LEARN SUDOKU
How to play Sudoku
Sudoku is a logic puzzle. You do not need arithmetic: each number is a symbol that must fit without repeating. Start with the clues already on the board and find one justified move at a time.
The rules
Fill the 9 × 9 grid with the numbers 1–9. Every row, every column and every outlined 3 × 3 box must contain each number exactly once. The starting clues cannot be changed. Sudoku-Y labels rows A–I and columns 1–9, so A1 is the top-left cell.
A digit can appear again elsewhere on the board: the restriction applies to its own row, column and box, not to the whole grid. These are the standard rules described by Nikoli.
Find your first digit using three rules
Find E5, the cell with the dashed outline. Inspect the evidence one unit at a time, then choose its value.
Row E already contains 4, 2, 6, 8 and 3. Removing them from 1–9 leaves {1,5,7,9}. There is no answer yet.
E5 candidates after this step
Green outline = evidence for the selected step · Dashed outline = E5 · Crossed out = eliminated candidate
Which digit belongs in E5?
Inspect the evidence and choose a digit. No guessing needed.
Read all three reasoning steps
- Row E already contains 4, 2, 6, 8 and 3. Removing them from 1–9 leaves {1,5,7,9}. There is no answer yet.
- Column 5 contains 7 at A5 and 9 at B5. Removing those from the previous set leaves {1,5}.
- The middle box (rows D–F, columns 4–6) contains 1 at D6. Its 8 and 3 were already removed. Only 5 remains.
LOOK · TRY · UNDERSTAND
Try it and see why
Where can a digit repeat?
A1 has a placed 5. Check its row, column and box.
Our own partial teaching board, not a full puzzle. Large digits are placed values; small digits are candidates. Dots mean the cell is not described here. Complete candidate lists are part of the stated scenario, not calculated from the dots.
Green outline = evidence · Purple “try” = an assumption · Crossed out = removed candidate
Which cell does not conflict with the 5 at A1? Check only the shown clue; this does not prove the cell’s answer.
See the reasoning
E5 shares none of A1’s three units. This 5 does not block it, but other clues still need checking.
Digit set 1/9 · A different digit set in the same pattern, not a new full puzzle.
Read the source principles: Nikoli — Sudoku
Start with pencil notes
Select an empty cell and check its row, column and box. Numbers already present in any of those three units cannot go in that cell. Write down the remaining candidates. For example, if the related units already contain 1, 2, 4, 5, 6, 7 and 9, the candidates are 3 and 8.
In Sudoku-Y, turn on Notes (+), then tap a number. Tap it again to eliminate it, and again to restore it. Fill all candidates writes these basic possibilities into every empty cell. It does not solve the puzzle or prove which candidate is the answer.
Two useful beginner techniques
- Naked single: a cell has only one candidate left. If 1–9 are all ruled out except 6, that cell must be 6.
- Hidden single: a number can fit in only one cell of a row, column or box. That cell must contain the number, even if your notes list other candidates there.
After filling a number, check its related cells again. One move may reveal another single. The Techniques tool also identifies naked pairs, pointing and box-line reductions, highlighting the source and target cells on your current board.
Practise singles, then learn how two cells can reserve a pair of digits. The distinction between a cell's only candidate and a digit's only position is also explained by HoDoKu's Singles guide.
What to do when no answer stands out
- Check the last entry. An incorrect placed digit can remove a valid candidate elsewhere. Start with the cells sharing its row, column or box.
- Check the notes are complete. One handwritten note does not prove a Single. Recalculate the possibilities from placed digits before treating a note as certain.
- Change what you scan. If every cell has several candidates, follow one digit through an entire row or box. Its only available position may be easier to spot.
- Look for an elimination. A Naked Pair may remove a candidate without immediately giving an answer. Update only the affected cells, then scan for singles again.
If you use a hint, first identify the evidence cells yourself. Being unable to find a move is not proof that guessing is required. If you do explore a what-if case, keep its entries separate until you can justify them.
Which board should you start with?
- 4×4: use 1–4 and 2×2 boxes to practise checking all three constraints with fewer cells.
- 6×6: use 1–6 and boxes two rows high by three columns wide. Follow the thick borders, not the 3×3 shape from Classic.
- Easy Classic 9×9: apply the rules on a standard board. Start with nearly completed units.
- 16×16: uses 1–16 and 4×4 boxes. More cells mean more scanning, not automatically a harder logical technique.
- Killer: adds cage totals and no repeated digit within a cage. Learn the difference between a fixed box and a sum cage before using arithmetic.
Hints and what-if cases
A hint explains its reasoning with steps on the board. Read the explanation before revealing the answer if you want to practice. Some difficult hints explore assumptions. Finding no immediate conflict does not prove an assumption is correct: a contradiction rules it out, while proving a unique answer requires eliminating the alternatives.
Use what-if cases to test alternatives without changing the main board. Add or switch cases, reset a case, or exit to restore the original board. Accept a case only when you are ready to use its entries.
Playing and saving
Choose Classic at one of four difficulty levels, try the daily puzzle, or progress through the Journey. The daily puzzle changes at midnight in Thailand, regardless of interface language.
You can play without an account. Games, notes, points and settings are saved in this browser. Switching between Thai and English on the same website keeps that data. Switching device or browser does not: cross-device sign-in and sync are not available yet. Clearing site data may erase your progress.
Are hints and extra chances free?
Automatic candidates are free. While reward ads are not enabled, hints and continuing after mistakes are also free. Once reward ads are enabled, each game includes 3 free hints and 3 free mistakes. Each completed ad gives either 1 additional hint or 1 extra mistake allowance, depending on the option you choose.
Continue with the Sudoku library, with examples and references.