Classic sudoku has exactly three rules. Everything else — every technique, every strategy — is just working out the consequences of those three rules under pressure. This page covers the base rules in full, then what changes in the variants you’ll see elsewhere.

The three rules

A completed 9x9 grid is valid when:

  1. Every row contains the digits 1 through 9, each exactly once.
  2. Every column contains the digits 1 through 9, each exactly once.
  3. Every 3x3 box contains the digits 1 through 9, each exactly once.

That’s the entire rule set. A puzzle gives you some of the 81 cells filled in already (the “givens”) and asks you to fill in the rest so all three rules hold everywhere at once. A properly made puzzle has exactly one way to do this — see uniqueness basics for why that matters and how it’s enforced.

There’s no arithmetic involved anywhere in classic sudoku, despite using digits 1-9. They’re just nine distinct symbols; a version using letters A-I would play identically.

Killer sudoku

Killer sudoku keeps all three base rules and adds “cages” — outlined groups of cells, each labeled with a target sum. The digits inside a cage must add up to that target, and (in standard Killer rules) can’t repeat within a cage. This turns sudoku into a genuine arithmetic puzzle on top of the logical one, and it’s usually solved starting from the smallest or most constrained cages, where only a few digit combinations can possibly add up to the target.

16x16 (and other sizes)

The same three-rule structure scales to other grid sizes, as long as the grid is a perfect square made of smaller square boxes. A 16x16 grid uses a 4x4 box and digits (or letters) 1 through 16 instead of 1 through 9; a 4x4 “kids” grid uses a 2x2 box and digits 1 through 4. The logic is identical at every size — only the box dimensions and symbol count change.

Other common variants

  • Diagonal (or “X”) sudoku adds a fourth constraint: both main diagonals must also contain 1-9 exactly once.
  • Hyper sudoku adds four extra 3x3 “bonus” boxes overlapping the grid, each with its own uniqueness constraint.
  • Jigsaw sudoku keeps the row/column rules but replaces the 3x3 boxes with irregularly-shaped regions of nine cells each.

What this site covers

Sudoku24x7 plays classic 9x9 sudoku, generated and graded by our own logical solver rather than a puzzle bank. If you came here from a variant and want the base logic first, how to play is the place to start.