Stuck on a Sudoku? 6 Techniques to Get Moving
When a Sudoku stops giving you easy numbers, pencil in every candidate for the empty cells, then look for patterns in the notes: a hidden single (a number that fits only one cell in a row, column or box), a naked pair (two cells that can only be the same two numbers), a hidden pair, or a pointing pair (a number confined to one line inside a box). Each one removes candidates until a single appears.
First: make sure you're really stuck
Most "stuck" moments are a missed easy move. Before reaching for advanced techniques, do one careful sweep:
- Scan every number from 1 to 9 across all nine boxes. It's easy to skip a number you already checked earlier, but new placements change everything.
- Check rows, columns and boxes with two or three gaps. List what's missing and test each gap.
- Look at the number you've placed most. If there are eight 4s on the board, the ninth is usually quick to find.
If you're on paper, also check for mistakes: a single wrong number earlier on can make the grid look impossible. If you're new to the basics (scanning and singles), start with how to play Sudoku.
1. Pencil in the candidates
Candidates (also called pencil marks or notes) are the small numbers that list what each empty cell could still be. For every empty cell, write every number that isn't already in its row, column or box. It feels slow, but on medium and hard puzzles almost every technique works by spotting patterns in these notes.
- Write notes in the same position each time (1 top-left, 9 bottom-right) so patterns are easy to see.
- When you place a number, immediately erase it from the notes of every cell in its row, column and box. Stale notes cause most mistakes.
- On GameLake, turn on Notes to pencil candidates in.
2. Hidden singles in the notes
With notes in place, scan each row, column and box for a number that appears in only one cell's notes. That number must go there, even if the cell has other candidates. Hidden singles are often invisible without notes and obvious with them.
3. Naked pairs
If two cells in the same row, column or box both have exactly the same two candidates, say 2 and 7, then those two numbers must go in those two cells, one each. You don't know which is which yet, but you do know that no other cell in that row can be 2 or 7. Cross them out everywhere else in the row.
The same logic works for three cells sharing three candidates (a naked triple), for example {3,4}, {4,7} and {3,7}: together they use up 3, 4 and 7.
4. Hidden pairs
A hidden pair is the mirror image. If two numbers appear in the notes of only the same two cells in a row, column or box, those cells must hold those two numbers, so you can erase every other candidate from those two cells.
5. Pointing pairs (box and line)
Look inside one box at where a particular number can go. If every possible cell for it lies in the same row (or the same column), then that number has to be in that row within this box, so it can't appear anywhere else in that row outside the box.
It also works the other way round (sometimes called claiming or box-line reduction): if a number's candidates in a row all fall inside one box, remove that number from the rest of the box.
6. A routine for hard puzzles
- Fill in all the easy numbers first, without notes.
- Pencil in candidates for every empty cell.
- Look for hidden and naked singles in the notes.
- Look for naked pairs, then hidden pairs, in every row, column and box.
- Look for pointing pairs and box-line claims for each number.
- Every time something is removed, go back to step 3: eliminations usually create new singles.
Very hard puzzles sometimes need more advanced patterns, such as the X-Wing (a number confined to the same two columns in two different rows), but master the techniques above first: they're the ones you'll use in almost every medium and hard puzzle.
Take a break. Coming back after a few minutes often makes a missed single jump out, because you stop re-reading the same cells in the same order.
On GameLake, Medium puzzles are built to need pairs and Hard puzzles to need box-line techniques like the ones above, so you can practise each step at the right level. If you're really stuck, the Hint button explains the next logical step rather than just filling in a number.
Frequently asked questions
What do you do when you get stuck on a Sudoku?
Re-scan every number 1 to 9, then pencil in candidates and look for hidden singles, naked pairs, hidden pairs and pointing pairs. Each removes options until a cell has only one.
Should you guess in Sudoku when stuck?
Avoid it. A proper Sudoku has one solution that can be found by logic. Guessing risks an error you won't notice until much later.
What is a naked pair in Sudoku?
Two cells in the same row, column or box with the same two candidates and nothing else. Those two numbers can be removed from every other cell in that unit.
What is a pointing pair?
When a number's only possible cells in a box all lie in one row or column, it must go there, so it can be removed from the rest of that row or column outside the box.
How do you solve hard Sudoku puzzles?
Use full pencil marks and work through pairs, pointing pairs and box-line reductions systematically. Re-check for singles after every elimination.