Sudoku is the one puzzle type where generating is genuinely hard and playing is easy. Every puzzle needs exactly one solution — and checking that is the part that separates a real generator from a grid shuffler.
1. Pick a size
| Size | Uses | Suits |
|---|---|---|
| 4×4 | 1–4 in 2×2 boxes | Ages 4–7 |
| 6×6 | 1–6 in 2×3 boxes | Ages 7–9 |
| 9×9 | 1–9 in 3×3 boxes | The standard |
| 16×16 | 1–9 and A–G | Enthusiasts, long sitting |
One caution on 16×16 in print: at that size the cells on a Letter page are small enough that pencil marks get awkward. If you publish them, give each one a full page.
2. Choose the difficulty
The difficulty setting in the sudoku generator controls how many numbers are removed. It is worth knowing that this is a proxy rather than the real thing — two puzzles with the same number of givens can differ enormously depending on which techniques the gaps force. What actually makes a sudoku hard goes into it.
Practically: generate two or three at your chosen level and solve the first few moves of each. You will feel the difference immediately.
3. The check that matters
Every 9×9 generated here is verified to have exactly one solution. After each number is removed, the puzzle is solved again exhaustively; if a second solution appears, the number goes back.
This is slow, which is why plenty of tools skip it — and it is invisible until it fails, at which point a solver has filled a grid legally and been told they are wrong.
We do not run this check on 16×16 — the search space makes it too slow to do in a browser. If you are publishing 16×16 puzzles, solve a sample yourself first.
Build a worksheet set, not a random stack
For a class or a puzzle book, generate in small labelled batches. Put one easier puzzle first so the solver learns the page layout, place the intended level in the middle, and finish with one step harder. Keeping the size constant matters: a jump from 4×4 to 9×9 changes the rules of the task as well as the difficulty.
| Position | Purpose | What to check |
|---|---|---|
| First | Warm-up | Several obvious opening moves |
| Middle | Target level | Progress without repeated guessing |
| Last | Stretch | Harder logic, still a single solution |
Five checks before handing it out
- The grid and its answer key carry the same title or number.
- Every symbol is dark enough to survive a normal photocopier.
- There is enough empty space in each cell for pencil notes.
- A sample puzzle begins with at least one understandable logical move.
- The answer key is stored separately so it is not printed on the reverse by accident.
Numbering the puzzle and key before downloading is the small habit that prevents mismatches later. A correct solution attached to the wrong grid is functionally no solution at all.
4. Print or play
Export the puzzle as PDF for paper. Print the answer keys separately if you are assembling a book — most layouts group them at the back, four to six per page.
Press Play to solve on screen instead. It has pencil notes built in and flags conflicts in red as you type, which is the difference between finding your mistake now and finding it twenty moves later.