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 16×16 Sudoku?
16×16 Sudoku is sudoku on the biggest board most solvers meet: 256 squares, the digits 1 to 16, and sixteen 4×4 boxes. Fill the grid so that every row, every column and every box holds each digit once. Nothing in the rules is new; what changes is how much each square has to answer to.
- There are 256 squares to fill, against 81 on a classic board.
- Each square shares a row, column or box with 39 others: 15 in its row, 15 in its column and 9 more in its box. On a 9×9 it is 20.
Every technique you know from 9×9 works here unchanged; each square just has about twice as many neighbours to check.
Each of 10 to 16 fills one square, like any other digit. From a keyboard, type 0 for 10 and the letters A to F for 11 to 16. That is not hexadecimal, where A would be 10: here A is 11, so glance at the square whenever you type a letter.
What does the 4×4 box change?
A box is crossed by four rows and four columns, and each of those eight 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. A naked single takes more finding on a board this size, because a square has to see fifteen different digits among its 39 neighbours before only one is left.
What do you do when singles run out?
On Hard and Expert they do: singles stalled on every one of our 40 sample boards at each level. The first step past them 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: 79 of the 80 had more than one answer, and on the 80th 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 promised X-Wing on Hard; what Hard really needed was a guess. Dealing was slow as well: one Easy board in our sample took 2 minutes 37 seconds to appear.
The generator now uses the same engine as the rest of the site, and a board is dealt 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 120 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 | 120 | singles | 100% | 0% |
| Medium | 86–98 | singles | 100% | 0% |
| Hard | 88–99 | singles and locked candidates | 0% | 0% |
| Expert | 89–98 | singles, locked candidates and naked pairs | 0% | 83% |
40 boards per level from this page’s generator, fixed seed; shares rounded. 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 94 givens. What changes is the hardest step you need: Medium never needs more than singles; on all 40 Hard boards singles stalled and locked candidates got past; and on 33 of the 40 Expert boards you need 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 counter written for the purpose: every one had exactly one answer, and it was the one the page stored.
Which level should you play?
Start on Easy, even if 9×9 is easy for you. At 120 givens nearly half the board is filled in, and the skill to learn is scanning sixteen digits across a board this size, with nothing harder than singles. Medium is the same skill on a much emptier board, about 94 givens, and still never beyond singles.
Hard is for solvers who know locked candidates: expect singles to stall, and locked candidates always get you past. Expert adds naked pairs, and most boards need one. No board on this page needs more than that, and none ever needs a guess.
A solving order that works here
- Open with the fullest boxes. A box that starts half full or more is usually where the first placements are.
- 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 four boxes. The four boxes side by side in one horizontal strip share its four rows, so a digit placed in three of them is pinned to one row of the fourth.
- Write notes where you are working. Sixteen marks a square is a lot to keep up to date; mark the box or line in front of you, not the whole board.
- Re-check a surprising move. On a board this long, a surprise is more often an earlier slip than a clever pattern.
What do the buttons do?
Select a square, then a digit from 1 to 16; 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 F for 11 to 16; 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
12×12 Sudoku is the step down, with boxes three rows tall and four columns wide, and Samurai Sudoku is another long solve, five 9×9 grids that share corners. If you have a 16×16 from a book or a newspaper, the 16×16 solver takes it typed in or pasted and checks whether it has one answer. For a 16×16 on paper, there are PDFs at four levels on our printable 16×16 page, and more in the printable sudoku collection.