Play Sudoku online for free
One puzzle, a quiet board, nothing tugging at your attention: play Sudoku right here in your browser — free, with no ads and no sign-up.
Sudoku is a logic puzzle on a 9×9 grid: every row, every column and every 3×3 box must contain the digits 1 through 9 exactly once. On CalmPlay you play Sudoku free in your browser — no ads, no sign-up — across four difficulty levels, with pencil notes, gentle error hints and puzzles verified to have exactly one solution.
Rules
Sudoku is played on a 9×9 grid divided into nine 3×3 boxes. Some cells come pre-filled. Your goal: complete the grid so that every row, every column and every 3×3 box contains the digits 1 through 9 exactly once.
That is the entire rule set — but three properties matter, because they carry the whole solving experience:
- There is no arithmetic. The digits are just symbols; Sudoku would work identically with letters or shapes.
- Every puzzle here has exactly one solution. If two digits ever look equally possible, you are missing a deduction — guessing is never required by design.
- A given clue is never wrong. If the grid stops working, the error is in one of your own entries. Our gentle error display marks conflicts without ever revealing the solution.
Strategy: the techniques that matter
Good Sudoku players don’t guess — they eliminate. These techniques are ordered by effort, and they are exactly the criteria our difficulty grading is built on.
Scanning (cross-hatching)
Take a digit that already appears often on the board and walk box by box: where is it still missing? Mentally strike out every row and column that already contains it. If only one cell in a box survives, you’ve placed a digit. Scanning alone carries you through most of an easy puzzle.
Naked single
A cell where, after removing everything its row, column and box already contain, only one candidate remains. The bread-and-butter deduction — easy to miss unless you check systematically.
Hidden single
The mirror image: a digit that fits in only one cell of a row, column or box — even if that cell could theoretically hold other candidates too. Here you reason from the digit, not from the cell.
Naked pair
Two cells in the same unit hold exactly the same two candidates. Those two digits are then locked into those two cells — and can be erased from every other cell in the unit. From Medium upward you’ll need this regularly; the notes feature exists precisely for it.
Box-line interaction
If a candidate appears in only one row (or column) within a box, it must live in that row inside the box — so it can be erased from the rest of the row. The reverse holds too. These eliminations are the signature of our Hard puzzles.
A practical rhythm: scan first, then hunt singles, then switch to notes and work pairs and box-line eliminations. When you stall, scan again — every placed digit creates fresh information.
Difficulty levels
Our four tiers are graded by the techniques a puzzle minimally requires — not merely the number of clues. Easy solves with scanning and singles alone; Medium demands pair techniques (notes become your friend); Hard requires box-line interactions and triples yet stays solvable with nameable logic; and the expert tier is deliberately built so the standard toolkit is provably not enough — what-if chains take over, while exactly one solution remains guaranteed.
Each tier has its own page with a matching puzzle ready to play and strategy notes for exactly that level:
- Easy Sudoku – the way in: scanning and singles carry you through.
- Medium Sudoku – pencil notes earn their keep; pairs come into play.
- Hard Sudoku – box-line interactions and triples, still pure logic.
- Expert Sudoku – beyond the standard ladder; chains take over.
You can also switch tiers any time right at the board; a fresh puzzle is ready instantly.
A short history
Sudoku’s roots reach back to the Latin squares studied by Leonhard Euler in the 18th century. The modern form with 3×3 boxes appeared in the USA in 1979 as Number Place, designed by puzzle constructor Howard Garns. It became a phenomenon in Japan from 1984, where publisher Nikoli coined the name Sudoku — shorthand for a phrase meaning “the digits must remain single”. A worldwide wave followed from 2004 via The Times of London, and Sudoku has been a fixture of logic puzzling ever since — a quiet, focused pastime that fits this site’s whole idea.
Frequently asked questions
Is Sudoku really free here?
Yes, completely. There are no ads, no paywall and no account. You open the page and play — that is CalmPlay's whole model.
Do I need to sign up to play?
No. There are deliberately no accounts and no registration. Pick a difficulty and the first puzzle is waiting.
Can every Sudoku be solved without guessing?
Every puzzle on this page is guaranteed to have exactly one solution. Up to the Hard level we verify by machine that the techniques described in the strategy section are enough — no guessing ever needed there. Our expert tier deliberately goes beyond those techniques and calls for what-if chains with pencil notes.
What do the difficulty levels actually mean?
They are graded by the techniques a puzzle demands, not just by how many clues you get. Easy needs only single-candidate moves, Medium adds pairs, Hard adds box-line interactions and triples, and the expert tier sits beyond that standard toolkit.
Does it work well on a phone?
Yes. The board and number pad are built for touch; on a computer you can also use the keyboard, including the arrow keys.
Is there a pencil-notes feature?
Yes. The Notes toggle lets you jot small candidates into empty cells. When you later commit a digit, matching notes in its row, column and box are tidied up automatically.