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 Killer Sudoku?
Killer Sudoku is Sudoku with the given digits taken away and replaced by sums. The grid is cut into cages, each outlined and printed with a total in its top-left corner, and you fill it so that:
- every row, column and 3×3 box holds 1 to 9 exactly once, as in classic Sudoku;
- the digits in each cage add up to its total;
- no digit appears twice in the same cage.
It also goes by sumdoku, sum doku and addoku and, in Japan where Tetsuya Nishio devised it from an idea of his student Miyuki Nisawa, samunanpure, “sum number place”. English readers met it in The Times in 2005. Our rules of Killer Sudoku guide covers the rules and their edge cases in full.
One thing about this page’s puzzles is worth knowing before you start. Traditional killer cages are two cells or more, but our generator also leaves some one-cell cages, whose total is simply the digit that goes there. They are free digits: type them in first.
How does a cage total give its digits away?
Different digits can only reach a total in a limited number of ways. Three cells can make 24 only as 7, 8 and 9, because anything smaller than 7 leaves the other two needing more than 9 and 8 can give. Three cells making 12 have seven ways to do it. The job at the start of every killer is telling the first kind of cage from the second.
That is why a cage lying inside a single box, row or column is worth checking first: whatever digits it must hold, the rest of that house cannot. The cages that pin their digits outright are rare but easy to learn, because they are always the extremes:
| Cage size | Totals with only one set | The set |
|---|---|---|
| 2 cells | 3, 4 · 16, 17 | 1 2 · 1 3 · 7 9 · 8 9 |
| 3 cells | 6, 7 · 23, 24 | 1 2 3 · 1 2 4 · 6 8 9 · 7 8 9 |
| 4 cells | 10, 11 · 29, 30 | 1 2 3 4 · 1 2 3 5 · 5 7 8 9 · 6 7 8 9 |
| 5 cells | 15, 16 · 34, 35 | 1 2 3 4 5 · 1 2 3 4 6 · 4 6 7 8 9 · 5 6 7 8 9 |
For each size, the two smallest and two largest possible totals are the only ones with a single set of different digits; every total in between allows at least two. Cages here never exceed five cells.
On this page’s boards, about one cage in nine of two or more cells is like that on Easy, falling to about one in fifteen on Hard and Expert. Every other total leaves a choice, and most of the solving is narrowing it; our Killer Sudoku combinations guide lists every total for every cage size.
What is the rule of 45?
The digits 1 to 9 add up to 45, so every row, column and box does too. Add the totals of the cages lying wholly inside one of them, take that from 45, and what is left belongs to the cells of the house that sit in cages crossing its edge. When that is a single cell, you have its digit.
The same works the other way round — cells of crossing cages that sit outside the house make up the excess — and over two or three houses at once, with 90 or 135 in place of 45. The rule of 45 guide works through both. On this page the rule is not decoration. We tried finishing 300 boards per level with it switched off: 4% of Medium boards, 29% of Hard and 41% of Expert could not be done without it, and on 11% of Expert boards only the version spanning several rows, columns or boxes would do.
An innie is a cell inside a row, column or box whose cage reaches out of it; an outie is a cell outside it whose cage reaches in. Once you know the totals of the cages lying wholly inside, the rule of 45 gives you the innies’ total, and the outies’ total, even when you know nothing else about those cells.
What do the Easy, Medium, Hard and Expert levels change?
The levels set how big the cages may be: one or two cells on Easy, two to three on Medium, up to four on Hard and up to five on Expert. Fewer, larger cages mean fewer totals that give anything away, and the rest follows from that:
| Level | Cage sizes | Cages per board | One-cell cages per board | Singles and cage totals alone finish | Needs the rule of 45 | Beyond our solver |
|---|---|---|---|---|---|---|
| Easy | 1–2 | 58 | 36 | 100% | 0% | 0% |
| Medium | 2–3 | 37 | 7 | 58% | 4% | 0% |
| Hard | 2–4 | 33 | 6 | 19% | 29% | 2% |
| Expert | 2–5 | 29 | 6 | 5% | 41% | 11% |
300 boards per level from this page’s generator, fixed random seed, averages rounded. “Needs the rule of 45” means our solver could finish the board with the rule and could not with it switched off. “Beyond our solver” boards still have exactly one answer; finishing them by hand takes longer chains than its techniques include.
Easy is barely a killer puzzle. Around 36 of its 58 cages are single cells, so 36 of the 81 digits arrive free and the rest come in pairs — a gentle way to learn how cages read. From Medium the cages grow, and the one-cell cages settle at six or seven a board, a few free digits to start from. Hard and Expert are where the rule of 45 earns its place. The singles in the table are the naked and hidden singles of ordinary Sudoku, and the techniques between singles and the rule of 45 are locked candidates and pairs, including a cage’s digits locking into one row or box as in the first picture.
Does every puzzle have exactly one answer?
Yes, and it is proved before the puzzle reaches you. The generator fills a grid, draws cages over it, and runs a solver that counts every possible completion. If it finds a second answer, or cannot settle the question quickly, that set of cages is thrown away and another is drawn.
We re-checked 1,200 boards, 300 per level, with a separate solver, plus every daily killer puzzle since the first on 11 April 2026 — 162 of them. All have exactly one answer. On our test machine most Expert boards were built and proved in about a fiftieth of a second, and none of the 300 took more than a sixth.
That matters more here than on most pages, because the game marks a digit red the moment it differs from the solution. With a single answer, red always means a real mistake, never a correct deduction that happens to differ from the stored grid.
Is Killer Sudoku harder than regular Sudoku?
It depends on the cages far more than the name suggests. Our Easy boards hand you about 36 digits as one-cell cages and never need more than singles and cage totals, which makes them a gentle place to start. Our Expert boards are another matter: two in five need the rule of 45 and one in nine goes beyond every technique our solver has. What stays constant is the kind of thinking. Classic Sudoku asks where a digit can go; killer adds how a total can be made, so you do a little arithmetic before the logic starts.
If you want to check a puzzle from a book or newspaper, our Killer Sudoku solver takes the cages and totals and tells you whether they have one answer.
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, column and box and of the rest of its cage; a wrong one clears nothing.
- Erase clears the selected cell; pressing its digit a second time does the same. Every cell is yours to fill, one-cell cages included.
- Undo takes back the last change to a cell, but not the notes a correct digit cleared elsewhere, and not the Mistakes count.
- Hint fills one empty or wrong cell with its answer, chosen at random rather than the next cell you could work out.
- Solve asks first, then shows the whole solution and stops the timer.
- New Game deals another board at the current level; the level buttons deal one at theirs.
Digits 1–9, the arrow keys, Backspace or Delete, and Ctrl+Z work from a keyboard. Your board is saved in the browser after every move, so it is waiting when you come back, timer included up to your last move. Each cage is tinted a different colour from the cages touching it, and selecting a cell also highlights the rest of its cage.
Daily and printable Killer Sudoku
The daily Killer Sudoku gives everyone the same board each day, from Easy on Mondays and Tuesdays to Expert at the weekend, built by this same generator. For paper, each level comes as a free PDF with answers: Easy, Medium, Hard and Expert, all described on the printable Killer Sudoku page. When you want a plan of attack for the harder levels, the Killer Sudoku tips and strategies guide puts the techniques in order.