That Solve Most Beginner and Easy Puzzles
Naked Singles
This is the most fundamental Sudoku technique and the one every player uses, often without realising it has a name.
A naked single occurs when a cell has only one possible digit. Every other digit from 1 to 9 is already present in that cell's row, column, or 3×3 box — so there is exactly one option left.
How to spot them: Go through each empty cell and mentally (or literally) cross off every digit that already appears in its row, column, and box. If only one digit remains, that is your answer.
On easy puzzles, naked singles are everywhere. You can often solve the entire grid just by cycling through cells repeatedly, filling in naked singles as they appear, and then cycling again as each new placement eliminates more candidates from neighbouring cells.
When naked singles stop appearing — when every remaining empty cell has two or more candidates — you need to move to the next technique.
Hidden Singles
A hidden single is slightly more subtle. It occurs when a digit can only go in one cell within a particular row, column, or box — even if that cell appears to have multiple candidates.
The difference from naked singles: with a hidden single, you are not looking at a cell and asking "what digit goes here?" You are looking at a digit and asking "where in this row/column/box can this digit go?"
Example: Say you are looking at the top-right 3×3 box. The digit 4 needs to go somewhere in that box. You look at the three rows that pass through the box — in two of them, a 4 already exists somewhere else in the row, ruling out those rows. That leaves only one row where 4 can go in that box. Now look at the columns — two of the three cells in that remaining row are already filled. The 4 must go in the one remaining empty cell.
That cell might have had 4, 6, and 9 as candidates. The 4 was hidden among them — but logic demanded it be there.
Tip: Scan each digit (1 through 9) through each row, column, and box systematically. Ask "where can this digit go?" rather than "what digit goes here?" — it reveals hidden singles that naked single scanning misses entirely.
The Box-Line Reduction (Pointing Pairs/Triples)
This one feels like a breakthrough moment when you first understand it, because it lets you eliminate candidates from cells you cannot yet fill.
Here is the idea: if a digit's only possible positions within a box all fall on the same row or column, then that digit cannot appear anywhere else in that row or column outside the box.
Concrete example: In the middle-left 3×3 box, the digit 7 can only go in one of three cells — and all three of those cells are in the same row (say, row 5). This means wherever 7 ends up in that box, it will be in row 5. Therefore: 7 cannot appear anywhere else in row 5. You can eliminate 7 as a candidate from every other empty cell in row 5.
You have not filled any cell. But you have reduced the possibilities elsewhere — which may create naked or hidden singles that were not visible before.
This is where Sudoku starts to feel like detective work rather than trial and error.
Level 2: The Strategies That Unlock Medium Puzzles
Naked Pairs (and Triples)
A naked pair occurs when two cells in the same row, column, or box each contain exactly the same two candidates — and only those two candidates.
Why this matters: Those two digits must go in those two cells. You do not know which order yet, but you know they are not going anywhere else in that row, column, or box. So you can eliminate both digits as candidates from every other cell in that shared unit.
Example: In a row, two cells each show only {3, 7}. Neither 3 nor 7 can appear in any other cell in that row. If another cell in the same row had {2, 3, 8} as candidates, you can reduce it to {2, 8}.
Naked triples work the same way but with three cells and three digits. The three cells do not each need all three digits — they just need to collectively contain only those three digits between them. For instance: {1,2}, {2,3}, and {1,3} form a naked triple on digits 1, 2, and 3.
Naked pairs appear frequently in medium puzzles. When you are stuck and scanning for singles is not helping, actively look for cells with only two candidates and check whether any other cell in the same row, column, or box shares the exact same pair.
Hidden Pairs (and Triples)
The mirror image of naked pairs — and considerably harder to spot.
A hidden pair occurs when two digits appear as candidates in exactly two cells within a row, column, or box — even though those cells may have other candidates too. Because those two digits can only go in those two cells, all other candidates in those cells can be eliminated.
Example: In a column, the digits 4 and 9 each appear as candidates in only two cells — cells A and B. Those cells might currently show {4, 6, 9} and {1, 4, 9} respectively. But since 4 and 9 can only go in cells A and B, you know nothing else can go there. You can strip both cells down to {4, 9}.
Hidden pairs require you to track where each digit can go across a unit, rather than looking at individual cells. Most players find them harder to see than naked pairs — which is exactly why hard puzzles rely on them.
Level 3: The Strategies That Crack Hard Puzzles
X-Wing
This is the first technique that genuinely feels like magic when you see it work.
An X-Wing applies when a single digit appears as a candidate in exactly two cells in each of two different rows — and those cells share the same two columns.
