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 Hyper Sudoku?
Hyper Sudoku is ordinary sudoku with four extra regions drawn inside the grid. Fill it so that:
- every row, column and 3×3 box holds 1 to 9 exactly once;
- and so does each of the four shaded windows, the 3×3 squares offset from the boxes.
That makes thirteen regions to satisfy instead of nine, on the same 81 cells. The windows sit at rows 2–4 and 6–8, columns 2–4 and 6–8, so each one straddles four different boxes — which is exactly where its usefulness comes from.
Sudoku logic works on houses: nine cells that must hold 1 to 9 once each. In ordinary sudoku every cell sits in three — its row, its column, its box. The 36 cells inside the windows sit in four. No new technique is needed for the extra one: a window is scanned exactly like a box, and everything you already know about singles and locked candidates applies to it unchanged.
Hyper Sudoku, Windoku, NRC Sudoku: one puzzle, three names
This variant picked up a different name in each market it reached. The Dutch paper NRC Handelsblad started printing it in 2005 and it became NRC Sudoku there; British and American publishers called it Hyper Sudoku; Japanese magazines called it Windoku, for the windows. You will also see Window Sudoku and Four-Box Sudoku. The rules are identical in every case — if you can solve one, you can solve all of them.
This page is the Hyper Sudoku and Windoku one. We keep a separate NRC Sudoku page because that name carries its own history and its own readers in the Netherlands, and the two link to each other rather than competing; the board they deal is the same.
What do the windows actually add?
A window is a house, so it places digits like a box does. Here the row, the column and the box between them leave three possibilities, and the window settles it:
How much they add depends on the level. We counted every board twice, once under hyper rules and once ignoring the windows. On Easy, 48% of boards still have exactly one answer without them — an ordinary sudoku with four shaded squares. From Medium up that changes: the windows are load-bearing on 92% of Medium boards and on every Hard and Expert board we generated. They are also the tightest part of the grid at first look, carrying about half a candidate less per cell than the rest, all the way up the levels.
What is a window-line lock?
Because a window overlaps four boxes, a digit confined inside it can eliminate along a row or column that an ordinary box could never reach. This is the move worth learning:
It works in reverse too: if a row’s last homes for a digit all fall inside one window, the rest of that window loses it. Both directions are ordinary locked candidates — the same technique that gives you pointing pairs in classic sudoku — applied to a region that overlaps the boxes instead of being one.
What do the four levels actually need?
We ran 40 fresh boards per level through a solver that knows one technique at a time, then took techniques away to see which boards stopped:
| Level | Given digits | Singles alone finish it | Needs locked candidates or a subset | Beyond our solver | Boards where the windows are load-bearing |
|---|---|---|---|---|---|
| Easy | 38 | 100% | 0% | 0% | 52% |
| Medium | 30 | 100% | 0% | 0% | 92% |
| Hard | 25 | 80% | 15% | 5% | 100% |
| Expert | 21 | 28% | 40% | 33% | 100% |
40 boards per level from this page’s generator, fixed seed, rounded. Each board counts under the hardest step it needed, from a solver that knows singles, locked candidates, naked pairs and triples, and the X-Wing. The last column is the share of boards that stop having a single answer if you ignore the four windows. “Beyond our solver” boards still have exactly one answer.
Easy and Medium are pure scanning — naked and hidden singles finished every board at both levels, with the windows quietly doing their share as four more regions to scan. Hard brings in locked candidates and naked pairs, and Expert leans on them hard.
The generator takes a finished grid and removes digits one at a time, keeping a removal only while the puzzle still has exactly one answer under hyper rules — and refusing one it cannot settle rather than dealing it anyway. We re-solved 160 fresh boards with a separate solver written for the purpose: every one had exactly one answer, and every level hit its clue count exactly. A board takes about 7 to 20 milliseconds to build, so it is ready before the page has finished loading.
Which level should you play?
Medium is where this variant is at its best. Every Medium board we generated falls to singles alone, yet 92% of them genuinely need the windows to have a single answer — so you are using the extra regions on every board without needing a technique you do not already have.
Hard is the step up: 80% still yield to singles, and the rest want locked candidates. Expert, at 21 given digits, is a different proposition — only 28% fall to singles, 40% need locked candidates or a subset, and a third go past everything our solver knows, which means chains longer than pairs, triples and the X-Wing. Every one of them has exactly one answer and none needs guessing, but if an Expert board feels impossible rather than hard, that is why.
What goes wrong on a Hyper Sudoku
- Placing a digit that is legal in its row, column and box but not its window. This is nearly every hyper mistake, and it happens because the windows are the one region your eye has not spent years learning.
- Treating the shading as decoration. On Hard and Expert it never is: not one board we generated at those levels has a single answer without the windows.
- Forgetting the cells between the windows. The row and column running down the middle of the grid belong to no window at all, so they stay looser for longer — they are usually the last part of the board to fall, not the first.
What do the buttons do?
Select a cell, then a digit from 1 to 9; the page opens on Medium. Under the grid:
- 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 your notes alone.
- Erase clears the selected cell, and pressing the same digit twice does the same.
- Undo takes back the last change to a cell, though not the notes a correct digit cleared elsewhere, and not the Mistakes count.
- Hint fills one cell with its answer, picked at random rather than the cell you could work out next.
- 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.
Digits 1–9, the arrow keys, Backspace or Delete and Ctrl+Z all work from a keyboard. The board is saved in this browser after every move, timer included, so it is waiting when you come back. The four windows are tinted, and selecting a cell highlights its row, column and box so you can see which regions it answers to.
Where to go next
Hyper Sudoku is one of the “add a region” variants, and they all read the same way once you accept the extra house. X-Sudoku adds the two diagonals instead of four windows and is the gentlest of them. Jigsaw Sudoku keeps nine regions but bends them into irregular shapes, so there is no box to fall back on. Samurai Sudoku overlaps five grids at the corners, and Killer Sudoku takes the given digits away and replaces them with sums.