How to Play Cosmic Sudoku
Cosmic Sudoku is the classic 9×9 logic puzzle reimagined against a drifting starfield. The rules are the ones that made sudoku a global pastime: fill every row, every column, and every 3×3 box with the digits 1 through 9 so each digit appears exactly once in each of those units. A handful of cells arrive pre-filled as your starting clues, and the rest is pure deduction — no arithmetic, just constraint logic. The four difficulties set how many clues you start with — Easy aims for 38, Medium 30, Hard 25, Expert 20 — and the generator only takes a clue away while the board still has exactly one valid solution. For fans of Sudoku.com and other online sudoku puzzles, this is a free version with no download and no account — play in your browser on desktop or mobile.
Click or tap a cell, then type a digit or use the number pad to commit it; each pad button tracks how many of that digit are still missing. Pencil Mode (the Pencil button or P) turns the same input into small candidate notes in a cell's corners, and placing a real digit clears that candidate from the whole row, column and box for you. Undo steps back through every placement, erase and pencil toggle you have made, as far as the opening position, at no cost to your score. Check highlights every wrong digit currently on the board for two seconds, while Auto-Check — which starts off on each new board — leaves them flagged the moment you place them. Neither changes the maths: a wrong digit counts in the ERRORS box and against your score whether Auto-Check is on or off. The Hint button fills one correct cell for you at a cost of thirty seconds on the clock and two hundred points.
Completing a board pays a base of 1,000 points on Easy up to 8,000 on Expert, plus a time bonus if you beat that difficulty's par, minus 100 for every mistake and 200 for every hint. A clean solve earns three stars; up to three mistakes with at most one hint earns two. Each difficulty keeps its own points leaderboard, and the menu screen remembers your games played, games won and best time per difficulty in your own browser.
Strategy Tips
- Hunt naked singles first. A naked single is a cell where only one digit can legally go once you account for the row, column and box it sits in. These are the free moves of sudoku — sweep every empty cell for them before attempting anything more complex. On Easy boards, naked singles alone will often carry you to the finish.
- Then scan for hidden singles. A hidden single is a digit that has only one legal home inside a given row, column or 3×3 box, even if that cell still shows several other candidates. Pick a digit — say 7 — look at each unit that doesn't yet contain it, and ask where 7 is still allowed. If there's exactly one cell, place it.
- Pencil before you commit. When the easy singles run out, switch to Pencil Mode and mark candidates across the hardest-looking region. The visual density of the notes exposes pairs, triples and locked candidates that are invisible to mental arithmetic.
- Start where the board is densest. Rows, columns and boxes that already hold six or seven digits have the fewest remaining possibilities and the highest chance of yielding singles. Solve the half-full regions first and the sparse ones often collapse on their own as neighbouring cells fill in.
- Never test a digit on the board. A wrong placement costs 100 points and a mark in the ERRORS box even with Auto-Check switched off, while pencil marks are free — try the candidate as a note instead. Save hints for a genuinely stuck Expert board: one hint still leaves you a two-star finish, a second drops you to one.
What Makes Cosmic Sudoku Different
Most sudoku sites wrap the puzzle in a paper-and-newsprint look that has barely changed since the 2005 boom. Cosmic Sudoku swaps that for a cool-toned deep-space palette with constellation-line box dividers, glowing digits, and a drifting starfield behind the grid — a calmer read for long evening solves. Underneath it is a full modern sudoku rather than a stripped-down demo, and every difficulty is unlocked from the first click. Sudoku.com's free tier is ad-supported and The New York Times keeps its sudoku archive behind a Games subscription; here there is no account to create and no puzzle held back.
A Brief History of Sudoku
The mathematical ancestor is the Latin square, studied by Leonhard Euler in the 18th century. The modern 9×9 form was designed by retired American architect Howard Garns and first published as "Number Place" in a 1979 Dell puzzle magazine. Japanese publisher Nikoli picked it up in 1984, named it sūdoku (数独, "single digit") and refined the conventions around symmetry and unique solutions. After The Times of London began printing the puzzle in 2004, it became a global phenomenon almost overnight.
Frequently Asked Questions
Is there a daily puzzle in Cosmic Sudoku?
Cosmic Sudoku does not have a dedicated daily puzzle mode — every time you start a game the board is generated fresh from the selected difficulty, with exactly one valid solution. That means you can play as many boards as you like in a session instead of waiting 24 hours for the next one, and each difficulty keeps its own leaderboard, ranked by the points a completed board scores.
What difficulty levels are offered?
Four tiers: Easy, Medium, Hard, and Expert. What separates them is how many clues the board opens with — Easy aims for 38, Medium 30, Hard 25 and Expert 20 — and the generator only takes a clue away while the board still has exactly one valid solution. At 38 clues, naked-singles scanning will usually carry an Easy board to the end; the sparser the grid gets, the more you lean on hidden singles and careful pencil-mark work.
Do hints cost points?
Yes. Each hint reveals one correct digit from the solution, but it adds thirty seconds to your timer and subtracts two hundred points from your final score. A fully clean solve — no mistakes, no hints — earns the maximum three-star rating and the highest score that board can pay out.
How does Pencil Mode work?
Press the Pencil button or tap P on the keyboard to enter Pencil Mode. Clicking a digit now writes it as a small candidate note in the cell's corner rather than committing it as your answer. When you later place a real digit, Cosmic Sudoku automatically clears that candidate from every related row, column, and box, so your pencil marks stay in sync with the board without manual cleanup.
Does the game work on mobile?
Yes. The grid scales to phone and tablet screens, the digit pad is large enough for thumb entry, and every control — pencil toggle, undo, hint, pause, auto-check — is exposed as a tappable button underneath the board. There is no app to install; the game runs in any modern mobile browser.
Can I undo mistakes?
Yes. The Undo button (or Ctrl+Z) reverses your last placement, erase, or pencil-mark toggle, and the history stretches back through the whole solve — you can unwind move by move all the way to the starting position if you want to. Undo itself carries no score penalty, so it's almost always preferable to guessing forward on a cell you're unsure about.