Picture four cells forming a rectangle: top-left, top-right, bottom-left, bottom-right. The digit appears only in these four cells across two rows and two columns. Wherever it ends up in the top row, it forces the other cell in the bottom row (because the columns cannot repeat the digit). This creates two possible solutions, both of which eliminate the digit from every other cell in those two columns.
Why it works: No matter which way the digit resolves — top-left + bottom-right, or top-right + bottom-left — it covers both columns. So the digit cannot appear in those columns anywhere outside the four rectangle cells.
The result: you can eliminate that digit from all other cells in those two columns, even though you cannot yet determine which of the four cells actually gets it.
X-Wing takes practice to spot. Start by focusing on digits that appear infrequently across the grid — a digit that appears in only six or eight empty cells is far more likely to produce an X-Wing than one with twenty possible positions.
Swordfish
Swordfish extends the X-Wing logic to three rows and three columns.
A digit forms a Swordfish when it appears in exactly two or three cells in each of three rows, and all those cells fall within the same three columns. The digit can be eliminated from those three columns everywhere outside the six or nine cells involved.
It sounds complicated — and honestly, it is harder to spot than X-Wing. But the underlying logic is identical: however the digit distributes itself across those three rows, it must cover all three columns, eliminating it elsewhere in those columns.
Most casual Sudoku players never need Swordfish. It appears in expert-level puzzles, and most hard puzzles can be solved with techniques up to X-Wing. But if you are tackling expert grids and hitting walls even after applying X-Wing, Swordfish is the next thing to look for.
XY-Wing
XY-Wing is my personal favourite advanced technique — elegant, satisfying, and genuinely useful on hard puzzles.
It requires three cells: a pivot cell with exactly two candidates {X, Y}, and two wing cells that each share one candidate with the pivot. Wing 1 has candidates {X, Z} and Wing 2 has candidates {Y, Z}. All three cells must be connected — the pivot sees both wings (shares a row, column, or box with each).
The logic: the pivot must be either X or Y. If it is X, then Wing 1 (which shares X with the pivot) cannot be X, so Wing 1 must be Z. If the pivot is Y, then Wing 2 must be Z. Either way, Z ends up in Wing 1 or Wing 2. Therefore: any cell that can see both Wing 1 and Wing 2 simultaneously cannot be Z — it can be eliminated.
You are not solving the pivot or the wings. You are using the relationship between them to eliminate a candidate from a third cell that might be far away in the grid.
When an XY-Wing clicks into place and opens up a cell you had been staring at for ten minutes, it is one of the most satisfying moments in puzzle solving.
The Order to Learn These Techniques
Here is the sequence I recommend, based on how often each technique appears and how much it unlocks:
Start here — solves easy puzzles:
Naked singles
Hidden singles
Learn next — solves most medium puzzles:
Box-line reduction (pointing pairs)
Naked pairs
Hidden pairs
Learn when medium puzzles stop feeling challenging:
X-Wing
XY-Wing
Learn if you want to tackle expert puzzles:
Swordfish
Hidden triples/quads
Most players find that naked and hidden singles plus naked pairs solve 80% of the puzzles they encounter. The advanced techniques are for when you want to go further — and for the specific satisfaction of cracking a hard puzzle without guessing.
A Few Practical Habits That Actually Help
Write candidates in pencil (or use the in-game notes feature). Trying to track all possible candidates in your head is genuinely hard and not a useful skill. The mental effort you save by writing candidates down is better spent on finding the logical patterns.
Scan by digit, not just by cell. Most players default to staring at empty cells asking "what goes here?" Alternate this with scanning each digit asking "where can this go?" — it reveals hidden singles and pairs that cell-by-cell scanning completely misses.
When stuck, revisit your eliminations. If you have been applying box-line reductions and pairs, some cells' candidate lists will have changed. Cycle through naked and hidden singles again before reaching for a harder technique — a previously invisible single may now be obvious.
Set the difficulty honestly. There is no prize for playing hard when you are not yet finding hidden pairs fluently. The techniques build on each other — time spent on medium puzzles until pairs feel automatic pays dividends when you move to hard.
Play and Practice
Reading about Sudoku strategies and using them are two different things. The techniques above will click into place much faster with a puzzle in front of you than with text alone.
Our free Sudoku game has beginner, intermediate, and hard difficulty levels — no account required, works on mobile. Play a medium puzzle and look specifically for naked pairs. Play a hard puzzle and see if you can spot an X-Wing.
The difference between a player who gets stuck and one who methodically works through hard grids is almost entirely technique — and all of these techniques are learnable with practice.