Learn Sudoku Techniques

Sudoku can be solved by logic alone, with no guessing. This page collects the seven techniques used most in real solving, in order of difficulty. For each one, read the explanation, study a worked example with the evidence marked, then practise on freshly generated puzzles.

Terms to know first

• Row, column and box: rows run across, columns run down, and the 3×3 areas inside the thick lines are boxes. Together they are called units, and every unit holds 1 to 9 exactly once.
• Candidate: a digit that could still go in an empty cell. A digit already in the cell's row, column or box cannot be a candidate.
• Cell names: cells are written as r3c5 for row 3, column 5. Boxes are numbered 1 to 9 from the top left to the bottom right.

The order to learn them in

The first three techniques (the two hidden singles and the naked single) place a digit directly. They alone solve most easy and medium puzzles.

The last four place nothing themselves. They remove candidates until a new single appears, which is what harder puzzles need once the singles run out.

The techniques

1. Step 1 · Beginner

   Hidden Single (box)

   If a digit has only one possible cell left inside a box, it goes in that cell.

2. Step 2 · Elementary

   Hidden Single (row or column)

   If a digit has only one possible cell left in a row or column, it goes in that cell.

3. Step 3 · Elementary

   Naked Single

   If a cell's row, column and box already hold eight different digits, the ninth is its answer.

4. Step 4 · Elementary

   Pointing

   If every possible cell for a digit inside a box lies on one line, the digit can be removed from the rest of that line.

5. Step 5 · Elementary

   Claiming

   If every possible cell for a digit in a row or column lies inside one box, the digit can be removed from the rest of that box.

6. Step 6 · Intermediate

   Naked Pair

   If two cells in a row, column or box hold the same two candidates and nothing else, those two digits can be removed from the rest of that unit.

7. Step 7 · Intermediate

   Hidden Pair

   If two digits can only go in the same two cells of a unit, every other candidate in those two cells can be removed.

How the practice works

• Every exercise comes from a freshly generated puzzle, at a point where the technique really applies.
• The row, column and box the deduction rests on are shown at full strength; the rest of the board is dimmed.
• A wrong answer is explained on the board, and when you are stuck you can ask for a hint and then the answer.
• Your record is kept in this browser only. No account is needed.