How to Solve a 9×9 Sudoku Step by Step

The 9×9 grid is the heart of Sudoku. Whether you're solving on paper or on your phone, the logic is always the same: eliminate, narrow down, place. This guide walks through the full process — from your first scan to the final cell.

Before You Start: What You're Working With

A standard 9×9 Sudoku board has 81 cells arranged in nine rows, nine columns, and nine 3×3 boxes. Each unit — row, column, box — must contain the digits 1 through 9, each exactly once.

The puzzle gives you a set of "given" numbers as starting clues. Your job is to fill in the rest using pure logic. Difficulty is determined by how many clues are given and how constrained the remaining cells are.

Step 1 — First Scan (Easy wins)

Before writing anything, spend 30 seconds scanning for obvious cells. Look for rows, columns, or boxes that have 7 or 8 clues already filled. The missing digit is determined by which numbers haven't appeared yet in that unit.

On Easy puzzles, this single pass often fills 10–15 cells. Always start here — it's free information.

Step 2 — Cross-Hatching

Cross-hatching lets you find where a specific digit must go inside a 3×3 box. Pick a digit — let's use 5 — and locate every row and column that already contains a 5.

Now look at a box that doesn't have a 5 yet. The rows and columns that already contain a 5 "cross out" those positions inside the box. If only one cell remains uncrossed, that's where the 5 goes.

Repeat for every digit (1–9) across every box. On Medium puzzles this technique alone can solve 70% of the board.

Step 3 — Candidate Notes

When a cell can hold more than one digit, write the candidates in small notation (pencil marks, or notes mode in an app). This turns an invisible problem into a visible one.

As you fill in neighbouring cells, remove the now-impossible candidates from your notes. Eventually a cell's note list shrinks to a single digit — and that's your answer.

Step 4 — Advanced Techniques (Hard & Expert)

When basic scanning and cross-hatching aren't enough, these techniques break the hardest puzzles open:

The Full Solving Loop

1. Scan all units for single missing digits and fill them in.
2. Cross-hatch all nine digits across all nine boxes.
3. Add candidate notes to unsolved cells.
4. Look for naked and hidden pairs in each unit.
5. Apply box-line reduction and pointing pairs.
6. Every new digit you place — go back to step 1.

Tips for Faster Solving

• Work the most constrained units first. Units with 7+ clues collapse immediately. Leave sparse units for later.
• Update notes as you go. Every placed digit invalidates candidates in its row, column and box. Remove them right away.
• Use a systematic order. Cross-hatch digit 1, then 2, then 3 — all the way to 9. This prevents you from missing easy placements.
• Pause before advancing difficulty. If Hard puzzles still feel overwhelming, spend a week on Medium until cross-hatching and naked pairs feel automatic.

The 9×9 board is where Sudoku becomes a genuine mental workout. With the techniques above, no puzzle should feel unsolvable — only more or less time-consuming. Practice consistently, and the harder levels will gradually feel like the easier ones used to.