Sudoku Made and maintained by Brian Hamilton and Karan Hamilton.
Brian builds the playable puzzle tools, while Karan helps test clarity, difficulty feel, and the player experience.
Playable puzzles and controls are tested in the browser for rule accuracy, usability, and a clear solving experience. See how we make and test puzzles, our editorial standards, or report a problem.
What is Dark Mode Sudoku?
Dark Mode Sudoku is classic sudoku on a board that opens in the dark theme: light digits on dark-blue squares, on a near-black page. Fill the 9×9 grid so that:
- every row holds the digits 1 to 9 once;
- every column does too;
- and so does every 3×3 box.
Only the colours change. The game, the four levels and the buttons are the classic page’s, and the two pages keep one saved game between them: start a board on one and it is waiting on the other. The diagrams below are drawn in the site’s own colours, so they come out dark here and light on a light page.
Every other page on the site follows your device’s light or dark setting until you choose one; this page and its six translations open dark whatever the device says. A choice made with the theme button is saved in this browser and wins everywhere, this page included: if you switched to light on another page, this one opens light until you switch back.
How does the theme button work?
It sits in the navigation bar, beside the language menu; on a phone it is inside the menu, so open that first. It shows a sun on the dark theme and a moon on the light one — the theme it will switch to. One press recolours the page at once, board, notes and diagrams included, and saves the choice in this browser, so every page on the site opens in it from then on. Nothing else changes: the board, the notes and the timer carry on as they were.
With nothing saved, this page stays dark even if your device changes theme while you play. Until October 2026 it did not: a phone set to switch to its light theme at sunrise turned this page light in the middle of a game. Once you have pressed the button there is no “follow my device” setting; the choice stands until you press it again or clear this site’s data in your browser.
How readable is the dark board?
We measured it. With this page open in Chrome, we put a real board into every state the game draws — a given digit, one of yours, a wrong one, pencil marks, a selected square — read the colours it actually uses, and worked out the contrast of each against the square behind it:
| On the board | Dark theme | Light theme |
|---|---|---|
| Given digit | 14.9:1 | 13.9:1 |
| Your digit | 8.6:1 | 4.3:1 |
| Wrong digit | 6.0:1 | 4.7:1 |
| Pencil mark | 7.9:1 | 3.3:1 |
| Box border | 6.5:1 | 5.9:1 |
| Thin line between squares | 2.6:1 | 2.4:1 |
| Selected square, against a plain one | 2.0:1 | 1.3:1 |
WCAG contrast ratios: 1:1 is no difference at all, 21:1 is black on white. The digits are large text, for which 3:1 is the usual floor; pencil marks are small, for which it is 4.5:1. Measured on this page’s own board in Chrome, October 2026.
On this board the dark theme is the easier one to read, not just the darker one. Pencil marks are 7.9:1 on dark and 3.3:1 on light, under the floor for small text, and your own digits 8.6:1 against 4.3:1. The selected square stands out more as well. The given digits are about as clear in both, and the thin lines between squares are faint in both, which is why the box borders are drawn thicker and brighter. As for glare: at the same screen brightness, an empty square on the dark theme gives off about a sixtieth of the light of one on the light theme.
Until October 2026, on the dark theme, a wrong digit was drawn in the same blue as a right one — on this page and on every standard board on the site. The rule for your digits’ colour outranked the rule for mistakes, so the Mistakes count rose but nothing on the board showed which square. A wrong digit is now red (6.0:1), as it always was on the light theme. It differs from a right one in colour more than in brightness.
How do you find where a digit goes?
One digit at a time. Pick a digit, find every copy of it already on the board, and look at a box that still needs it: each copy rules out its own row and column. When only one empty square of the box is left, the digit goes there, whatever else that square could hold on its own. That is a hidden single:
Select a square holding the digit and every copy of it on the board lights up, which makes that sweep quick. The same sweep works along a row or a column: a digit with one place left in a line goes there. And a square whose row, column and box already hold eight different digits takes the ninth. Between them, those two moves finish every Easy and Medium board on this page.
What do Hard boards add?
Locked candidates. When singles run out, put the digits each empty square can still hold in its pencil marks — Notes draws them small, and Start Notes fills in every square at once — and look inside one box at a time. If a digit’s places in the box all lie in one row or column, the box’s copy of that digit is that line’s copy too, so it comes out of the rest of the line:
It works the other way round as well: a digit confined to one box within a row or column comes out of the rest of that box. Hard boards are dug so that singles and locked candidates finish them.
And Expert boards?
Naked pairs. Two squares of one row, column or box that can each hold only the same two digits will take those two between them, whichever way round, so no other square of that row, column or box can have either:
Expert boards are dug so that singles, locked candidates and naked pairs finish them, and they start with the fewest digits.
