🚀 Cosmic Sudoku

How to Play Sudoku

A complete beginner's guide

Sudoku is a logic puzzle that anyone can learn in a couple of minutes — no math, no luck, just clear thinking. This guide walks you through the rules and gives you a simple method for solving your very first grid.

The one rule that matters

A Sudoku puzzle is a 9×9 grid, split into nine smaller 3×3 boxes. Some cells already contain numbers — these are your clues. Your job is to fill in every empty cell so that:

That's the whole game. The digits are just symbols — you never add, subtract, or do any arithmetic. If a number can't be repeated in its row, column, or box, you've followed the rule.

Key idea: Every well-made Sudoku has exactly one solution, and you can always reach it with logic alone. In Cosmic Sudoku, every one of the 777 levels is generated to have a single unique answer — so you never have to guess.

How to read the grid

Before placing anything, get comfortable looking at the board in three ways at once: the row a cell sits in, the column it sits in, and the 3×3 box that surrounds it. A number you want to place must be allowed in all three of those at the same time. Training your eye to flick between row, column, and box is most of the skill.

A simple method for your first puzzle

  1. Scan for nearly-full groups. Look for a row, column, or box that already has lots of numbers. The fewer the gaps, the easier it is to work out what's missing.
  2. Hunt one digit at a time. Pick a number — say 5 — and look across the board for boxes that don't have a 5 yet. Often a 5 can only legally go in one cell of a box because the other empty cells already "see" a 5 in their row or column. Place it.
  3. Fill the forced cells. Sometimes a single empty cell in a row, column, or box has only one possible value. Those are free — place them immediately, because each one you fill makes the neighbours easier.
  4. Repeat and cascade. Every number you place removes possibilities elsewhere. Keep cycling through digits and groups, and the puzzle slowly unlocks itself.

Using notes (pencil marks)

When you can't immediately see a cell's value, switch to notes mode and jot down the candidates — the small numbers that could still legally go in that cell. Pencil marks turn a fuzzy grid into a clear one: when a cell ends up with only one candidate left, that candidate is the answer. In Cosmic Sudoku, tap the notes toggle, then tap numbers to add or remove them from a cell.

Hints, mistakes, and difficulty

Stuck? Cosmic Sudoku gives you 10 free hints per day. A hint reveals the correct value for a well-chosen cell to get you moving again. Wrong entries are highlighted so you can spot and fix a slip before it spreads. Levels rise gently in difficulty from Easy to Hard as you travel from Earth to the edge of the universe, so the early levels are a comfortable place to practise the method above.

Ready to play?

The best way to learn Sudoku is to solve one. Start at level 1 — it's free, there's no sign-up, and it runs right in your browser.

Play Cosmic Sudoku free →

Frequently asked questions

Do I need to be good at math?

No. Sudoku uses the digits 1–9 as symbols only. It's a placement puzzle, not an arithmetic one.

Is there always a logical next move?

Yes. Proper Sudoku puzzles never require guessing — there is always a deduction available, even if it takes a moment to spot. Want sharper techniques? Read our Sudoku strategy and tips.

Is Cosmic Sudoku really free?

Completely. All 777 levels, the daily puzzle, streaks, and 10 hints per day are free for everyone.