A fresh Sudoku, ready to print.

How to play Sudoku

Sudoku is a pure logic puzzle — no arithmetic, no guessing if you do it right. You fill a 9×9 grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once.

A puzzle starts partly filled. Those printed numbers are the givens, and they never change. Your job is to work out the one digit that belongs in each empty square using only what the givens force to be true. A well-made puzzle — like every one this generator produces — has exactly one solution, so you should never have to flip a coin.

Three rules cover everything:

• Each row of nine squares holds 1–9 with no repeats.
• Each column of nine squares holds 1–9 with no repeats.
• Each of the nine 3×3 boxes holds 1–9 with no repeats.

That single "no repeats" idea, applied across all three shapes at once, is the entire game. The skill is in seeing where those three constraints overlap to leave only one possibility.

What easy, medium, and hard actually mean here

Most generators set difficulty by simply deleting more numbers. That is misleading — a grid with very few givens can still be easy, and a grid with many givens can be nasty. Freshdoku grades each puzzle the way a person solves it: by the hardest technique you need to finish without guessing.

• Easy — solvable start to finish with "singles" alone (see below). Good for warming up or for kids.
• Medium — at some point you must use a pairs-level idea such as locked candidates or a naked pair. A satisfying coffee-break puzzle.
• Hard — needs deeper deductions like triples or an X-wing, and sometimes a long chain of reasoning. Bring a pencil.

Every generated puzzle is verified to have a unique solution before it reaches you, so the difficulty label reflects genuine reasoning effort rather than the number of blanks.

Solving strategies, from beginner to brain-bending

You can solve almost any easy or medium puzzle with the first four ideas below. The last two are what separate hard puzzles from the rest.

1. Scanning (cross-hatching)

Pick a digit — say 5 — and look at where it already appears. Each 5 "blocks" its whole row, column, and box. Slide along those lines and you will often find a box where only one square is still open for a 5. Place it. Scanning is fast, requires no notes, and clears out the easy placements first so the grid is less cluttered for everything else.

2. Naked single

Look at one empty square and ask which digits are still legal for it. If every digit except one is already present in its row, column, or box, that remaining digit is forced. It is "naked" because the square itself has only one option left.

3. Hidden single

This is the mirror image. Instead of a square with one option, look for a digit that has only one home left inside a row, column, or box — even if that square superficially looks like it could hold other numbers. The digit is "hidden" among other candidates, but the unit leaves it nowhere else to go.

4. Locked candidates

Sometimes a digit's only possible squares inside a box all sit in the same row (or column). You may not know which of them gets the digit, but you know it lands somewhere on that line inside this box — so it cannot appear in that same line anywhere else. Erase it from those other squares. This "pointing" elimination, and its reverse where a line restricts a box, is the gateway to medium puzzles.

5. Naked and hidden pairs

If two squares in the same unit can each only hold the same two digits — say {4, 7} and {4, 7} — then those two digits are spoken for. Neither 4 nor 7 can appear anywhere else in that unit, even though you don't yet know which square takes which. The hidden version is the same logic seen from the digits' side: two digits that can only live in the same two squares push every other candidate out of those squares.

6. X-wing

The classic "aha" technique for hard puzzles. Find a digit that, in two different rows, can go in only two columns — and crucially, the same two columns in both rows. Those four squares form a rectangle. Whichever diagonal of the rectangle ends up holding the digit, it occupies both of those columns — so the digit can be erased from every other square in those two columns. The same pattern works with rows and columns swapped.

Printing tips

The Print button strips away everything except the puzzle, so you get a clean sheet with no menus, ads, or page furniture. A few things that help:

• In the print dialog, set margins to Default and scale to 100% (or "Fit to page") for a large, comfortable grid.
• Turn off "Headers and footers" to lose the browser's date/URL line.
• Tick Include the solution before printing if you want an answer key — it prints on a second page so you can keep it separate.
• Prefer a file? PDF gives you a crisp, scalable sheet; PNG is handy for sharing in a message or pasting into a document.

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