How to Play Sudoku: Rules and Your First Solve

Fill the grid so every row, every column, and every 3x3 box contains the digits 1 through 9 exactly once. That's the whole rule. Everything else in sudoku is figuring out where each digit goes.

You don't need math. The digits could be letters, colors, or animals and the puzzle would work the same way. You only need to notice what's missing.

The words you'll see

• Cell: one square. There are 81.
• Row / column: a horizontal or vertical line of 9 cells.
• Box: one of the nine 3x3 squares, outlined in bold.
• Unit: any row, column, or box. Every unit holds 1 through 9 once.
• Givens: the digits printed at the start. You can't change them.
• Candidates (or notes): the small digits you pencil into a cell to track what could still go there.

Your first solve, step by step

1. Pick a digit and scan for it. Start with whichever digit appears most often in the givens, say 5. Look at a box that doesn't have a 5 yet. Check which rows and columns running through that box already contain a 5. Those lines are blocked. If only one empty cell in the box is left unblocked, the 5 goes there.

This is called scanning or cross-hatching, and it solves most of an easy puzzle on its own.

2. Work through every digit, then repeat. Go 1 through 9. Each digit you place blocks more cells for the next pass, so a second sweep usually finds things the first one missed.

3. When scanning stalls, look at crowded units. Find a row, column, or box that's almost full. If a row has eight digits, the ninth is obvious. If it has seven, check which two digits are missing and which cells can take them.

4. Start using notes. On medium puzzles, scanning alone won't finish the job. Write every possible candidate into the cells of one crowded box. Then look for a cell with only one candidate left. That's a naked single, and it's the next thing to learn.

Mistakes almost everyone makes

• Guessing. A well-made sudoku has exactly one solution and can be solved by logic alone. If you're guessing, you've missed a deduction. Step back and scan again.
• Writing a digit you're not sure of. One wrong placement poisons everything built on it. Only place a digit when you can point to the reason.
• Forgetting to update notes. When you place a digit, erase it from the notes of every cell that shares a row, column, or box with it. Most sudoku apps can do this for you, but it's worth understanding why.
• Starting on Hard. Difficulty in sudoku is about which techniques a puzzle requires, not how many givens it has. Start on Easy until scanning feels automatic.

Where to go from here

Once Easy puzzles take you under ten minutes, move to Medium and learn the two singles. Read Naked Singles and Hidden Singles, then work through the Technique Journey: 15 short lessons, from singles all the way to x-wing and swordfish.

Ready? Play an Easy puzzle now. No ads, no account needed. Or try today's daily: everyone gets the same grid, and the leaderboard shows how many hints each solver used.