Sudoku tips for beginners
Sudoku looks like a number puzzle but it's really a logic puzzle: there are no sums, only the question of which number can go where. Every puzzle on this site has exactly one solution and can be solved by logic alone, without guessing. These are the techniques you need.
The rules
A classic Sudoku is a 9×9 grid split into nine 3×3 boxes. Some squares start with numbers. Fill in the rest so that every row, every column and every box contains the numbers 1 to 9 exactly once. Smaller versions work the same way: a 4×4 grid uses 1 to 4 in 2×2 boxes, and a 6×6 grid uses 1 to 6 in 2×3 boxes.
Technique 1: the only number that fits
Pick an empty square and check which numbers are already in its row, column and box. If eight of the nine numbers are taken, the last one must go there. This is called a naked single. It's easiest to spot in rows, columns or boxes that are nearly full, so start there.
Technique 2: the only place a number fits
Now turn it around. Pick a number, say 5, and a box that doesn't have one yet. Look along the rows and columns that pass through the box: any that already contain a 5 rule out their squares in this box. If only one square is left, the 5 goes there. This is called a hidden single, and the scanning method is called cross-hatching.
Work through the numbers 1 to 9 like this, box by box. Together with naked singles, it solves most easy and medium puzzles, including every puzzle on this site.
Using notes
When a square has two or three possible numbers, pencil them in with Notes. As you fill in other squares, cross options off; when only one is left, that's your answer. Notes are most useful on the 9×9 grid. Avoid filling every square with notes at the start, which is slow and hard to read.
Common mistakes
- Guessing. If you're stuck, look again for a single; there's always at least one.
- Forgetting the boxes. Beginners often check the row and column but not the box.
- Not updating notes after placing a number, which leaves wrong options behind.
- Rushing. A clash shown in red costs more time than checking first.
From 4×4 to 9×9
Young children and complete beginners should start with the 4×4 grid, where the whole puzzle fits in mind at once. Once that takes under a minute, the 6×6 grid introduces rectangular boxes and more numbers. The 9×9 game uses exactly the same techniques, just more of them. A typical 9×9 puzzle here takes about ten minutes; with practice many players get under five.