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 purely about scanning. Solving Hard and Expert puzzles requires specific logical techniques. Here are the 6 techniques that take you from “30-minute Easy puzzles” to “5-minute Expert puzzles.”
Technique 1: Singles (Naked, Hidden)
A naked single is a cell that can only contain one digit. If a cell has digits 1, 2, 3 ruled out by its row, column, and 3×3 box, only the remaining digits are 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.
90% of Easy and Medium puzzles solve with just these two techniques applied iteratively.
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 sharing the same three candidates.
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 A3-A9 (the rest of row A).
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.
If you can spot Swordfish without writing pencil marks, you’re at roughly 2000-rated Sudoku. World-class solvers see them in 10-15 seconds.
What to skip
Don’t memorise 18 named techniques. Naked singles, hidden singles, naked pairs, pointing pairs, and X-wing will solve every Sudoku in our app up to Expert level. Past that, you’re into hand-crafted puzzles where the techniques have proper names because the techniques themselves are aesthetic.
Solve speed milestones
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. Expert under 20 minutes: you’re seeing 3-4 techniques at once and chaining them.
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.
For more logic puzzles, see our games like Sudoku roundup.
