Sudoku — Rules, Techniques and Online Play
Every row, column and 3×3 box holds 1 to 9 exactly once
What this level trains
Sudoku is a logic puzzle played on a 9×9 grid split into nine 3×3 boxes. One rule governs everything: each row, each column and each box must contain the digits 1 to 9 exactly once. You fill the grid by pure deduction, without ever falling back on guessing.
Solving works by eliminating a cell's candidates until only one remains: naked and hidden singles first, then box-line reduction, then pairs, and on the hardest grids advanced patterns such as triples, X-Wing, XY-Wing and forcing chains. Each step removes options you can rule out, so the board closes in.
This site makes Sudoku the eighth level of its brain-training set. The eight difficulties are ranked by the technique required rather than the number of blanks, because a puzzle with many blanks can still fall to simple elimination, while one with plenty of givens may need chain reasoning.
Each set deals a single puzzle with a guaranteed unique solution, and filling all 81 cells clears it; a shorter time scores higher, and the server compares your final grid against the stored solution so a wrong entry never passes, which keeps every submission honest.
How to play
- Read the boardFor every empty cell, list the numbers that could go there. This candidate list is the shared basis of every Sudoku technique, so build it before you start placing.
- Fill the singlesIf a cell has one candidate left, place it. Do the same when a number has only one possible spot left in a row, a column or a box.
- Use box-line reductionIf a number in one box can only appear somewhere in a single row or column, remove that number from the rest of that row or column to narrow the field.
- Close the netAfter every placement, update the row, column and box it affects and keep going until the board is full. A well-formed puzzle never needs a guess at any point.
8 difficulty levels and score thresholds
The eight levels are ranked by the technique they require, not by blank count. Levels 1 and 2 need only naked and hidden singles, levels 3 and 4 add box-line reduction, levels 5 and 6 need pairs, level 7 calls for triples, fish, XY-Wing or colouring, and level 8 needs forcing chains. Excellent times run from three minutes at level 1 to fifteen minutes at level 8.
| Level | Techniques required | Full-mark line | Excellent | Good | Pass | Max score | Notes |
|---|---|---|---|---|---|---|---|
| Level 1 | Solvable with naked and hidden singles alone | 2 min 6 s | 3 min | 4 min 12 s | 6 min | 3000 | Intro |
| Level 2 | Still basic techniques, but fewer clues and more rounds of elimination | 2 min 48 s | 4 min | 5 min 36 s | 8 min | 4000 | Basic |
| Level 3 | Requires locked candidates (pointing / claiming) | 3 min 30 s | 5 min | 7 min | 10 min | 5000 | Advanced |
| Level 4 | Locked candidates combined with basic techniques | 4 min 12 s | 6 min | 8 min 24 s | 12 min | 6000 | Intermediate |
| Level 5 | Requires naked / hidden pairs | 4 min 54 s | 7 min | 9 min 48 s | 14 min | 7000 | Pairs |
| Level 6 | Pairs and locked candidates interleaved — you plan the order | 5 min 57 s | 8 min 30 s | 11 min 54 s | 17 min | 8000 | Cross |
| Level 7 | Requires triples / fish / XY-Wing / colouring | 7 min 42 s | 11 min | 15 min 24 s | 22 min | 9000 | Fish & chains |
| Level 8 | Requires forcing chains (assume, propagate, contradict) | 10 min 30 s | 15 min | 21 min | 30 min | 10000 | Master |
You can only reach the maximum score at or under the full-mark line. Past the pass line the score keeps decaying, down to 0 at twice the pass line. The maximum rises from 3000 to 10000 across the levels.
Tips and common mistakes
- Start where candidates are fewestFilling the cells with only two candidates beats sweeping the grid from the top. The fewer the options, the faster the deduction tends to be.
- Update candidates as you goDo not wait until you are stuck to refresh your notes. Updating after each placement lets many answers surface on their own before you go looking.
- Flip the questionWhen you stall, switch from asking what this cell can hold to asking where this number can go. It often breaks the deadlock at once.
- Never guessA wrong guess forces every later deduction to be rebuilt from scratch. A proper Sudoku puzzle can always be finished by logic alone.
Frequently asked questions
- Does Sudoku have a unique solution?
- A well-formed puzzle does. If a grid admits several answers it usually has too few givens and counts as a flawed puzzle. Every puzzle on this site is served with a guaranteed unique solution, which the server checks against your final grid.
- Do I need to be good at maths for Sudoku?
- No. Sudoku only uses the symbols 1 to 9 and involves no arithmetic at all. It trains pure logical elimination, which is why it appeals to solvers well beyond mathematicians. It is a logic puzzle, not a maths test, so you never have to calculate anything.
- What happens if I fill a cell wrongly?
- The server compares your completed grid with the puzzle unique solution, so a wrong entry will not clear the level. You can undo the move or simply change that cell and keep solving. Only a full grid that matches the solution will finish the puzzle.
- Can I play Sudoku on a phone?
- Yes. The input works by selecting a cell and then tapping a number pad, so a 9×9 board fits portrait screens. Arrow keys and number keys are also supported on a desktop keyboard. On a phone, tap a cell and then a digit, and the board scales to fit the width.
- Is there a time limit for each puzzle?
- There is no hard clock that stops you, but every puzzle is graded on your total time. The excellent benchmark runs from three minutes at level 1 to fifteen minutes at level 8, with good at 1.4 times that and pass at twice it, so a quicker solve always scores higher.
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Content updated on 2026-10-03 · Level 8 · Logic