The best sudoku tips for beginners: learn the one rule (each row, column and box holds 1 to 9 once), scan one digit at a time, then add light pencil marks. Look for squares with only one possible digit and for pairs that rule out others. Avoid guessing, and let every step be forced by logic.
This guide covers the rules, a step-by-step solving routine, a worked mini example of naked singles and naked pairs, common mistakes and how to move up to harder puzzles.
Classic sudoku is a 9 by 9 grid split into nine 3 by 3 boxes. Your goal is to fill every empty square so that each row, each column and each box contains the digits 1 to 9 exactly once. There's no math involved. The numbers could just as easily be letters or colors.
A well-made puzzle has exactly one solution, and the fewest starting clues a proper sudoku can have is 17. The modern form first appeared in 1979 in Dell puzzle books under the name Number Place.
In the examples below, rows are numbered 1 to 9 from top to bottom and columns 1 to 9 from left to right. Box 1 is the top-left box, and the boxes are numbered left to right, top to bottom.
Start with the boxes. SudokuWiki, a sudoku strategy site, says it's best to start with boxes because they're the easiest shape to work with. Once you've scanned the boxes for obvious answers, check the rows and columns.
Scan one digit at a time. Pick a digit that already appears often, say 5. For each box that's missing a 5, look along its rows and columns for 5s elsewhere that block squares. If only one square is left, the 5 goes there. This is called a hidden single.
Finish nearly full units. A row, column or box with only one or two gaps is quick to complete. Check which digits are missing and where each one can go.
Add pencil marks when scanning stalls. In each empty square, write small digits for every number not already in that square's row, column or box. Write them lightly, or in the same corner each time, so they're easy to read and erase.
Hunt for naked singles and naked pairs. These two patterns, explained below, are the first ones to learn after scanning.
Update after every placement. When you write in a digit, erase it from the pencil marks in its row, column and box straight away. Stale marks are an easy way to get stuck or make an error.
Worked example of scanning. Box 1 needs a 5. Rows 1 and 2 each already have a 5 in another box, so the 5 must go in row 3 of box 1. Row 3, column 3 is already filled, and column 2 has a 5 lower down. That leaves only row 3, column 1, so write the 5 there.
SudokuWiki defines a naked single as the last remaining candidate in a square. If only one digit is possible, it must go there.
Naked single example. Row 1 has every digit except 8 and 9, with gaps in column 2 and column 7. Column 7 already has an 8 further down. So row 1, column 7 can only be 9. Write it in, and the 8 goes in column 2.
A naked pair is two squares in the same row, column or box that have the same two candidates and nothing else. Those two digits must go in those two squares, so you can erase both digits from every other square in the unit they share.
Naked pair example. Row 5 already has 3, 4, 6 and 8, so it still needs 1, 2, 5, 7 and 9. Its five empty squares have these pencil marks:
Column 1: 2, 7
Column 4: 2, 7
Column 6: 2, 5, 7
Column 7: 1, 9
Column 9: 1, 5, 9
Columns 1 and 4 can only hold 2 or 7. Whichever way round they go, those two squares use up the row's 2 and 7. So erase 2 and 7 from every other square in row 5.
Column 6 drops from 2, 5, 7 to just 5. That's a naked single, so write 5.
Erase 5 from column 9. It drops from 1, 5, 9 to 1, 9.
Columns 7 and 9 now form a second naked pair, 1 and 9. Clues from their columns or boxes will decide which is which.
If the two squares of a pair also share a box, you can clear those digits from the rest of that box too. Here, columns 1 and 4 sit in different boxes, so the pair only works on the row.
Guessing. A wrong guess can spread through the grid, and you won't know where it went wrong. If you're stuck, scan again or check your pencil marks.
Checking only one direction. Every digit has to fit its row, its column and its box. Check all three before writing.
Pencil-marking everything at the start. Scan first. It finds the easy digits faster and leaves fewer marks to manage.
Forgetting to erase. Old pencil marks lead to false pairs and wrong answers.
Writing in pen. Use a pencil or an erasable pen until you're confident.
Racing the clock. Speed comes from good habits. Accuracy first.
Easy: SudokuWiki notes you'll find plenty of singles in easier puzzles. Stay here until you can finish without guessing.
Medium: expect to need pencil marks and naked pairs.
Harder: learn the next basic strategies on SudokuWiki's list, hidden candidates and intersection removal, one at a time. Tougher ones like the X-Wing come after those.
Habit: one puzzle a day at a level that makes you think, but doesn't make you guess, is a good pace.
Sudoku is one kind of logic puzzle. For nonograms, grid puzzles and more, see our guide to logic puzzles for adults. And for an honest look at what puzzles can do for your brain, read Do brain games work?
Scan the boxes for digits that already appear several times on the grid. Those often give you quick placements before you need any pencil marks.
Yes, once simple scanning stops finding answers. Pencil marks are how you spot naked singles and pairs.
It's better not to. A well-made puzzle has one solution you can reach by reasoning, and a wrong guess can ruin the grid.
At least 17 for a proper sudoku with exactly one solution.
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