Sudoku Strategies for Beginners: From Scanning to Pairs
Sudoku looks like a maths puzzle, but you do not need to add or multiply anything. It is a puzzle of pure logic. Every well-made Sudoku has exactly one solution, and you can reach it step by step without ever guessing. Once you learn a few simple techniques, puzzles that once looked impossible start to fall into place.
This guide walks you through the main beginner strategies in the order most people learn them.
The rules in one minute
- The grid has 9 rows, 9 columns and 9 boxes of 3×3 cells.
- Some cells already have numbers. These are the "givens".
- Fill every empty cell with a number from 1 to 9.
- Each row, each column and each box must contain every number from 1 to 9 exactly once.
That is all. Everything else is technique.
Strategy 1: Scanning
Scanning is the first thing to do with any puzzle, and it solves many cells in easy puzzles all by itself.
Pick a number, say 5. Look at every 5 already on the grid. Each 5 "blocks" its own row and column. Now look at a box that does not yet have a 5. Rule out every cell in that box whose row or column already contains a 5. If only one cell is left, the 5 must go there.
Go through numbers 1 to 9 this way, box by box. Start with numbers that already appear many times on the grid, since they block the most cells and are easiest to place.
Cross-hatching
This kind of scanning is often called cross-hatching, because you imagine lines running across the rows and down the columns from each existing number. The cells those lines do not touch are the only possible spots.
Strategy 2: Pencil marks
When scanning stops giving answers, it is time for pencil marks (also called candidates or notes). For each empty cell, write small numbers showing every value that could still go there, meaning numbers not already in that cell's row, column or box.
Pencil marks feel slow at first, but they turn the puzzle into something you can see. Most online Sudoku games, including ours, have a notes mode for this.
Tips for pencil marks:
- Fill them in carefully. One missing candidate can lead you to a wrong answer.
- Each time you place a number, remove that number from the pencil marks in the same row, column and box.
- Keep your marks in the same position in every cell (for example, 1 in the top left, 9 in the bottom right). This makes patterns easier to spot.
Strategy 3: Naked singles
A naked single is a cell that has only one possible candidate left. Every other number is already used in its row, column or box. If a cell's pencil marks show only one number, that number goes in.
Naked singles are easy to spot once you have pencil marks, and placing one often creates more, as removing candidates elsewhere leaves other cells with only one option.
Strategy 4: Hidden singles
A hidden single is a little sneakier. Here, a cell might have several candidates, but one of those numbers can go nowhere else in that row, column or box.
For example, look at a row. Suppose the number 7 appears as a candidate in only one cell of that row. Even if that cell also has 2 and 4 as candidates, the 7 must go there, because the row needs a 7 and there is no other place for it.
To find hidden singles, pick a row, column or box and ask, for each number, "where can this go?" If the answer is "only one place", you have found one.
Strategy 5: Pairs
Pairs are the first step into more advanced solving, and they are very satisfying to use.
Naked pairs
Suppose two cells in the same row each have exactly the same two candidates, say 3 and 8, and nothing else. Those two cells must contain 3 and 8 between them, in some order. That means no other cell in that row can be 3 or 8. You can remove 3 and 8 from the pencil marks of every other cell in the row.
The same works for a column or a box. Removing those candidates often reveals new singles.
Hidden pairs
A hidden pair is the reverse. In a row, column or box, two numbers can each only go in the same two cells. Even if those cells have other candidates, they must hold those two numbers, so you can remove all other candidates from those two cells.
A simple solving routine
- Scan for easy placements, number by number.
- When scanning stops working, add pencil marks to every empty cell.
- Look for naked singles.
- Look for hidden singles in each row, column and box.
- Look for naked and hidden pairs and remove candidates.
- Repeat from step 3 until the puzzle is solved.
Common beginner mistakes
- Guessing. If you guess and it is wrong, you might not notice for many moves. Logic is always faster in the end.
- Forgetting to update pencil marks after placing a number.
- Only checking rows. Remember that boxes and columns matter just as much.
- Jumping to hard puzzles too soon. Build confidence on easier ones first.
Practise online
The best way to learn these strategies is to use them. Try our free Sudoku game and start with an easier level. Focus on scanning first, then add pencil marks as puzzles get harder. You will be surprised how quickly the patterns start jumping out at you.
If you like logic puzzles, you might also enjoy Minesweeper or the slide puzzle, and our article on brain games as a daily habit. Sudoku fans in the TNChatZone app are always happy to share a tricky puzzle or two.