How to play sudoku: the rules, step by step

Sudoku looks like a math puzzle, but it isn’t: there’s nothing to add up. It’s a logic puzzle with three rules you can learn in a minute, and a few simple habits that get you through your first grid. This guide covers both. Once you’re comfortable, our sudoku strategies guide takes you from hard to expert puzzles.

The rules of sudoku

• The grid has 9 × 9 squares, split into nine 3 × 3 boxes.
• Some squares start with a number already in them (the givens). You can’t change those.
• Fill every empty square with a number from 1 to 9 so that each row, each column and each box contains every number exactly once.

That’s the whole game. A finished sudoku has every number once in all 9 rows, all 9 columns and all 9 boxes, and a well-made puzzle has only one way to get there.

How to start: find a number that can only go one place

The quickest way into any sudoku is scanning: pick a number and look for the box where it can only fit in one square.

1. Choose a number that’s already on the grid a lot, say 7 appears five times.
2. Find a box that doesn’t have a 7 yet.
3. Look along the rows and columns that run through that box. Every row or column that already has a 7 somewhere else rules out its squares in this box.
4. If only one empty square in the box is left, the 7 goes there. Write it in.
5. Move on to the next box missing a 7, then to the next number.

Every number you place rules out more squares for the rest, so the puzzle speeds up as you go.

A worked example

Imagine the top-left box is missing a 5. Inside it, three squares are still empty: one in the top row, one in the middle row and one in the bottom row.

• The top row already has a 5 further to the right, so the 5 can’t go in that row’s empty square.
• The middle row has a 5 too, in the box next door.
• That leaves the bottom-row square, the only place left in the box for the 5. Write it in.

That’s exactly how most of an easy sudoku gets solved: one number, one box, a quick look along the lines.

When scanning stops: count what’s missing

Sometimes no box has an obvious spot for any number. Then flip the question around: instead of asking “where does this number go?”, ask “which number goes in this square?”

1. Pick an empty square in a busy part of the grid.
2. List the numbers already in its row, its column and its box.
3. If eight different numbers show up, the missing ninth is the answer.

Rows, columns or boxes with only one or two empty squares are the best places to try this.

Use notes (pencil marks) on harder puzzles

On medium and hard puzzles you’ll reach a point where nothing can be placed for sure. That’s when you start writing notes: the small candidate numbers that could still go in each square.

• Note every number that isn’t already in the square’s row, column or box.
• Each time you place a number, erase it from the notes in the same row, column and box.
• A square with only one note left is solved.

From there, patterns like pairs and X-wings help you remove notes faster. Those are covered in the sudoku strategies guide.

Common beginner mistakes

1. Guessing. If you guess and it’s wrong, you might not find out until much later, and then the whole grid is spoiled. There’s always a logical next step.
2. Only checking boxes. Many easy placements show up when you look along a row or column that has just one or two gaps.
3. Forgetting to update notes. One stale note can hide a solved square or lead you into a mistake.
4. Starting on the hardest numbers. Begin with the numbers that already appear most; they’re the easiest to finish.
5. Picking puzzles that are too hard. Start on Easy until scanning feels quick, then move up.

Sudoku difficulty levels

• Easy: lots of givens. You can solve it just by scanning.
• Medium: you’ll need to count what’s missing and use a few notes.
• Hard: notes are essential, along with simple patterns like pairs.
• Expert: few givens, and you need the advanced techniques.

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, and every puzzle has exactly one solution. It has notes, unlimited undo and hints for when you’re stuck. It’s free in your browser, on a phone or a computer, with no ads.