All Sudoku Variants
9×9 · Constraint

Odd-Even Sudoku

Also known as Even-Odd Sudoku

Shaded cells restrict parity: grey squares must hold even digits, circles must hold odd ones.

4 6 7 3 8

An example Odd-Even puzzle: grey squares hold even digits (4, 6); circled cells hold odd digits (7, 3).

Odd-Even Sudoku shades or marks some cells to tell you the parity of the digit that belongs there. A cell drawn as a grey square must hold an even digit; a cell drawn as a circle must hold an odd one. Not every cell is marked, but the ones that are cut their candidate list in half before you place a thing.

The rule: squares are even, circles are odd

There are four even digits — 2, 4, 6 and 8 — and five odd ones — 1, 3, 5, 7 and 9. A square can only ever take one of the four evens; a circle only one of the five odds. Everything else is ordinary Sudoku: rows, columns and boxes still need 1–9 exactly once.

6 3 8 5◯◯

Grey squares hold even digits (6, 8); circles hold odd digits (3, 5).

The hidden count: four evens, five odds

Because there are exactly four even and five odd digits, every row, column and box contains precisely four even cells and five odd cells. When a whole unit is marked, that becomes a hard count: four squares and five circles, no more and no less. Even a partially marked unit gives it away — spot five circles and the four remaining cells must all be even, and vice versa.

◯◯◯◯◯

A fully-marked box always has four squares (even) and five circles (odd) — the split is fixed.

Using parity as a filter

The real value of the marks is candidate pruning. A square instantly drops 1, 3, 5, 7 and 9; a circle drops 2, 4, 6 and 8. Combine that with a normal scan and hidden singles appear far sooner: if a box needs a 7 and only one of its cells is a circle, the 7 goes there without further thought. Parity also pairs beautifully with sum clues — in a marked cage, knowing which cells are odd or even often fixes the exact combination.

Solving strategy

  • Prune first. Pencil only evens into squares and only odds into circles before any scanning.
  • Count the parities. Four evens and five odds per unit — use it to place the last cell of a nearly-marked row or box.
  • Target odd or even digits. When you need to place a specific digit, its parity tells you which marked cells can even hold it.
  • Feed the grid. Each placement behaves like a given for its row, column and box.

Common mistakes

  • Swapping the shapes. Squares are even, circles are odd — reversing them breaks every deduction.
  • Assuming unmarked means anything. An unmarked cell has no parity clue, but it still obeys the four-even/five-odd count for its unit.
  • Forgetting 5 is odd. The middle digit trips people up — it belongs in circles, not squares.
  • Ignoring the count. The fixed 4-5 split per unit is a clue in its own right, not just a consequence.

Can I play it here?

Not yet — Odd-Even Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile: