Guide
How to play Sudoku
Sudoku is a pure logic puzzle, no math and no guessing required. Learn the single rule and a handful of techniques below, and you will be able to solve any board, from Easy to Expert.
Know the one rule
Fill the 9x9 grid so that every row, every column, and every 3x3 box contains the digits 1 through 9 exactly once. That single constraint drives every move you make.
Scan for the obvious
Pick a digit and scan the rows, columns, and boxes where it already appears. Wherever those lines cross out every empty cell in a box but one, that cell is forced.
Use pencil marks
When a cell is not obvious, note every digit that could still legally go there. These candidate marks turn a fuzzy board into a set of small, solvable puzzles.
Find the singles
A naked single is a cell with only one candidate left. A hidden single is a digit that fits only one cell in a unit. Both are guaranteed, so place them with confidence.
Core solving techniques
These are the exact techniques ZenSudoku uses when you ask for a hint. Learn to spot them yourself and you will rarely be stuck.
Scanning
Follow a digit across the board and eliminate cells it cannot occupy. The fastest way to open up a fresh puzzle.
Naked single
Every other digit already appears in the cell row, column, or box, so only one digit remains. It must go there.
Hidden single
A digit can legally land in only one cell of a row, column, or box, even if that cell still shows other candidates.
Elimination
Placing a digit removes it as a candidate everywhere it sees. Keep your pencil marks tidy and new singles appear on their own.
Common questions
What are the rules of Sudoku?+
Fill the 9x9 grid so every row, every column, and every 3x3 box contains the digits 1 to 9 with no repeats.
What is a naked single?+
A cell where only one digit is still possible because every other digit already appears in its row, column, or box. That digit must go there.
What is a hidden single?+
A digit that can legally go in only one cell within a row, column, or box, even if that cell looks like it has other candidates.
How do pencil marks help?+
Pencil marks (notes) let you record the candidate digits for a cell so you can spot singles and eliminations without holding everything in your head.
Ready to try it?
Start on Easy and work your way up. A hint is one tap away whenever you need it.