Learn the rules, master the techniques, and become a competitive Sudoku solver.
The Rules
Sudoku is played on a 9×9 grid, divided into nine 3×3 boxes. Some cells come pre-filled with numbers — these are the clues. Your job is to fill in the remaining empty cells so that:
Each row contains 1–9 exactly once
Each column contains 1–9 exactly once
Each 3×3 box contains 1–9 exactly once
That's it — three simple rules. No math is involved. Every puzzle can be solved using logic alone, without any guessing. The pre-filled numbers give you enough information to deduce the solution step by step.
Solving Techniques
Here are the core techniques that will take you from beginner to competitive solver:
1
Scanning
Start by scanning rows, columns, and boxes for digits that appear frequently. If a number already appears in two rows within a band of three boxes, the third row can only place that number in one remaining box — narrowing your options quickly.
2
Pencil Marks (Candidates)
Use pencil marks to note all possible digits for each empty cell. As you fill in numbers, eliminate candidates from related cells. When a cell has only one candidate left, that's the answer. This is your most powerful tool for harder puzzles.
3
Naked & Hidden Singles
A Naked Single is a cell with only one possible candidate. A Hidden Single is when a digit can only go in one place within a row, column, or box — even if that cell has other candidates. These two techniques alone can solve most Easy and Medium puzzles.
4
Naked Pairs, Triples & Pointing
When two cells in a row/column/box share the same two candidates, those digits can be eliminated from all other cells in that group. Similar logic extends to triples. Pointing techniques eliminate candidates when a digit in a box is confined to a single row or column.
These four techniques will solve most Easy and Medium puzzles. For harder puzzles, you'll need advanced techniques like X-Wings, Unique Rectangles, XY-Chains, and more. Our platform supports 28 solving techniques in total — plus a full set of Killer-specific cage strategies. Visit the Learning Hub for visual, step-by-step guides, for both Classic and Killer.
Controls on Sudoku Challenge
Our interface is designed to be intuitive whether you're on desktop or mobile:
Select a Cell
Click/tap any empty cell, or use arrow keys to navigate
Enter a Number
Click a number button or press 1-9 on your keyboard
Pencil Marks (Notes)
Turn on the pencil, then type or tap digits to note candidates. The 12 button switches corner and centre notes; Candidates fills them all in
Erase
Click the eraser button or press Delete/Backspace to clear a cell
Undo / Redo
Step backward and forward through your moves with unlimited undo/redo
Multi-Select
Hold Ctrl and click multiple cells, or click-drag to select a range — then enter a number to fill all at once
Same-Digit Highlight
Select a cell holding a digit and hold Shift to light up every cell with that digit
Game Modes
Daily Challenge
New puzzles every day: Easy, Medium and Hard in both Classic and Killer, the Classic Daily Challenge with its own leaderboard, and the Daily Triple — three elite puzzles with one combined leaderboard. Solve one a day to keep your streak alive.
Classic Mode
Unlimited Classic puzzles at your chosen difficulty. Play at your own pace with no time pressure.
Killer Sudoku
A twist on Classic: the grid is divided into dashed "cages", each with a target sum, and no digit repeats inside a cage. Play it across all five difficulty tiers, with a live cage-combinations helper and the same Next Step teacher. Also available as a daily practice puzzle.
Tournament
Create or join a lobby — Easy, Medium or Hard, from 15 minutes to an hour — and race other players through timed puzzles. Signed-in players earn rating points.
Duel
Challenge a specific player by name or join the Pool Match queue. Direct challenges are available to rated players within 200 rating points. Same puzzle, same clock — the faster solver wins.
Training Mode
Practice puzzles across five difficulty levels: Easy, Medium, Hard, Extreme, and Diabolical. Each has its own score range, from 100 (Easy) to 9,999 (Diabolical). The Next Step button is your personal solving coach — it finds the next logical deduction and walks you through it with highlighted candidates and arrows.
Tips for Beginners
Start with Easy puzzles and gradually increase difficulty as your confidence grows.
Focus on one 3×3 box at a time — complete the easiest boxes first.
Look for "low-hanging fruit" — cells with only one or two possible candidates.
Use pencil marks generously. They are your most powerful tool on harder puzzles.
Never guess. Every puzzle on Sudoku Challenge has a unique solution reachable by logic alone.
Take breaks if you get stuck. Fresh eyes can spot patterns you missed.
Use Training Mode's Next Step feature to learn techniques interactively — it explains each deduction as you go.
Play Duels to sharpen your speed under pressure.
Remember: Sudoku is a game of pure logic. No mathematics required — just patience, systematic thinking, and practice. The more you play, the faster you'll get!