How to Solve Jigsaw Sudoku: Beginner Strategies Step by Step

If you want to know how to solve Jigsaw Sudoku, the good news is that you already know most of it. The rows and columns work exactly as in classic Sudoku. Only the 3 x 3 boxes are replaced by nine irregular regions, and this page teaches three methods that handle them step by step, each one checked on a real puzzle made by the LogikLand generator.

Read the regions first

A Jigsaw grid has nine regions, each with nine cells in one connected piece, outlined with a thick line. Before you write anything, take a pencil and trace each region once, from its first cell to its last. Then say to yourself: a digit may appear once in every row, once in every column and once in every region. A cell therefore looks at three groups, not at one square.

Method 1: scan one digit through a region

Pick a digit and a region, and ask where that digit can still go. Take the region on the lower left of the puzzle above: column 1 from row 5 to row 9, row 8 column 2, and row 9 from column 1 to column 4. It has no 3 yet. Go through its nine cells:

• Row 6, column 1 holds 5. Row 8, column 1 holds 4. Row 9, column 1 holds 1. Row 9, column 3 holds 9. Row 9, column 4 holds 2. Five cells are already full.
• Row 5, column 1 is in row 5, which already has a 3 in column 2. It is out.
• Row 8, column 2 and row 9, column 2 are in column 2, which has the same 3. Both are out.

One cell is left, row 7, column 1, so the 3 goes there. One single 3 removed three cells at once because the region cuts across a row and a column. That is the strength of irregular regions: they share more cells with other groups than a square box does.

Method 2: count what a cell sees

The second method starts from a cell instead of a digit. List the digits that its row, column and region already show. If eight different digits appear, the ninth is the answer. Try row 1, column 5 in the same puzzle:

Together these show 1, 2, 3, 5, 6, 7, 8 and 9. Only 4 is missing, so the cell is a 4. Method 1 finds a place for a digit, method 2 finds a digit for a place. When one stops giving results, switch to the other.

Method 3: use a row to clean a region

On harder grids you will reach a point where neither method finds anything. The next tool is an elimination. Suppose that, in some row, a digit can only go in cells that all belong to the same region. Then that digit must be in that row, inside that region, and no other cell of the region can hold it. Here is a hard puzzle where this works from the very first look:

In row 1, the 8 cannot go in column 3, because there is an 8 lower in that column (row 6). It cannot go in columns 5 and 6, because there is an 8 in row 2, column 6. It cannot go in columns 7 and 8 either, because there is an 8 in row 4, column 7. Columns 4 and 9 already hold digits. That leaves row 1, column 1 and row 1, column 2, and both cells are in the same region, the one that also holds row 2, column 1 and row 3, columns 1 and 2.

So the region's 8 is in row 1. Cross the 8 out in row 3, columns 1 and 2, with a small pencil mark. You have not placed a digit, but you have made two other cells simpler, and that often leads to a new method 1 or method 2 result a few steps later.

Habits that prevent mistakes

• Write small pencil marks only in cells where you have two or three candidates left. A cell full of marks is hard to read.
• Every time you place a digit, check its region first. It is the group people forget.
• Work one digit at a time through the whole grid before you change digit. Scanning for all the 3s is faster than looking for anything.
• If a row ends with no place for a digit, stop and look for the earlier mistake. A correct grid never gets stuck that way.

Which level should you start with?

The Jigsaw assistant has four levels. The number of digits given is a minimum, and the technique needed is the hardest one that the generator accepts for that level.

Start with easy, then medium, and only then try hard. When you are ready for expert grids, read advanced Jigsaw Sudoku techniques. For the exact rules and how the regions are built, see Jigsaw Sudoku rules.

Questions