How to Solve Sudoku

What is Sudoku?

Sudoku is a logic puzzle played on a 9×9 grid divided into nine 3×3 boxes. The grid starts with some cells already filled in — called givens — and your job is to fill in the remaining empty cells with digits 1 through 9.

Despite the numbers, Sudoku requires no arithmetic. It is purely a puzzle of logic and elimination. The same techniques work whether you write the digits 1–9 or the letters A–I.

The One Rule

Every row, every column, and every 3×3 box must contain each digit from 1 to 9 exactly once. That's the entire ruleset. Everything else is strategy.

Technique 1 — Scanning

Scanning is the first thing every solver learns. The idea is simple: look at a digit already placed in the grid and trace where else it cannot go.

For example, suppose the digit 5 already appears in row 2 and in column 4. Now look at the 3×3 box that sits at the intersection of that row and column area — any cell in that box that shares row 2 or column 4 is immediately ruled out for a 5. If only one empty cell remains in the box, it must be the 5.

Scan each digit 1–9 in turn. For popular digits (those that appear frequently in the givens) you will often find a cell that can only hold one value just from scanning.

Technique 2 — Elimination

Elimination works from the cell's perspective rather than the digit's. For each empty cell, list the digits that cannot go there based on what already appears in its row, column, and box. Whatever is left is a candidate.

If a cell has only one candidate remaining, place it immediately — this is called a Naked Single. It is the most common placement you will make in beginner and intermediate puzzles.

Many solvers write small pencil marks (candidate digits) in the corner of each cell to track possibilities. Our solver supports this mentally, and the step-by-step hint mode shows you exactly which cell to focus on next.

Technique 3 — Hidden Singles

Sometimes a digit has multiple candidates across a row, column, or box — but only one of those candidate cells can actually hold it. Even if that cell has other candidates listed, the digit is forced there because no other cell in the group can take it.

This is called a Hidden Single. It is hidden because the cell looks like it has choices, but once you survey the whole row (or column or box), only one home exists for that digit. Scanning helps you spot these: if a digit's possible positions in a box collapse to exactly one cell, that's your hidden single.

A Simple Solving Loop

Combine these three techniques into a repeating cycle:

1. Scan each digit 1–9 across rows, columns, and boxes.
2. Eliminate impossible candidates from every empty cell.
3. Place any Naked Singles (cells with one candidate left).
4. Place any Hidden Singles (digits with one possible cell in a group).
5. Repeat from step 1 with the updated grid.

For easy and medium puzzles this loop is enough to reach a complete solution. Harder puzzles require the strategies covered in the Advanced Guide.

Common Beginner Mistakes

Guessing early. Pure Sudoku never requires a guess. If you feel stuck, it means there is a logical deduction you haven't spotted yet — scan again before assuming you need to try numbers.

Ignoring one group type. Beginners often focus on rows and forget boxes, or vice versa. Always check all three constraint types (row, column, box) when scanning or eliminating.

Placing without checking all three. Before you write a digit down, confirm it doesn't appear elsewhere in the same row, same column, and same box. Skipping one check is the most common source of errors.

Tips for Getting Started

Start with digits that appear most frequently in the givens — they constrain the grid the most. Look for boxes, rows, or columns that are nearly full; one or two missing digits are often easy to deduce. Work systematically rather than jumping around, and double-check each placement before moving on.

If you want to see these techniques in action, open our solver and use Step-by-step hint mode. Each hint reveals one logical deduction at a time, so you can follow along and understand exactly why each digit goes where it does.