How to Solve Sudoku — 6 Techniques From Easy to Expert
Sudoku looks like a number puzzle but isn’t really: it’s a constraint-satisfaction puzzle that happens to use digits 1-9 as labels. Solving Easy puzzles is mostly about scanning. Solving harder puzzles requires specific logical techniques. Here are the 6 techniques that take you from slow, careful Easy solves to confident work on the hardest grids.
The short version of how to solve sudoku: every row, column and 3×3 box must contain 1 to 9 exactly once. Start by finding cells where only one digit fits, write candidate digits in the empty cells once that dries up, then use patterns among those candidates to rule digits out.
Key takeaways
- Naked and hidden singles solve most Easy and many Medium grids on their own.
- Pencil marks (candidate lists) are what make pairs, pointing pairs and X-wing visible.
- Work one technique at a time, from simplest to hardest, and go back to singles after every new digit.
- A properly built sudoku has exactly one solution, so logic alone is always enough.
The rules and the vocabulary
A sudoku grid has 81 cells in nine rows, nine columns and nine 3×3 boxes. Some cells start filled (the “givens”); you fill the rest so that each row, column and box holds every digit from 1 to 9 once. Rows, columns and boxes are all called units, and the cells that share a unit with a given cell are its peers.
The candidates of an empty cell are the digits that do not already appear among its peers. In this guide rows are lettered A to I from top to bottom and columns numbered 1 to 9, so A1 is the top-left cell.
Before you start: scanning
On a fresh grid, do not write anything yet. Pick a digit that already appears often, say 5, and look at each box that is missing it. The 5s in neighbouring rows and columns cross out cells in that box. If only one cell is left, that is where the 5 goes. Players call this cross-hatching. Run through all nine digits, then repeat until a pass finds nothing.
Technique 1: Singles (Naked, Hidden)
A naked single is a cell that can only contain one digit. Rule out every digit that already appears in the cell’s row, column, and 3×3 box; whatever is left are the candidates. If only one candidate remains, fill it in.
A hidden single is a digit that can only go in one cell within a row, column, or box. Even if that cell has several candidate digits, if it’s the only cell in the unit where, say, 7 can possibly go, you write 7.
Most Easy and many Medium puzzles solve with just these two techniques applied over and over.
Pencil marks: when and how to use them
Once singles stop appearing, write the candidates for each empty cell in small digits. Every technique from here on works by spotting patterns in those candidate lists.
Fill them in for every empty cell, and every time you place a digit, erase it from the candidates of all its peers. Stale candidates are the most common cause of wrong answers.
Technique 2: Naked Pairs / Triples
If two cells in a unit both have exactly the same two candidate digits (and nothing else), those two digits must occupy those two cells. You can eliminate those digits from every other cell in the same unit. Example: cells A1 and A2 both have candidates {4, 7}. Then no other cell in row A can be 4 or 7.
Naked triples are the same idea with three cells whose candidates, taken together, are only three digits.
The mirror image is the hidden pair: two digits that can only go in the same two cells of a unit. Those cells must hold those two digits, so you can erase every other candidate from them.
Technique 3: Pointing Pairs
Look inside a 3×3 box. If a particular digit can only appear in cells within one row or one column of that box, then that digit must come from those cells, so you can eliminate the digit from the rest of that row or column outside the box.
Example: digit 5 in box 1 can only go in cells A1 or A2. So 5 must be in row A, somewhere in box 1. That means 5 can’t be in cells A4-A9 (the rest of row A outside the box).
Technique 4: Box-Line Reduction
The reverse of pointing pairs. If a digit in a row or column can only appear in cells within one 3×3 box, eliminate that digit from the rest of the box.
Technique 5: X-Wing
This one feels magical the first time. Find a digit that can only appear in two cells within two different rows. If those four cells form a rectangle (same columns), the digit must occupy two opposite corners of that rectangle. So you can eliminate the digit from those two columns outside the rectangle.
In practice: scan each digit in each row. If digit 7 appears as a candidate in only two cells of row A and those two cells line up with the only two candidate cells in row F, you have an X-wing on 7 across rows A and F in those two columns.
Technique 6: Swordfish (advanced)
Same idea as X-wing but with three rows and three columns. The digit appears in only those cells across three rows; the cells line up with three columns; the digit must be in three of those nine cells, eliminating the digit from those three columns elsewhere.
