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What to Try When You’re Stuck on a Sudoku

A sudoku with one answer always has a next move. When you can’t see it, the move usually belongs to a check you haven’t tried yet. Work down this list from the top, and stop at the first step that finds something. After every move you make, start again from step one.

This guide uses colors, because Bubble Sudoku plays with nine colors instead of the numbers one to nine. Every step works the same way with numbers. Just read “color” as “number.”

Twelve steps, easiest first

1. Look for a row, column, or block with one empty cell

   If eight of its nine cells are filled, the ninth color is the one that is missing. It is the easiest move in sudoku, and it is also the easiest one to miss late in a puzzle, when the board is nearly full.

   Lesson: Full House

2. Count the colors around one empty cell

   Pick an empty cell in a crowded part of the board. Look at its row, its column, and its block, and name every color already there. If you can see eight different colors, the ninth must go in the cell. Crowded rows, columns, and blocks give answers fastest, so start there.

   Lesson: Obvious Single: Last Possible

3. Follow one color across the board

   Pick one color and find every cell where it is already placed. Each one rules that color out of its row, its column, and its block. Now look at a block that still needs the color. Often there is only one cell left in it where the color can go.

   In the board shown, only Red is drawn. The shaded rows and columns already hold a Red, so the middle block on the left has just one cell left for Red.

   Lesson: Hidden Single: Last Remaining

4. Write notes

   When counting and following colors stop finding moves, write notes. A note is a small dot for a color that could still go in a cell. Give every empty cell a dot for each color its row, column, and block still allow. It takes a few minutes, but many moves can only be seen in notes.

   Only write a note you can prove, and remove a dot as soon as a color is placed in the same row, column, or block.

   Lesson: Notes

5. Read your notes for singles

   Look for a cell with only one dot left. That color is the answer. Then look for a color that shows in the notes of only one cell in a row, column, or block. That cell must take it, even if it shows other dots too.

   Lessons: Obvious Single: With Notes · Hidden Single: With Notes

6. Look for a color that can only go along one line

   Inside a block, if a color can only go in cells along one row or column, it must go on that line inside the block. So you can rule it out of the rest of that row or column. It also works the other way: if a row or column can fit a color only inside one block, rule the color out of the rest of that block.

   Lessons: Pointing Pairs · Claiming Pairs

7. Look for matching groups

   When two cells in the same row, column, or block show exactly the same two colors, those two colors belong to those two cells. Rule them out of every other cell there. Three cells that hold only three colors between them work the same way.

   Lessons: Obvious Pairs · Obvious Triples

8. Look for hidden groups

   Sometimes two colors can only go in the same two cells of a row, column, or block, but those cells show other dots too. The two cells belong to those two colors, so every other dot in them can come off. Three colors in three cells work the same way.

   Lessons: Hidden Pairs · Hidden Triples

9. Look for patterns across the whole board

   Pick one color and look at the rows where it fits in only two cells. If two of those rows have their two cells in the same two columns, the color must use those columns in those rows. You can rule it out of every other cell in both columns. With three rows and three columns, the same idea is called a Swordfish. Both also work with rows and columns swapped.

   Lessons: X-Wing · Swordfish

10. Check for a mistake

    If nothing above finds a move, look for a color that appears twice in the same row, column, or block. One wrong bubble placed early can leave a board with no next move at all. Check your notes too: a dot removed by mistake can hide the answer.

    In Bubble Sudoku, Auto Check Mistakes marks a wrong bubble the moment you place it, as long as the setting is on.

11. Test one color, and keep only what you prove

    Pick a cell with just two dots and try one of the two colors. Follow the moves it forces. If the board breaks, with a color that has nowhere left to go or a cell with no colors left, the other color is the answer. Take the test bubbles back off either way.

    Bubble Sudoku’s puzzles are built to be solved with the techniques each level lists, so you should not need this there. It helps with puzzles from anywhere else.

    Lesson: Trial and Error (Test a Bubble)

12. Take a short break

    Step away for a few minutes, then come back and start again from step one. After a break it is often easier to see a move you missed.

What each level can ask for

Each Bubble Sudoku level asks only for the techniques listed next to it, and the levels build on each other. If you are stuck on an Easy puzzle, the next move is an Obvious Single somewhere, so you can skip the rest of the list.

Let the game show you

In Bubble Sudoku, tap Hint. It finds the next move on your own board, names the technique, and walks you through the reason one step at a time. You can stop as soon as you see it, and the next time the same pattern comes up, you will know what to look for.