How to Solve Sudoku: A Step-by-Step Guide for Beginners
2026-06-22
Short version: you almost never need to guess. Most Sudoku puzzles fall to three simple ideas — scanning, singles, and pencil marks — applied in order. Here's how, and you can try each step on a free Sudoku board as you read.
The one rule
Fill the grid so every row, every column, and every 3×3 box contains 1–9 exactly once. That's it. Everything below is just a faster way to figure out what goes where.
1. Scan for the obvious (cross-hatching)
Pick a number — say 5 — and look at where 5s already are. A row or box that has a 5 can't take another. Slide your eyes across rows and down columns and you'll often find a box where only one empty cell can hold the 5. Pencil it in. Doing this number by number clears the easy cells fast.
2. Look for "singles"
- Naked single: a cell where only one number is still legal (every other digit already appears in its row, column, or box). It has to be that number.
- Hidden single: a number that can only go in one cell within a row, column, or box — even if that cell looks like it has other options.
Hidden singles are the ones beginners miss most, so hunt for them deliberately.
3. Use pencil marks when you're stuck
When scanning dries up, jot the candidates (the still-legal numbers) into each empty cell. Now patterns pop out: a cell with one candidate is a naked single; a candidate that appears once in a box is a hidden single. ToolKoala's board has a Notes button for this, and an Auto-notes button that fills every candidate for you so you can focus on the logic.
4. When you truly get stuck
Take a hint to reveal one correct square and keep momentum — then try to continue on your own. Guessing-and-checking works too, but a well-made puzzle never requires it: every ToolKoala puzzle has exactly one solution, so the answer is always reachable by logic.
Practice
Open the Sudoku board, start on Easy, and use Notes + Auto-notes while the techniques sink in. Want it on paper? You can also print a PDF pack. Newer to it? Try the gentler 4×4 and 6×6 grids first.
FAQ
Do I ever have to guess in Sudoku? No. A proper Sudoku has a single solution reachable by logic alone. ToolKoala only removes squares while one solution remains, so you never have to guess between two valid answers.
What's the fastest beginner technique? Scanning (cross-hatching) one number at a time clears the easy cells quickest. Then switch to pencil marks to find singles.
What are pencil marks? Small candidate numbers you note in a cell to track which digits are still legal there. Use the Notes button, or Auto-notes to fill them all at once.
How do I get better? Solve a little every day and force yourself to find hidden singles before guessing. The daily puzzle is a good habit — everyone gets the same board, so you can compare times.
— Milo 🐨