Loona Sudoku Generator — Puzzles with Exactly One Solution
A Sudoku with two solutions is a broken puzzle: a student can reason perfectly and still "fail". This generator removes numbers one at a time and re-checks after every removal that only one solution remains, so the puzzle you print is always fair.
Sudoku — Solution
How to Use It
- Choose a size: 4 × 4 uses 2×2 boxes (numbers 1–4), 6 × 6 uses 2×3 boxes, 9 × 9 is the classic.
- Choose difficulty — it controls how many numbers are removed. Harder puzzles remove more, but never at the cost of a second solution.
- Generate (hard 9 × 9 can take a few seconds — every removal is verified).
- Print the puzzle, and print the solution separately when you need it.
How It Works
Solutions are built from the classic Sudoku pattern, shuffled by bands, stacks, rows, columns and digits so every puzzle differs. A backtracking solver with a most-constrained-cell strategy then counts solutions during removal: a number is only removed permanently if the puzzle still has exactly one solution. The solver stops counting at two, which keeps even 9 × 9 generation practical in a browser.
Frequently Asked Questions
What makes Sudoku good for students?
Pure logic with no language barrier: deduction, elimination and patience. The 4 × 4 version teaches the exact same reasoning to six-year-olds.
Why does hard 9 × 9 take a few seconds?
Because fairness is expensive: dozens of removals, each one proven not to create a second solution. A few seconds of generating buys a puzzle that can never ambush a student.
Can I check a student's finished grid against the solution?
Yes — print the solution page, or keep it open on screen. Every given number and every answer matches the one unique solution.