Swordfish is hard to spot without pencil marks. Write candidates in first, then scan one digit at a time.
Sudoku techniques at a glance
| Technique | What you look for | Needs pencil marks? | Typical level |
|---|---|---|---|
| Scanning / hidden singles | One place left for a digit in a unit | No | Easy |
| Naked singles | One digit left for a cell | Helpful | Easy to Medium |
| Naked and hidden pairs | Two cells sharing two digits | Yes | Medium |
| Pointing pairs, box-line reduction | A digit locked into one line of a box | Yes | Medium to Hard |
| X-wing | A rectangle of candidates for one digit | Yes | Hard |
| Swordfish | The same pattern across three lines | Yes | Very hard |
A solving routine that works
- Scan every digit for hidden singles until a full pass finds nothing.
- Fill in pencil marks for every empty cell.
- Look for naked singles in the marks, then hidden singles in each unit.
- Look for pairs and triples, then pointing pairs and box-line reductions.
- Only then hunt for X-wings and Swordfish, one digit at a time.
- Every time anything changes, start again at step 3. Simple techniques almost always reopen after a hard one.
Common mistakes
- Guessing. A wrong guess can poison the whole grid before you notice.
- Forgetting to update candidates. One stale pencil mark can make a pair or X-wing look real when it is not.
- Jumping to advanced patterns too soon. Most stuck moments are a missed single, not a missing technique.
What to skip
Don’t memorise 18 named techniques. Naked singles, hidden singles, naked pairs, pointing pairs, and X-wing will carry you a long way through our Easy to Hard levels. The top settings in our app (Very Hard, Insane, Inhuman) give you fewer starting digits and can demand longer chains of the same ideas, plus more patience. Past that, you’re into hand-crafted puzzles where the techniques have proper names because the techniques themselves are aesthetic. Our advanced sudoku techniques guide covers the next tier when you want it.
Solve speed milestones
Treat these as rough personal targets, not official benchmarks. If you can solve Easy in under 5 minutes, you’ve internalised singles. Medium under 7 minutes: you’ve got pairs and pointing pairs. Hard under 12 minutes: you’ve got X-wing. Very Hard under 20 minutes: you’re seeing 3-4 techniques at once and chaining them.
How our Sudoku helps you practise
Our Sudoku generates a fresh puzzle with a single solution every time. The six levels set how many digits you start with: about 62 givens on Easy, 53 on Medium, 44 on Hard, 35 on Very Hard, 26 on Insane and around 22 on Inhuman. Fewer givens usually means harder logic.
The app marks a wrong digit in red as soon as you enter it and counts your errors, which is handy while learning. To practise real solving, try a puzzle without looking at the red marks, then press Check at the end. There is also a Hint button that fills the cell you have selected, plus Undo. The app has no pencil-mark mode, so keep candidates on paper for harder grids.
Check your work with a sudoku solver
When you are stuck, Sudoku Solver can help without just handing over the grid. Enter or paste a puzzle to check it for errors and for more than one solution, then see each solving step explained in plain English, so you can learn the technique you missed.
Frequently asked questions
What is the first step to solve a sudoku?
Scan for hidden singles. Take one digit at a time and check each box for the only cell where it can go. Repeat until nothing new appears, then start writing candidates.
Do you ever have to guess in sudoku?
Not in a puzzle with a unique solution, which is how well-made sudoku are built. If you feel stuck, you have usually missed a single or have an out-of-date pencil mark.
What is the minimum number of clues a sudoku can have?
Seventeen. A computer search published in 2012 showed that no standard sudoku with 16 clues has a unique solution. Our hardest level sits a few givens above that floor.
Who invented sudoku?
The modern puzzle is usually credited to Howard Garns, whose “Number Place” puzzles appeared in American puzzle magazines in the late 1970s. The Japanese publisher Nikoli popularised it in Japan and gave it the name sudoku. Wikipedia’s sudoku article covers the full history.
Practice
Play our Sudoku at Easy difficulty until you can solve in 5 minutes. Then Medium. Then Hard. Don’t skip difficulties: each level reinforces a different technique. Our easy-to-evil sudoku progression plan maps out that climb in more detail.
For more logic puzzles, see our games like Sudoku roundup, or try Picross and Minesweeper, which use the same kind of elimination logic. Between puzzles, rest your brain with a run of the Chrome Dino game.
