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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. On ToolKoala every Easy and Medium puzzle falls to exactly those; Hard and Expert sometimes need the two extra moves in step 4. 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 — and whenever you place a number, it's crossed off the notes in its row, column and box — so you can focus on the logic.

4. When singles run out (Hard and Expert)

Every ToolKoala Easy and Medium puzzle can be finished with the moves above. Hard and Expert usually can too, but now and then the singles dry up. Two more moves crack most of those boards. Both work on the pencil marks — they cross candidates off instead of placing numbers:

  • Pointing pair: inside one box, a candidate (say 7) only appears in cells that share a row. That box's 7 must be in that row, so cross 7 off the rest of the row outside the box. It works the other way too: if a row's only 7s sit inside one box, cross 7 off the rest of that box.
  • Naked pair: two cells in the same row, column or box have exactly the same two candidates, say 3 and 8. Those two numbers belong to those two cells, so cross 3 and 8 off every other cell in that row, column or box.

Each crossing-off often leaves a new single behind. A few Expert boards need rarer patterns still (X-wings, chains) — that's where the next step comes in.

5. 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 careful logic always gets there — on the toughest Expert boards it just takes techniques beyond this guide.

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. Easy and Medium need nothing beyond singles; some Hard and Expert boards need pointing pairs, naked pairs or rarer techniques.

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 🐨