Sudoku Strategies and Techniques

The techniques below go roughly from the ones you will use in every game to the ones you will need only on the toughest grids. You do not have to learn them all at once. Most players pick up one new trick, use it for a week, and then come back for the next.

Before You Start: Pencil Notes

Almost all sudoku techniques work with candidates: the numbers that could still go in a square. On paper you write them small in the corner; on our board you switch on Notes and tap the numbers. A square with notes 2, 7 and 9 means "this is 2, 7 or 9, I just do not know which yet". As soon as you place a number, remove it from the notes in the same row, column and box. Our board does that for you.

Simple Techniques

Naked Single

A naked single is a square with only one possible candidate. Look at the row, the column and the box that pass through the square and collect every number they already hold. If eight different numbers are taken, the ninth is the answer.

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Practice on Hard

Hidden Single

A hidden single turns the question around. Instead of asking what goes in a square, ask where a number can go in a unit. Pick a box that is missing, say, a 4. Every 4 in the rows and columns crossing that box rules out some of its squares. When only one square is left, the 4 goes there, even if that square still has other candidates.

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Practice on Hard

Intermediate Techniques: Hard Sudoku Strategies

When singles run out, you need to remove candidates until a single appears again. These three hard sudoku strategies do exactly that, and together they are the answer to how to solve hard sudoku grids on our Hard level.

Pointing

Sometimes all the candidates for a number inside one box line up in a single row or column. You do not know which square will get the number yet, but you know it will be in that line. So the number cannot appear anywhere else in that row or column, outside the box.

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Practice on Hard

Naked Pair

If two squares in the same unit have exactly the same two candidates, those two numbers belong to those two squares, one each. No other square in that unit can hold them, so you can cross them out everywhere else in the unit.

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Practice on Hard

Hidden Pair

A hidden pair is the mirror image. Two numbers appear as candidates in only two squares of a unit, although those squares also have other notes. Since the two numbers must go somewhere, they take those two squares, and every other candidate in them can be removed. A hidden pair is easy to miss because the extra notes disguise it, which is why it pays to scan each unit number by number.

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Practice on Hard

Advanced Sudoku Techniques

The two patterns below look at a single number across several rows and columns at once. You will never need them on Easy, Medium or Hard, but they are standard advanced sudoku techniques for Expert puzzles.

X Wing

Choose a number and look for two rows where it can go in only two squares each, and where those squares sit in the same two columns. The four squares form a rectangle. The number must take two opposite corners, one in each row and one in each column, so it can be crossed out of every other square in those two columns. The same works with columns and rows swapped. Spotting the X Wing sudoku pattern takes practice, so start by checking one number at a time.

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Practice on Expert

Swordfish

A Swordfish is an X Wing with three rows and three columns. Find three rows where a number can go in only two or three squares each, and where all those squares fall within the same three columns. Three copies of the number must fill those three columns, one per row, so the number can be removed from every other square in the three columns.

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Practice on Expert

What to Do When You're Stuck

Everyone hits a wall sometimes. Here is how to solve sudoku when stuck, in the order we would try things:

1. Check your notes. Make sure every empty square in the area has a complete, up-to-date set of candidates. Missing notes are the most common reason a puzzle seems impossible.
2. Scan number by number. Go through 1 to 9 and, for each number, look at every box where it is missing. A hidden single is often waiting in plain sight.
3. Look for pairs. Search each row, column and box for two squares with the same two candidates, then for two numbers that live in only two squares.
4. Try pointing. For each box, check whether a number's candidates line up in one row or column.
5. Take a break. Press Pause, make a cup of tea and come back. A fresh look solves more puzzles than any technique.
6. Use one hint. There is no shame in it. A single revealed number often restarts the whole chain of reasoning.

Sudoku Tips and Tricks

A few habits make every technique above easier. These sudoku tips work at any level:

• Start with the busiest units. Rows, columns and boxes with six or seven numbers already filled are the quickest wins.
• Work one number at a time. Following a single number around the whole grid is faster than jumping between squares.
• Notice the numbers that are almost complete. The count on each button of our number pad shows how many of that number are still missing. When only one or two are left, finding them is often easy.
• Never guess. If you have to choose between two numbers, there is a logical reason for one of them that you have not found yet.

Some people call these sudoku tricks, but there is nothing tricky about them: they are just good ways of looking at the grid. Combine them with the sudoku techniques on this page and the hardest puzzles become a pleasant challenge rather than a wall. If you are new to the game, start with our guide on how to play Sudoku, or go straight to the board at the Medium level.