Sudoku Basics: Rules and How to Solve Your First Puzzle

Sudoku is a logic puzzle played on a 9x9 grid. The grid is divided into nine 3x3 boxes, and some cells already contain numbers — these are called givens. To solve it, you fill every empty cell so that each row, each column, and each 3x3 box contains the digits 1 through 9 exactly once. No arithmetic is involved; you never add or multiply anything. You only use the givens and the numbers you place to deduce what goes where. This guide covers the rules, the first techniques to learn, and how to check your work.

The Grid and the Givens

A standard Sudoku grid has 81 cells arranged in 9 rows and 9 columns. Thick lines divide it into nine 3x3 boxes (also called regions or blocks).

• Rows run left to right and are usually labeled 1–9 from top to bottom.
• Columns run top to bottom and are usually labeled 1–9 from left to right.
• Boxes are the nine 3x3 squares, three across and three down.

The givens are the pre-filled numbers. They are the starting information and are fixed — you never change them. The number of givens varies by puzzle. A well-formed puzzle has exactly one solution, which means every empty cell can be deduced by logic alone. You should never need to guess.

The Three Core Rules

Every placement must satisfy all three of these conditions at once:

1. Each row contains the digits 1–9 exactly once.
2. Each column contains the digits 1–9 exactly once.
3. Each 3x3 box contains the digits 1–9 exactly once.

A digit may appear only once in its row, only once in its column, and only once in its box. If placing a number would repeat a digit in any of those three groups, the placement is wrong.

How to Start: Scanning for Easy Numbers

Begin with the digit that appears most often among the givens — it usually has the most constraints and is easiest to place. Then work digit by digit.

For each digit, scan the grid to find where it can legally go:

• Look at a row or column that already contains the digit. That digit cannot appear again anywhere in that row or column.
• Look at a box that already contains the digit. It cannot appear again in that box.
• Cross-reference the two: if a row and a column both block a box, often only one cell in that box remains for the digit.

Repeat this for every digit, then cycle back through them. Each number you place creates new constraints and unlocks more placements.

Beginner Techniques

Naked Single

A naked single is a cell where only one digit is possible. If a cell's row, column, and box together already contain eight different digits, the ninth is forced. This is the simplest placement and often the first thing to look for.

Hidden Single

A hidden single is a digit that can only go in one cell within a row, column, or box — even if that cell could technically hold other digits too. For example, if within a single box the digit 7 fits in only one empty cell, then 7 must go there, regardless of what else that cell might allow. Hidden singles are the workhorse of beginner solving.

Pencil Marking

Pencil marking means writing the candidate digits (the digits still possible) lightly in each empty cell. This turns the puzzle into a visible map of possibilities.

• Mark candidates only after you have placed the obvious singles, so you are not erasing constantly.
• When you place a digit, erase that digit from the candidates in its row, column, and box.
• Pencil marks make hidden singles and naked singles much easier to spot.

If you solve on paper, use a pencil so you can erase. If you solve online, most interfaces have a notes or candidates mode.

Checking Your Work and Recovering from a Mistake

When the grid is full, verify it before assuming you are done:

• Check every row contains 1–9 with no repeats.
• Check every column contains 1–9 with no repeats.
• Check every 3x3 box contains 1–9 with no repeats.

If you find a conflict, you made an earlier error. Because a wrong entry can stay hidden for many moves, work backward:

1. Find the cell where the conflict appears.
2. Trace which of your placed digits caused it.
3. Undo placements from the most recent backward until the contradiction disappears.
4. Re-examine the cell you just cleared — you likely misread a candidate or missed a constraint.

A useful habit: after placing a digit, immediately scan its row, column, and box for any cell that now has only one candidate left.

Where to Practice

You can practice with daily printable or online puzzles. The site Daily SUDOKU Puzzles (sudoku.friko.net) offers a Sudoku online solver that can generate random puzzles, solve a given or generated puzzle, and support solutions and printing. That combination lets you generate a fresh grid, attempt it yourself, and check the solution when you are stuck — useful while the basic techniques are still becoming automatic.

Start with easier puzzles that have more givens. As scanning and pencil marking become faster, move to puzzles with fewer givens, which require more advanced techniques.