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 NRC Sudoku?
NRC Sudoku is sudoku with four extra boxes. Fill the 9×9 grid so that every row, every column and every 3×3 box holds 1 to 9 once — and so do the four shaded windows, 3×3 squares set one square in from each corner.
- The windows cover rows 2–4 and 6–8, crossed with columns 2–4 and 6–8.
- Each window straddles four boxes, and no two windows touch.
- That makes 31 groups that must each hold 1 to 9, against 27 in classic sudoku.
The same puzzle is also published as Hyper Sudoku and as Windoku, and our Hyper Sudoku page plays it too and walks through the two window moves on a real board: a digit a window places, and a window that confines a digit to one row. This page is about the name, and about what the four windows force without saying so.
The Dutch puzzle maker Peter Ritmeester devised the variant, with its four extra grey boxes, in 2005 and offered it to the Dutch daily NRC Handelsblad, where the first Saturday sudoku appeared on 8 October 2005. When he and Will Shortz made a book of sudoku variants that same year, they needed an English name for “the NRC variant”: Ritmeester suggested Sudoku Extra, Shortz suggested Hyper Sudoku, and Hyper Sudoku won. In the Netherlands it kept the newspaper’s name.
What do the windows force?
Most of the information on a hard board. Keep the same givens but ignore the windows, and a board usually stops having one answer: 55% of our Easy boards had more than one, and so did every Medium board but one and every Hard and Expert board. Remove the windows from our solver instead and it stalls on 22 of 40 Easy boards and on every Medium one.
They also force groups that no rule mentions. Take rows 2 to 4: between them they hold every digit three times. The two windows in those rows hold every digit once each, so twice between them. That leaves each digit exactly once for the rest of those three rows — the nine squares in columns 1, 5 and 9. Those nine behave like a tenth box:
The same count works for rows 6 to 8, and turned sideways for columns 2 to 4 and columns 6 to 8, which gives four such groups. What is left of rows 1, 5 and 9 — the four corners, the middle square of each edge and the centre — makes a fifth. So an NRC grid has 36 groups that hold 1 to 9, not 31. We call the five extra ones hidden regions. The page does not shade them, so you have to remember where they are.
How do you use a hidden region?
The way you use a box: find where a digit can still go inside it. The givens that decide it come from all over the grid, through rows, columns, boxes and windows, which is why the move is easy to miss:
That is a hidden single, and on this page it is not a rare one. At first look, before anything is placed, a hidden region offered at least one single that nothing else on the board gave on 37 of our 40 Easy boards, 26 of 40 Hard boards and 19 of 40 Expert boards.
Were the boards on this page fair?
Yes, but for a long time they were slow. Even before the rewrite, the generator took a digit out only when a full search proved the board still had one answer, so every board has had exactly one. The trouble was that search: it tried all nine digits in every empty square, and measured on the identical Hyper Sudoku generator, a board took a median of 1.6 to 16 seconds to appear depending on the level, and up to 30, with the page frozen while it worked.
The fix in September 2026, shared with Hyper Sudoku, brought that down to a few milliseconds. The search now stops after 4,000 steps, and a digit it cannot settle in that time stays on the board, so the cap can only ever keep a clue.
How do the four levels differ?
By the number of given digits, which the generator hits exactly: 38, 30, 25 and 21. We ran 40 fresh boards per level through a solver that knows one technique at a time, with the hidden regions and without:
| Level | Given digits | Singles alone finish it | With the hidden regions | Beyond our solver |
|---|---|---|---|---|
| Easy | 38 | 100% | 100% | 0% |
| Medium | 30 | 100% | 100% | 0% |
| Hard | 25 | 80% | 88% | 8% |
| Expert | 21 | 30% | 50% | 43% |
40 boards per level from this page’s generator, fixed seed; shares rounded. “With the hidden regions” is singles alone with the five hidden regions counted as groups. The solver knows singles, locked candidates, naked pairs and triples and the X-Wing; given the hidden regions too, it finishes three more Expert boards, leaving 35% beyond it.
The hidden regions matter most where the board is hardest. On Easy and Medium, singles finish every board either way. On Hard they lift singles from 32 of our 40 boards to 35, and on Expert from 12 to 20. Every board, on every level, still has exactly one answer; “beyond our solver” means it needs a technique the solver does not know, not a guess.
A digit comes out of the grid only while a search proves the board still has one answer. We re-counted 160 fresh boards with a separate counter written for the purpose: every one had exactly one answer, and every level hit its number of givens exactly.
Which level should you play?
Start on Easy if windows are new to you. Singles finish every Easy and Medium board, and nearly every Easy board has a hidden-region single waiting at first look, so it is the place to get used to seeing those five extra groups. The page opens on Medium.
Hard is where singles stop being enough: 8 of our 40 Hard boards needed more, 3 of them more than our solver knows. Expert is for solvers who know pairs, triples and beyond: even with the hidden regions, a third of our Expert boards needed something our solver does not have.
A solving order that works here
- Scan the windows first. They are shaded, and a digit placed inside one rules itself out of a row, a column, a box and a window at once.
- Then the hidden regions. Columns 1, 5 and 9 across rows 2–4, and again across rows 6–8; rows 1, 5 and 9 across columns 2–4, and again across columns 6–8; and the nine squares where rows 1, 5 and 9 meet columns 1, 5 and 9.
- Sweep one digit at a time. Select a square holding that digit and every copy of it lights up, which shows at a glance the rows, columns, boxes and windows that still need it.
- Use the overlaps. A window shares squares with four boxes. If a box can put a digit only in the squares it shares with a window, the rest of that window cannot have it, and the same works the other way round: locked candidates, with a window as one of the two groups.
- Re-check a surprising move. With 36 groups in play, a surprise is more often a slip than a clever pattern.
What do the buttons do?
Select a square, then a digit; the page opens on Medium. Selecting a square highlights its row, its column, its box and, if it has one, its window, and a wrong digit is marked the moment it goes in. Under the grid:
- Notes turns pencil marks on and off. A correct digit clears itself from the notes of its row, its column, its box and its window; a wrong one leaves the other squares’ notes alone.
- Erase clears the selected square, and pressing the same digit twice does the same.
- Undo takes back the last change to a square, though not the notes a correct digit cleared elsewhere, and not the Mistakes count.
- Hint fills one square with its answer, picked at random rather than the square 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.
From a keyboard: 1–9, the arrow keys, Backspace or Delete and Ctrl+Z. The board is saved in this browser after every move, so it is waiting when you come back.
Where to go next
X Sudoku adds the two long diagonals instead of windows, and Jigsaw Sudoku replaces the boxes altogether.