A Classic Sudoku board: 30 givens, and every remaining cell reachable by logic alone.
Classic Sudoku is the original puzzle and the perfect starting point for newcomers — yet it stays endlessly challenging for experts. The board is a 9×9 grid divided into nine 3×3 boxes. A handful of cells are pre-filled (the givens); your job is to logically deduce every remaining cell. A correct puzzle always has exactly one solution and never requires guessing.
The rules
Every row must contain the digits 1–9 exactly once.
Every column must contain the digits 1–9 exactly once.
Every 3×3 box must contain the digits 1–9 exactly once.
A typical Classic 9×9 — grey cells are the givens you start with
How to play, step by step
Pick a cell by tapping it. Its row, column and box highlight so you can see what's already used.
Enter a number from the on-screen pad (1–9) or your keyboard. A wrong number turns the cell red and counts as an error.
Use Notes (
) to pencil in small candidate digits when a cell could still take several numbers — then erase them as you rule options out.
Work the dense areas first — rows, columns and boxes that already have the most numbers are where deductions come easiest.
Hint (
) reveals one correct cell if you get truly stuck, at a small score cost.
Solving techniques
These are the core logical tools for Classic Sudoku — and the foundation for every other variant.
1. Naked Single
The simplest technique: if a cell has only one possible candidate — because the other eight digits already appear in its row, column or box — that digit must go there. Placing it often frees up neighbouring cells, so re-scan after every move.
The row already has 1 2 3 4 6 7 8 9 — only 5 is missing, so the amber cell is a Naked Single.
2. Hidden Single
Look at it from the digit's side: if a number can legally go in only one cell of a row, column or box (every other cell is blocked), it must go there — even if that cell still shows other candidates. Scanning each digit across the board (“cross-hatching”) is the fastest way to find them.
Digit 7 fits only the amber cell in this box — all others are blocked by their row or column.
3. Naked Pair
An elimination technique. If two cells in the same unit share exactly the same two candidates {X, Y} and nothing else, then X and Y are claimed by those two cells — remove X and Y from every other cell in that unit. This often triggers fresh singles.
Indigo: the pair {3,7}. After removing 3 and 7 elsewhere, the green cells collapse to {5,9}.
4. X-Wing
An advanced single-digit pattern. When a digit appears in exactly two cells of two different rows, and those cells share the same two columns (forming a rectangle / “X”), the digit can be eliminated from the rest of those two columns. The same works with rows and columns swapped.
Digit 5 forms an X-Wing (amber corners); it can be removed from the rest of both columns (red ✗5).
5. Forcing Chains
The expert's last resort when patterns run out. Pick a cell with two candidates and follow the consequences of each: “if this cell is A, then… ; if it is B, then…”. If both chains force the same digit into some other cell, that conclusion is certain regardless of which branch is true. Powerful, but slower — exhaust the simpler techniques first.
Always work from simplest to hardest: Naked Singles → Hidden Singles → Naked Pairs → X-Wing → Forcing Chains. After every placement or elimination, re-scan for new singles — most of the board falls in cascades.
Best techniques for Classic Sudoku
These are the solving techniques that pay off most in Classic Sudoku — each links to a full, illustrated guide.
Your score builds up from 0 as you place correct numbers, up to the full base when the board is finished. Then two bonuses are added:
base × your progress × (1 + combo bonus + time bonus) × 1
Progress: the live score climbs from 0 to the base as the grid fills with correct numbers.
Combo bonus (up to +50%): keep a clean run for the full bonus. Each error or hint is a combo break that shrinks the bonus — but it never takes Sparks away below the base.
Time bonus (up to +50%): beat the normal time and you earn extra, proportional to how much faster you were. Finishing slower than normal costs nothing — there is no time penalty.
The total is multiplied by ×1 and never drops below 0.
Coins
The coins you earn reflect your score — the more Sparks, the more coins (about 1 coin for every 45 Sparks, minimum 1). Spend coins in the Shop on hints and Jokers.
Tip: a clean, hint-free solve under the normal time scores the highest. The Daily Challenge is always Classic and fully scored.
Classic Sudoku — FAQ
What is Classic Sudoku?
Classic Sudoku is a 9×9 grid divided into nine 3×3 boxes. The goal is to fill every cell with a digit from 1 to 9.
What are the rules of Classic Sudoku?
Each row, each column and each 3×3 box must contain the digits 1 to 9 exactly once, with no repeats. Some cells are filled in at the start as clues.
How do I start solving a Sudoku?
Look for rows, columns or boxes that are almost full and find the only digit that can fit a cell. Then use pencil marks to track the remaining candidates for harder cells.
Is Classic Sudoku good for beginners?
Yes. Start on Easy or Medium, which can be solved with simple logic and never require guessing — every SudokuStreak puzzle has exactly one solution.
Do I ever need to guess?
No. Every puzzle has a single unique solution that can be reached by pure logic, so guessing is never required.