Sudoku rules
Short rules card: 1–9 once per row, column, and box. Practice on the live grid above — Check flags conflicts.
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Sudoku rules
The rules fit in one breath: place digits 1 through 9 so every row, every column, and every 3×3 box contains each digit exactly once. Givens (pre-filled cells) stay put. A fair puzzle has one solution.
This page is the short rules card with a live grid above. If you want first-move coaching and longer “how do I start” prose, use how to play sudoku. If you want habits and named techniques, use sudoku strategy.
When
When to use
You searched sudoku rules / rules of sudoku; you want a checklist, not a novel; you are teaching someone the constraint set.
When not
When not to
First-move walkthrough → how to play sudoku. Tips list → sudoku tips. Killer sums → killer sudoku (extra cage rules).
Defaults
Assumptions
Classic 9×9. Mini / 16×16 / samurai use related ideas on later pages — not this URL. No diagonal requirement unless a future variant page says so (standard classic does not).
Steps
How
- Memorize the three houses: row, column, box.
- Never repeat a digit inside a house.
- Practice on the live grid; Check flags conflicts.
- Ready for habits → strategy or tips.
Worked notes
Examples
- Digit 7 already in a row → no other 7 in that row, even if the box looks empty of 7s elsewhere
- You place two 5s in one box → broken rules; Check / conflict should light up
Watch-outs
Gotchas
- Sudoku is not a math crossword — no cage sums on this classic rules page (that is Killer).
- Guessing is not a rule; uniqueness work prefers logic.
- Difficulty labels do not change the rules — only how sparse the givens feel.
Nearby pages
Compare
Longer how-to: how to playTips: sudoku tipsStrategy: sudoku strategyPlay: play sudoku
Controls
Quick controls
- Hint — a cell/digit, technique name, and short reason.
- Check — flags conflicts without revealing the full grid.
- Pencil — candidate notes for empty cells.
- New puzzle — same difficulty band, fresh seed.
- Daily — fixed local seed for the calendar day.
- Erase — clear the selected cell (not givens).
FAQ
Common questions
What are the three constraints?
Every row, every column, and every 3×3 box must contain each digit from 1 through 9 exactly once. That is the whole classic rule set.
Can I change givens?
No. Pre-filled cells stay locked. Erase only clears digits you placed yourself.
Diagonal required?
Not in classic 9×9 on this site. Unless a future variant page says otherwise, diagonals are optional flavor elsewhere — not a rule here.
Killer rules?
Killer keeps these digit rules and adds cages with printed sums (no repeat inside a cage). Open killer sudoku for that mode.