Were the boards on this page fair?
They always had exactly one answer: when this page went up on 30 April 2026 the generator already checked every digit it took out with a full search, and we re-counted 160 of its boards with a separate counter to confirm it. What it never checked was how a board would be solved, though the page told players “When you are stuck, avoid random guessing”.
Each level only had a number of digits to leave, so the names meant less than they said. Easy, Medium and Hard always had exactly 40, 32 and 26; Expert aimed at 22, managed it on 4 boards in 40 and left a median of 25, much the same as Hard’s 26. Singles alone finished 22 of 40 old Hard boards and 15 of 40 old Expert ones, while a solver that knows singles, locked candidates, pairs, triples and the X-Wing could not finish 26 of the 160 boards — 3 Medium, 12 Hard and 11 Expert. On those, not guessing was not always possible.
The generator was rewritten in October 2026. It is the classic one, so the fix reached the classic page, its level pages and the daily puzzles too, and it is the same fix the letter pages got. It now takes a digit out only while the level’s own steps still solve the board, which also proves the board has exactly one answer. Two keyboard faults went at the same time: Ctrl+1 (Cmd+1 on a Mac) typed a 1 into the selected square instead of switching browser tabs, and counted a mistake if 1 was wrong there; and Ctrl+Shift+Z never redid, because the code looked for a small z and Shift makes it a capital. Ctrl+Y always worked.
How do the four levels differ?
By how many digits are given, and by the steps a board is dug for. We dealt 40 fresh boards per level from the new generator, and 40 per level from the old one:
| Level | Given digits | Dug for | Singles alone finish it | Old boards beyond our solver |
|---|---|---|---|---|
| Easy | 40 | singles | 40 of 40 | 0 of 40 |
| Medium | 32 | singles | 40 of 40 | 3 of 40 |
| Hard | 26 | + locked candidates | 37 of 40 | 12 of 40 |
| Expert | 22–25 | + naked pairs | 31 of 40 | 11 of 40 |
40 boards per level, fixed seed. Expert digs toward 22 digits as far as its steps allow and keeps the fewest of up to six tries: 22 to 25, median 23. On the new boards nothing past naked pairs was ever needed: 3 Hard boards could not be finished without locked candidates, and of the Expert boards, 6 needed locked candidates and 3 naked pairs.
At the top the new levels are easier than the old ones, and fair all the way down. Most Hard and Expert boards now fall to singles alone, and what makes them harder is having fewer digits to start from — 26, and about 23 — against Medium’s 32. The old Hard and Expert were harder mainly because more than a quarter of their boards needed more than any level here teaches.
Start on Easy if sudoku is new to you, and on Medium if you know it; the page opens on Medium. Easy gives 40 digits and Medium 32, and both fall to singles alone. Hard and Expert add the two moves above, on boards with fewer digits to start from.
A digit comes out only while the level’s steps — singles on Easy and Medium, plus locked candidates on Hard, plus naked pairs on Expert — still fill the grid, and each step only strikes digits that cannot go where they are struck. We re-counted all 160 new boards with a separate counter written for the purpose: every one had exactly one answer, the stored one.
What do the buttons do?
Select a square, then a digit, from the pad under the grid or the keyboard. Selecting a square highlights its row, its column and its box, and every copy of its digit. A wrong digit is marked the moment it goes in, and so is one that repeats a digit already in its row, column or box. Under the grid:
- Undo takes back the last change to a square, its digit or its notes, though not the notes a correct digit cleared elsewhere, and not the Mistakes count.
- Redo puts back what Undo took.
- Erase clears the selected square, and pressing the same digit twice does the same.
- Notes turns pencil marks on and off. A correct digit clears itself from the notes of its row, its column and its box; a wrong one leaves the other squares’ notes alone.
- Start Notes fills every empty square with the digits that still fit it, replacing any notes there; one Undo takes them all back.
- Hint fills one square with its answer, picked at random rather than the square you could work out next; Undo can take it back.
- Solve asks first, then fills the grid and stops the timer.
- New Game deals another board at the level you are on; the level buttons deal one at theirs.
From a keyboard: 1–9, the arrow keys, Backspace or Delete, Ctrl+Z to undo and Ctrl+Y or Ctrl+Shift+Z to redo (Cmd on a Mac); other shortcuts, such as Ctrl+1, are left to the browser. The timer starts the moment a board is dealt and runs while the page is open, even in a background tab, and there is no limit on mistakes: the count only counts. The board is saved in this browser after every move, so it is waiting when you come back.
Where to go next
The daily puzzle is the same game, one board a day for everyone. For paper there are printable puzzles, and for a variant, Killer Sudoku adds sums and Jigsaw Sudoku bends the boxes. Every page takes the theme you last chose with the button.