Stuck on Sudoku? A 7-Step Checklist

When should you use this checklist?

Use this checklist when you have been staring at the grid but cannot find your next number. Most of the time, you are stuck because you missed a place to apply a method you already know, rather than because you need a new technique. Work through the seven steps in order to catch what you missed before moving on to more involved methods.

Seven steps

1. Check for wrong numbers. This app increases the on-screen “Mistakes” count as soon as you enter a number that differs from the solution. If “Highlight duplicates” is enabled in settings (the default), it also marks cells red when the same number appears twice in a row, column or 3×3 box. If you see red cells, or have filled cells since your mistake count increased, check those first. Continuing from a wrong number will not get you unstuck.
2. Scan the boxes one number at a time. Pick a number from 1 to 9 and check each box to see whether it has only one place for that number. The example in this article demonstrates this step.
3. Look for rows, columns or boxes with few empty cells. If only two or three cells are empty, only two or three numbers remain, making their positions easier to determine.
4. Pick one cell and check all three areas. This is “Finding your first number ①” in Sudoku Rules: A 3-Minute Guide. Make sure you have not missed its row, column or box.
5. Write down candidates. Work through one row or column at a time, using Notes to record the candidates in empty cells. See How to Use Sudoku Candidate Notes for the details.
6. Look for a Naked Pair. If two cells in the same row or column have exactly the same two candidates, you can remove those numbers from the other cells in that row or column.
7. Use Hint. If you are still stuck, reveal one cell's answer with Hint. You get 3 uses per puzzle by default, and each use deducts 100 points from your score.

Scanning a box for a number — an example

In the example below, looking only at r6c4 leaves four candidates: 1, 2, 4 and 7. You cannot decide yet. This time, focus on the number 1 instead of the cell, and look for its place in box 5, the center box.

c1c2c3c4c5c6c7c8c9

r1r2r3r4r5r6r7r8r9

4

8

1

5

9

5

9

8

9

7

2

6

3

8

1

3

7

5

3

8

9

6

1

1

3

2

2

6

9

8

7

5

Box 5 has six empty cells: r4c6, r5c4, r5c5, r5c6, r6c4 and r6c6.

• Row 5 already has a 1 at r5c7, so r5c4, r5c5 and r5c6 cannot be 1.
• Column 6 also has a 1 at r8c6, so r4c6 and r6c6 cannot be 1 either.

Only r6c4 remains, so box 5's 1 must go in r6c4. Focusing on the cell left you with four candidates; focusing on the number gives you the answer immediately. If you skip step 2 and start by writing candidates, it takes longer to find cells like this.

When this method does not work

• If scanning a box for a number leaves two or more possible cells, you cannot decide at this step. Move on to the next number or box.
• If you have scanned every box for every number from 1 to 9 without finding an answer, move on to step 3 and beyond.
• A difficult puzzle may need relationships more complex than a Naked Pair. In that case, using Hint at step 7 can save you time.

Common mistakes

• You skip step 1. If a number is already wrong, all your later reasoning can go off track. When you get stuck, check for red cells first.
• You only focus on individual cells. Even when a cell has many candidates, focusing on a number can often reveal that it has only one possible place.
• You write candidates in every cell from the start. If you can still find numbers with steps 2–4, writing candidates takes extra time.

Practice puzzle

c1c2c3c4c5c6c7c8c9

r1r2r3r4r5r6r7r8r9

2

5

6

3

1

3

2

8

6

9

2

9

2

8

6

9

9

1

3

5

2

8

5

6

9

4

7

2

3

2

9

1

Where does 5 go in box 9, the bottom-right box? For each empty cell in that box, check whether its row or column already contains 5.

Show answer

r9c9. Box 9 has four empty cells: r7c7, r7c9, r9c8 and r9c9. r7c7 and r7c9 cannot be 5 because row 7 already has a 5 at r7c6. r9c8 cannot be 5 because column 8 has a 5 at r6c8. Only r9c9 remains.