Windoku, also called Hyper Sudoku, is a free online Sudoku variant with four extra shaded 3×3 windows. As in ordinary Sudoku, every row, every column and every 3×3 box must hold the digits 1 to 9 exactly once, and each of the four windows must hold them once too. The extra boxes add constraints, so a Windoku board can start with far fewer given digits and still have only one answer. Tap a square and pick a digit, or type it. Pencil in notes, undo, and use up to three hints that explain their reasoning. Twelve levels go from boards with about 40 givens to sparse grids that need lined-up digits and pairs, and every board is generated, has exactly one answer and is solved by logic alone before you see it. No download, no account.
How to play Windoku
- The board is a 9×9 Sudoku grid split into nine 3×3 boxes by the heavy lines. Four more 3×3 areas are shaded blue inside dark frames: these are the windows. The digits in black are given and cannot be changed.
- Fill every empty square with a digit from 1 to 9 so that every row, every column, every box and every shaded window holds each digit exactly once.
- Tap a square to select it, then tap a digit on the pad, or type it on a keyboard. The arrow keys move the cursor, and Backspace, 0 or the ⌫ key clears a square. Tapping the digit a square already holds takes it off again.
- Turn on Notes (or press Space or N) to pencil small candidate digits into a square instead. Writing a digit clears it from the notes of every square in the same row, column, box and window.
- A digit that appears twice in a row, column, box or window turns red, so a clash shows at once.
- The puzzle is solved the moment every square is filled and nothing clashes. Undo steps back, Clear removes everything you added, and New Puzzle deals another board on the same level.
- The ? button fills one square and says why: a wrong digit is put right first, otherwise it fills the next square that can be worked out from the board, naming the row, column, box or window that forces it. Three per board.
Tips
- Scan the windows first. A window overlaps four boxes, two row bands and two column bands at once, so a digit that already sits in nearby rows and columns often has only one square left in a window long before it is pinned in a box.
- Every square inside a window sees more of the board than a square outside: its row, column, box and window. So they tend to have fewer candidates left, which makes them good starting points on the sparse late levels.
- Advanced trick: the windows create five more hidden groups that are not drawn. Rows 1, 5 and 9 inside columns 2 to 4 hold 1 to 9 once (columns 2 to 4 hold each digit three times, and the two windows in them account for two), and the same goes for those rows inside columns 6 to 8, for columns 1, 5 and 9 inside rows 2 to 4 and 6 to 8, and for the nine squares where rows 1, 5, 9 meet columns 1, 5, 9. No board here needs them, but they can shortcut a hard spot.
- From level 8, use notes. When a digit's only places in a window all lie in one row, it cannot go anywhere else in that row outside the window — the same "lined up" step you use with boxes works with every pair of crossing groups.
- On levels 11 and 12 look for pairs: two squares in one row, column, box or window that can only take the same two digits. Those digits belong to those two squares, so cross them off everywhere else in that group.
FAQ
Is Windoku free to play?
Yes. It runs in the browser with no download, no sign-up and nothing to buy. All twelve levels are included and unlock as you solve them, and New Puzzle deals more boards on any level you have reached.
What is Windoku?
Windoku is a Sudoku variant, also known as Hyper Sudoku. The usual rules apply — every row, column and 3×3 box holds 1 to 9 once — and four extra shaded 3×3 windows, covering rows 2 to 4 and 6 to 8 crossed with columns 2 to 4 and 6 to 8, must hold 1 to 9 once as well.
How is Windoku different from normal Sudoku?
It adds four more groups that must each contain 1 to 9, so 36 of the 81 squares belong to four groups instead of three. The extra rules mean a board can start with fewer given digits and still have one answer: the sparsest "only place" boards here start with about 20 givens, and the same givens read as an ordinary Sudoku have more than one answer.
Does every puzzle have exactly one answer?
Yes, and it is checked rather than assumed. Each board is built from a complete grid, and a solver that only makes forced deductions — never a guess — must finish it before it is shown, which means there is only one answer. The tests also count every possible answer by brute force and find exactly one.
Will I ever have to guess?
No. Levels 1 to 4 need only one step: a digit with just one square left in a row, column, box or window. Levels 5 to 7 also need a square with just one digit left. Levels 8 to 10 need digits that line up where two groups cross, and levels 11 and 12 need pairs of squares that share the same two digits.
What are the hidden regions in Windoku?
Because the windows sit where they do, five more groups of nine squares must also hold 1 to 9: rows 1, 5 and 9 inside columns 2 to 4, the same rows inside columns 6 to 8, columns 1, 5 and 9 inside rows 2 to 4 and inside rows 6 to 8, and the nine squares where rows 1, 5, 9 meet columns 1, 5, 9. They follow from the rules rather than being extra rules, and the tests check every answer keeps them. No board here needs them.
Does Windoku work on mobile?
Yes. Tap a square, then a digit on the pad under the board; the board fills the width of a phone held upright. On a computer you can play with the mouse or entirely from the keyboard.