How to Play Sudoku
The Basic Rules
Sudoku is played on a 9x9 grid divided into nine 3x3 sub-grids called boxes. The puzzle starts with some numbers already placed. Your goal is to fill every empty cell so that the following three rules are satisfied simultaneously:
- Every row must contain the digits 1 through 9, each exactly once.
- Every column must contain the digits 1 through 9, each exactly once.
- Every 3x3 box must contain the digits 1 through 9, each exactly once.
That is the complete rule set. No arithmetic is involved. You never add, subtract, multiply, or divide. The challenge is purely logical — eliminating possibilities until only one number can fit each cell.
Playing on This Site
- Click any empty cell on the grid to select it. The selected cell, its row, column, and box will be highlighted.
- Press a number key (1–9) on your keyboard, or tap a button on the number pad below the grid.
- Press Backspace or Delete to erase the number in the selected cell.
- Use the arrow keys to move between cells without lifting your hands from the keyboard.
- Press N or tap Notes OFF to toggle pencil-mark mode. In this mode, numbers are written as small candidate notes rather than filled answers.
- Tap Erase to clear the selected cell's value or notes.
- If you enter a wrong number it will show with a strikethrough and your mistake count will increase.
- The puzzle is complete when all 81 cells are correctly filled.
Quick Reference
| Click cell | Select |
| Keys 1 – 9 | Enter number |
| Backspace / Del | Erase cell |
| Arrow keys | Move selection |
| N key | Toggle notes |
| New Game | Fresh puzzle |
Difficulty Guide
Easy — 45 filled cells. Relies on single-candidate logic only. Recommended for newcomers.
Medium — 35 filled cells. Requires scanning rows and columns to narrow down possibilities.
Hard — 27 filled cells. Demands advanced techniques such as naked pairs and box-line reduction.
Beginner Strategies
Scanning
Scan each row, column, and box to find cells where only one number is missing. Fill those first. This alone solves most easy puzzles.
Single Candidates
For each empty cell, list which numbers are already present in its row, column, and box. If only one number is absent, that number belongs in that cell.
Pencil Marks
Use Notes mode to write small candidate numbers in each cell. As you fill cells, update and remove candidates. This is essential for medium and hard puzzles.
Naked Pairs
If two cells in the same row, column, or box each contain only the same two candidates, those two numbers can be removed from all other cells in that unit.
Box-Line Reduction
If a candidate number in a box appears only in one row or column within that box, eliminate that candidate from the rest of that row or column outside the box.
Practice
There is no substitute for repetition. Solve one puzzle per day. Start with Easy, progress to Medium when comfortable, then tackle Hard once your logic is sharp.
About ZeroGlitch Sudoku
Our Mission
ZeroGlitch Sudoku was built with a single purpose: provide a clean, fast, and fully functional Sudoku game that works on any device and requires no account, no app download, and no unnecessary complexity.
We believe a good Sudoku site should get out of the way. The focus belongs on the puzzle, not the interface. Every design decision — the black and white theme, the newspaper-style typography, the minimal controls — reflects that belief.
A Brief History of Sudoku
The modern Sudoku puzzle was designed by Howard Garns, an American architect and freelance puzzle constructor, and first published in 1979 by Dell Magazines under the name "Number Place." The puzzle gained international popularity after Maki Kaji, president of the Japanese puzzle company Nikoli, introduced it in Japan in 1984 under the name "Suuji wa dokushin ni kagiru," which translates roughly as "the digits must remain single." This was shortened to Sudoku.
Sudoku reached global audiences after being published in British newspaper The Times in 2004. Today it is among the most widely recognised puzzle formats in the world, appearing in newspapers, books, apps, and dedicated websites.
What Makes a Good Puzzle?
Every puzzle generated on this site satisfies two fundamental properties. First, it has a unique solution — there is exactly one correct way to complete the grid. Second, it is solvable by logic alone — no guessing is ever required. These properties are verified algorithmically at generation time.
Technical Notes
Puzzles are generated entirely in your browser using a backtracking algorithm. No puzzle data is sent to or stored on any server. The timer and score exist only in your current session. There are no accounts, no leaderboards, and no persistent tracking of any kind.