The Goal of Sudoku in One Sentence
Fill every empty cell in the 9×9 grid so that each row, each column, and each 3×3 box contains the digits 1 through 9 exactly once.
The 3 Rules
Every Sudoku puzzle follows the same three constraints. If you remember these, you know the rules:
Row rule: Each row of 9 cells must contain 1–9 with no repeats.
Column rule: Each column of 9 cells must contain 1–9 with no repeats.
Box rule: Each 3×3 box (there are 9 of them in the grid) must contain 1–9 with no repeats.
That is genuinely everything. No addition, no multiplication, no guessing. Every puzzle is solved through pure logic.
A Simple Step-by-Step Approach
When you are staring at a fresh puzzle, here is a reliable method to get started:
Step 1: Scan the grid for rows, columns, or boxes that already have many numbers filled in. The more clues present, the fewer options remain for the empty cells.
Step 2: Pick a nearly-full row, column, or box. Count which numbers are missing.
Step 3: For each empty cell in that group, ask: which missing numbers could go here? Check the row, column, and box that intersect at that cell.
Step 4: If only one number is possible for a cell, write it in. This is called a "naked single" — the most common solve technique.
Step 5: After placing a number, re-scan nearby rows, columns, and boxes. Your new number may unlock another cell.
Step 6: If no cell has an obvious single answer, use pencil marks (also called candidates). Write small numbers in a cell to track what is still possible.
Step 7: Repeat. As you fill cells, the puzzle gets easier because constraints tighten everywhere.
A Worked Mini-Example
Imagine Row 4 has these values: 3, _, 7, 1, _, 9, 8, 2, _. Three cells are empty, at Columns 2, 5, and 9. The missing numbers for this row are 4, 5, and 6.
Look at Column 2. Suppose it already contains 4 and 6 elsewhere. That means the empty cell at Row 4, Column 2 can only be 5.
Now Column 5 might already have 5 and 6. That leaves only 4 for Row 4, Column 5.
By elimination, Row 4, Column 9 must be 6.
That is the core loop: narrow down, eliminate, place. Every cell in every Sudoku puzzle is solvable this way — no guessing needed.
Common Beginner Mistakes
Forgetting the box rule. New players often check rows and columns but forget that each 3×3 box is also a constraint. Always check all three.
Guessing instead of deducing. If you are not certain a number goes somewhere, do not place it. Use pencil marks instead and wait until you can prove it.
Not scanning the full grid. Beginners sometimes fixate on one area. After placing a number, look across the whole grid — your new placement may create a solve somewhere far away.
Placing a number that already exists in the row, column, or box. Slow down and double-check before writing. Our online player highlights conflicts in red to help.
Overloading pencil marks. Writing every possible number in every cell creates visual noise. Start with pencil marks only in cells where you have narrowed it down to 2 or 3 options.
Giving up too early. If you feel stuck, step away for a minute. Fresh eyes often spot something you missed. The "stuck" checklist below helps too.
If You Are Stuck, Do This
Scan all rows for any with just 1 or 2 empty cells. Then scan all columns the same way. Then scan all 3×3 boxes. Check your pencil marks for any cell that has been reduced to one option. Look for a number that appears 8 times in the grid — the 9th placement is forced. If nothing works, take a break and come back.
How to Practice Efficiently
Start with easy puzzles. They have more given numbers and require only basic scanning. Play our Daily Sudoku every day to build a habit, or grab a printable easy pack for offline practice.
Once easy puzzles feel comfortable in under 10 minutes, move to medium. The jump is gentle — you will just need pencil marks more often.
For a deeper understanding of strategies beyond scanning, check out our tips page and the rules reference.
A Note for Seniors and Large Print
If small grids are hard on your eyes, we offer large print printable puzzles with one puzzle per page and bigger numbers. Pair them with a pencil and eraser for a relaxing pen-and-paper experience.
Frequently Asked Questions
Do I need to be good at math to play Sudoku?
Not at all. Sudoku uses the digits 1–9 as symbols, but you never add, subtract, or do any arithmetic. You could replace the numbers with letters or shapes and the puzzle would work the same way.
Is guessing ever necessary?
No. Every properly constructed Sudoku puzzle has exactly one solution reachable through logic alone. If you find yourself guessing, try a different technique or use pencil marks to narrow down possibilities.
What are pencil marks?
Pencil marks (also called candidates) are small numbers you write in a cell to track which values are still possible there. They are a tool, not a solution — you erase them as you eliminate options.
How long does a Sudoku puzzle take?
Easy puzzles typically take 5–15 minutes for a beginner. Medium puzzles take 10–30 minutes. Hard and expert puzzles can take 30 minutes to over an hour depending on your experience.
Where should I start as a complete beginner?
Try our Deluxe Player on Easy difficulty. It has conflict highlighting, hints, and an undo button so you can learn without frustration.