Read the boxes before you make a move
Mini Sudoku uses a six-by-six board. Each box is two rows tall and three columns wide. In the practice grid below, the upper-left box covers rows 1 and 2, columns 1 to 3. The box beside it covers the same two rows and columns 4 to 6. A dot marks an empty square. R1C1 means row 1, column 1.
| Row | C1 | C2 | C3 | C4 | C5 | C6 |
|---|---|---|---|---|---|---|
| 1 | · | · | 1 | 6 | · | · |
| 2 | · | 3 | 4 | · | · | · |
| 3 | 2 | · | · | 3 | · | · |
| 4 | 4 | · | · | · | · | 2 |
| 5 | · | · | · | · | 6 | 5 |
| 6 | · | · | · | · | · | · |
This is a board from the game's generator, with ten starting digits and one solution. Try the first move before reading on. You can copy the six rows onto paper and draw the box boundaries after rows 2 and 4 and between columns 3 and 4.
Start with a square that has one possible digit
Look at R1C1. Row 1 already has 1 and 6. Column 1 already has 2 and 4. The upper-left box already has 1, 3 and 4. Put those restrictions together and the only digit left is 5. Place it at R1C1. This is a cell with one candidate, often called a naked single.
Now R1C2 has to be 2. The 1, 5 and 6 in its row and the 3 and 4 in its box leave one choice. The upper-left box now has five digits, so its last empty square, R2C1, must be 6. This sequence shows why it pays to look around each digit you place. One entry can reduce several nearby choices at once.
Finish this pass by looking at R2C6. Row 2 contains 6, 3 and 4, so it is missing 1, 2 and 5. Column 6 contains 2 and 5. Put 1 at R2C6, then pause. A square with several possible digits can still be the only place for one of those digits.
Switch from looking at a square to looking for a digit
Find where 3 can go in row 4. Columns 1 and 6 already have entries in this row. Column 2 has a 3 at R2C2. Columns 4 and 5 in row 4 are in the middle-right box, which already contains the 3 at R3C4. That leaves only R4C3 for 3. The square could allow other digits if you looked at it in isolation, but the row needs its 3 here. This is a hidden single.
Use that new 3 to inspect R5C3. Column 3 contains 1, 4 and 3. Row 5 contains 6 and 5. Place the remaining 2. Then find where 6 belongs in row 4. Columns 4 and 5 are blocked by the 6s at R1C4 and R5C5. The remaining place for 6 is R4C2. From here, check the middle-left box and then return to row 1 as the new digits narrow its choices.
Finish the grid and check your answer
Continue alternating the two scans. First look for an empty square whose row, column and box rule out five digits. If you cannot spot one, choose a missing digit in a partly filled row or box and check each empty square for it. After every placement, cross that digit off any notes in the same row, column or box. This keeps your notes useful when a long scan brings you back to an earlier square.
| Row | C1 | C2 | C3 | C4 | C5 | C6 |
|---|---|---|---|---|---|---|
| 1 | 5 | 2 | 1 | 6 | 3 | 4 |
| 2 | 6 | 3 | 4 | 2 | 5 | 1 |
| 3 | 2 | 1 | 5 | 3 | 4 | 6 |
| 4 | 4 | 6 | 3 | 5 | 1 | 2 |
| 5 | 3 | 4 | 2 | 1 | 6 | 5 |
| 6 | 1 | 5 | 6 | 4 | 2 | 3 |
In Mini Sudoku, turn on Notes or press N to keep candidates in a square. A placement that repeats a digit is refused immediately. A digit that fits so far can still lead to a dead end later, which is why each placement in the worked grid has a reason. For more examples on a larger board, read the Sudoku strategies guide.
