What is Sudoku? A Complete Guide from Beginner to Expert

Sudoku is a logic-based number puzzle played daily by millions worldwide. It requires no math — only logic, patience, and systematic thinking. This guide covers what Sudoku is, grid structure, rules, and how to solve your first puzzle step by step.

Sudoku is played on a 9x9 grid. The grid is divided into nine 3x3 boxes — 81 cells total. Some cells are pre-filled with numbers at the start; these are called 'givens.' Your job is to fill the remaining empty cells with the numbers 1 through 9.

There are three basic rules: Each row must contain the numbers 1–9 exactly once. Each column must do the same. Each 3x3 box must contain 1–9 with no repetition. These three constraints together mean there is exactly one correct answer for each cell.

Rows run horizontally — nine cells from left to right. Columns run vertically — nine cells from top to bottom. Boxes are 3x3 blocks, separated by thicker lines on the grid. A cell belongs to one row, one column, and one box at the same time.

Given numbers cannot be changed — they are the puzzle's foundation. You can write 'candidate' numbers in empty cells — using pencil marks or draft mode. When only one number can fit in a cell, you place it.

First step: Look at the rows, columns, and boxes with the most filled cells. Find the missing number and check where it can go. Often only one possibility remains in a cell — that's a 'naked single.'

If stuck: Use pencil marks. Write the possible numbers for each empty cell. Eliminate candidates as you go. When a number can only go in one cell within a row, column, or box — a 'hidden single' — place it there.

Sudoku requires no guessing. Every puzzle is solvable by logic alone. If you feel you must guess, you've missed something. Step back, review your candidates, and look for elimination patterns.

Difficulty depends on the number of givens and the techniques required. Beginner puzzles with 35–40 givens are easy. Expert puzzles with 25 or fewer givens and techniques like X-Wing and XY-Wing are challenging.

Sudoku is a learnable skill. The rules are simple; the depth is endless. Check our How to Play page and techniques guide to solve your first puzzle. Happy solving!