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 25×25 Sudoku?
25×25 Sudoku is sudoku on the largest board on this site: 625 squares, the digits 1 to 25, and twenty-five 5×5 boxes. Fill the grid so that every row, every column and every box holds each digit once. The rules are the ones you know from 9×9; what grows is the bookkeeping.
- There are 625 squares, more than seven and a half 9×9 grids, and 75 rows, columns and boxes to keep track of, against 27.
- Each square shares a row, column or box with 64 others: 24 in its row, 24 in its column and 16 more in its box. On a 9×9 it is 20.
Twenty-five is odd, and so is five, so unlike 16×16 this board has a true middle: a centre box, a centre row and column, and a centre square in every box. Over a long solve, “two boxes left of the centre” is an easier landmark than a count of squares from the edge.
Each of 10 to 25 fills one square, like any other digit, so sixteen of the twenty-five digits are two characters wide. From a keyboard, type 0 for 10 and the letters A to O for 11 to 25: A is 11, E is 15, J is 20 and O is 25. That is not hexadecimal, where A would be 10.
What does the 5×5 box change?
A box is crossed by five rows and five columns, and each of those ten lines can rule a digit out of part of it. Pick a digit, find it in the rows and columns through a box, and see which of the box’s empty squares are left. When only one is, that square is the digit, whatever else its pencil marks say:
That is a hidden single, and on Easy and Medium boards here it is all you need, together with naked singles: every one of those boards is dug so that singles alone finish it. Work it the way the diagram does, one digit at a time. With twenty-five digits you cannot hold a box’s missing list in your head, and a half-remembered list is where mistakes start.
What do you do when singles run out?
On Hard and Expert they run out early. On the Hard board below, singles placed 12 digits and stopped with 347 squares still open, and they stalled on every one of our 40 sample boards at both levels. The next step is locked candidates: when a digit’s places in a box all lie in one row or one column, the box’s copy of that digit is the line’s copy too, so it comes out of the rest of the line.
Expert goes one step further. When singles and locked candidates have both stalled, look for a naked pair: two squares in one row, column or box that can each hold only the same two digits. Those two digits must go in those two squares, so they come out of everything else in that row, column or box.
Were the boards on this page fair?
No. The old generator filled a grid, blanked random squares until the level’s count was reached, and never checked what was left. We dealt 20 boards at each level from it and counted their answers with a separate solver: 67 of the 80 had more than one answer, and on the other 13 our counter ran out of steps before it could finish. Not one could be shown to have exactly one.
That matters more than it sounds. The page marks a digit wrong when it differs from the one answer it stored, so on a board with two answers you could place a digit that breaks no rule and still be charged a mistake. No technique can finish such a board either, since logic cannot choose between two answers that both work: every one of the 20 old Easy boards went beyond everything our solver knows. The old page said these boards came apart with the tools that finish a Medium 9×9; what they really needed was a guess.
The grids were not what they seemed, either. Every one was the same pattern grid with its digits relabelled and its rows and columns shuffled. Looking down the columns of each box, an old board had only five different groups of five digits, repeated in all twenty-five boxes; a new grid has 123 to 125 different groups out of 125. The generator now fills a fresh random grid every time, and deals a board only once a fixed set of steps can finish it, which proves it has one answer. Any level deals in under a second on a desktop computer.
How do the four levels differ?
Every level is dug the same way: digits come out one at a time in a random order, and a removal is kept only while the level’s steps can still finish the board. The levels differ in which steps those are. Easy stops at 325 givens; the other three dig as far as their steps allow. We then ran 40 fresh boards per level through a separate solver that knows one technique at a time.
| Level | Given digits | Dug so these finish it | Singles finish it | Needs a pair or triple |
|---|---|---|---|---|
| Easy | 325 | singles | 100% | 0% |
| Medium | 252–273 | singles | 100% | 0% |
| Hard | 254–271 | singles and locked candidates | 0% | 0% |
| Expert | 254–273 | singles, locked candidates and naked pairs | 0% | 100% |
40 boards per level from this page’s generator, fixed seed. The solver knows naked and hidden singles, locked candidates, naked pairs and triples, and the X-Wing, and it finished all 160 boards.
Harder does not mean fewer digits here. Medium, Hard and Expert all end at about the same count, a median of 263 or 264 givens. What changes is the hardest step you need, and on this board the levels separate cleanly: Medium never needs more than singles; all 40 Hard boards needed locked candidates; and all 40 Expert boards needed a naked pair or triple as well.
No search is involved. A board is dealt only if its level’s steps, applied until they stop, fill every square, and each of those steps only ever rules out a digit that cannot go. A board they finish therefore has exactly one answer. We re-solved 160 fresh boards with a separate solver written for the purpose, one that also uses only sound steps: it finished every one, and each time its answer was the one the page stored.
Which level should you play?
Start on Easy, even if 16×16 is easy for you. At 325 givens just over half the board is filled in, and the skill to learn is keeping your place across 75 rows, columns and boxes, with nothing harder than singles. Medium is the same skill on an emptier board, about 264 givens, and still never beyond singles.
Hard is for solvers who know locked candidates, and Expert adds naked pairs; in our samples every board at those levels needed them. No board on this page needs more than that, and none ever needs a guess. The board saves after every move, so a 25×25 can take as many sittings as you like: stop at a finished box or band, not halfway through a chain of reasoning you will not remember.
A solving order that works here
- Mark selectively. Writing all 25 candidates into every empty square before your first placement is hours of work and an unreadable board. Note candidates in a box or line that is already more than half full, where an elimination will resolve something, and leave the empty regions for later.
- Sweep one digit at a time. Select a square holding that digit and every copy of it on the board lights up, which shows at a glance which rows, columns and boxes still need it.
- Work a band of five boxes. The five boxes side by side in one horizontal strip share its five rows, so a digit placed in four of them is pinned to one row of the fifth.
- Place a digit only when you can say why. “the 14 goes here because a 14 in its row or column covers every other square of this box” is a reason; “nothing else seems to fit” is how a wrong digit gets in.
- Distrust a surprise. The page marks a wrong digit, but not a wrong note: on a board this long, a move that looks too easy more often comes from a mark struck by mistake than from a clever pattern.
What do the buttons do?
Select a square, then a digit from 1 to 25; the page opens on Easy. Selecting a square highlights its row, column and box, 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 and its box; 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, then 0 for 10 and A to O for 11 to 25; 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
16×16 Sudoku is the step down, with 4×4 boxes on 256 squares, and 12×12 Sudoku is smaller again, with boxes three rows tall and four columns wide. Samurai Sudoku is another long solve: five 9×9 grids that share their corners. For a 25×25 on paper, there are PDFs at four levels on our printable 25×25 page, and more in the printable sudoku collection.