🔢 Classic Sudoku
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.
Strategy summary
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.
🏆 Difficulties & scoring
This type can be played on all five difficulties. The score multiplier for Classic is ×1. The "normal time" below is tuned for this type.
| Difficulty | Base score | Max (×1) | Normal time | Hints |
|---|---|---|---|---|
| easy | 150 | 300 | 5 min | 5 |
| medium | 300 | 600 | 10 min | 4 |
| hard | 600 | 1200 | 15 min | 3 |
| expert | 1000 | 2000 | 20 min | 2 |
| evil | 1500 | 3000 | 30 min | 1 |
How the score is calculated
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 × (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 points 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 points, the more coins (about 1 coin per 20 points, 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.