Sudoku strategies: how to solve sudoku step by step

Sudoku needs no math, just logic. These strategies take you from your first puzzle to the hard ones, in the order you’ll need them.

How do you play sudoku?

• The grid is 9 × 9, divided into nine 3 × 3 boxes.
• Some cells start filled in. Fill every empty cell with a digit 1 to 9.
• Each row, each column and each box must contain every digit exactly once.

That’s all the rules. There’s no math: the digits could just as well be nine colors or nine letters. The puzzle is figuring out which digit can only go in one place.

Your first move, step by step

1. Look for a digit that already appears often, say there are six 5s on the board.
2. Find a box with no 5. The 5s in the rows and columns that cross that box rule out some of its cells.
3. If only one empty cell in the box is left for the 5, write it in.
4. Do the same with the other boxes, then the next digit. Every digit you place gives you new information for the next one.

On an easy puzzle, these steps (called scanning, below) are enough to finish the whole grid.

Beginner: scanning

Pick a digit, say 5. For each box that doesn’t have a 5 yet, look at the rows and columns that already contain a 5: they block those cells. If only one cell in the box is left that could hold a 5, it goes there. Repeat for every digit. Scanning alone solves most easy puzzles.

Beginner: naked singles

Look at a single empty cell and list the digits already in its row, column and box. If eight digits are taken, the ninth is the answer. This works best in busy areas of the grid.

Intermediate: pencil marks

When scanning dries up, write the candidates in each empty cell: the small digits that could still go there. Every time you place a digit, erase it from the notes in its row, column and box. In Kepo Sudoku, the Notes mode does this for you.

Hidden singles

A digit that appears as a candidate in only one cell of a row, column or box must go there, even if that cell has other candidates too.

Naked pairs

If two cells in the same row, column or box both have exactly the same two candidates (say 3 and 7), those two digits must go in those two cells. Remove 3 and 7 from every other cell in that row, column or box. The same works with three cells and three digits (naked triples).

Pointing pairs

If, inside a box, a digit’s candidates all sit in one row (or column), then that digit must be in that row within the box, so you can remove it from the rest of that row outside the box.

Advanced: X-wing

Find a digit that, in two different rows, can go in only two cells, and those cells sit in the same two columns. The digit must take one corner in each row, so you can remove it from every other cell in those two columns. (It also works with columns and rows swapped.)

Tips to solve faster

1. Start with the most common digits. A digit that’s already on the board 6 or 7 times is the easiest to finish.
2. Don’t guess. If you’re stuck, scan again or look for a pair: there’s always a logical next step.
3. Check boxes, rows and columns in turn. Many people only scan boxes and miss easy row-based placements.
4. Use notes on hard puzzles only. On easy ones, scanning is quicker; on hard ones, notes are essential.

Play sudoku online

Kepo Sudoku has puzzles at four levels (Easy, Medium, Hard and Expert) plus a daily puzzle that’s the same for everyone. Every puzzle has exactly one solution. It’s free in your browser with no ads, and it has notes, unlimited undo, and hints if you get stuck.