How to Play Sudoku

New to Sudoku? This guide covers everything - From understanding the grid to solving your first puzzle confidently, without guessing.

The One Goal

Fill every cell in the 9×9 grid with a digit from 1 to 9 so that:

• Every row contains the digits 1–9 exactly once
• Every column contains the digits 1–9 exactly once
• Every 3×3 box contains the digits 1–9 exactly once

The pre-filled numbers (the clues) are fixed and cannot be changed. They are the starting point for your logic.

Anatomy of the Grid

Element	Count	Rule
Rows (horizontal)	9	Each must contain 1–9 once
Columns (vertical)	9	Each must contain 1–9 once
3×3 Boxes	9	Each must contain 1–9 once
Total cells	81	9 rows × 9 columns
Given clue cells (Easy)	40–45	Pre-filled, cannot change

The Three Core Techniques

Every Easy and most Medium puzzles can be solved using only these three methods. Master them first.

1. Last Free Cell
   Find a row, column or box with only one empty cell. The missing digit (the one not yet in that unit) is your answer. This is always the fastest win - Scan for it first every time.
2. Last Possible Number (Elimination)
   For a blank cell, list every digit that already appears in its row, its column, and its 3×3 box. Cross those out. If only one digit is left, it must go in that cell.
3. Hidden Single (Candidate Counting)
   Look at a unit (row, column or box) and ask: for a specific digit, how many blank cells could it go in? If only one cell is valid, fill it - Even if that cell appears to have multiple candidates.

Your Step-by-Step First Solve

1. Start with the most-filled units. Look for rows, columns or boxes with 7 or 8 digits already placed. One or two blanks left means near-instant fills.
2. Scan each digit 1–9. For the digit 1: where are all the 1s already placed? Look at which rows and columns are already covered. In remaining boxes, some cells are ruled out - If only one valid cell remains in a box, fill it.
3. Fill and immediately re-check. Every time you fill a cell, re-scan its row, column and box. Your new entry often creates a Last Free Cell in an adjacent unit right away.
4. Switch perspective when stuck. If row/column scanning stalls, try box scanning instead. The same grid looks different from different angles.
5. Use Notes mode for harder puzzles. Enable pencil marks (the Notes button) to record which candidates each cell still allows. Update these marks as you fill cells.
6. Never guess. Any Sudoku on Sudoku.by can be solved by pure logic. If you feel the need to guess, return to systematic scanning - You've missed something.

Common Mistakes - And How to Fix Them

Mistake	What to Do Instead
Only checking the row, forgetting the column	Always verify all three units: row + column + box
Guessing when stuck	Return to technique 3 - Scan for Hidden Singles
Not updating notes after each fill	Delete that digit from all peers immediately
Starting with random cells instead of most-filled units	Always start with the unit closest to completion
Ignoring digits that are already placed 7+ times	High-count digits are nearly done - Finish them first

Difficulty Progression Guide

Once you can reliably solve Easy puzzles in under 10 minutes, you're ready for the next step. Here's the roadmap:

Level	New Skill Required	Typical Time
Easy	Basic elimination	5–15 min
Medium	Hidden Singles	10–20 min
Hard	Naked/Hidden Pairs	20–45 min
Expert	X-Wing pattern	30–60 min
Master	Candidate chaining	45–90 min
Extreme	All advanced techniques	60–120+ min