Math sudoku variants (math is the US spelling, maths the Commonwealth one) add a sum to the classic rules. Digits still obey rows, columns and boxes, but a clue now says what some group of digits must add up to. Killer Sudoku, also sold as Sum Sudoku, is the main example and the only one Sudoku69 hosts. This page is for players who know classic sudoku and want to see where Killer sits among its relatives and how hard it really is.
Every variant below keeps the sudoku grid except Kakuro, which drops it. They differ in where the sum clue sits and whether a digit may repeat inside the summed group.
| Variant | Sum clue | Repeats inside the group | On Sudoku69 |
|---|---|---|---|
| Killer (Sum Sudoku, Sumdoku) | Outlined cage with a total in its corner | No | Yes |
| Little Killer | Arrow outside the grid, sum of a whole diagonal | Yes | No |
| Sandwich | Number outside a row or column, sum of the digits between the 1 and the 9 | Not applicable | No |
| Arrow | Circle cell equals the sum of the digits along its arrow | Yes, if the cells share no house | No |
| Kakuro | Sum of each horizontal or vertical run of white cells | No, within one run | No |
Two points explain most of the differences. In Killer a cage is a small extra unit: its digits are distinct, so the usual subset logic works inside it. In Little Killer and Arrow the summed cells can repeat a digit, so you lose that shortcut and lean on sum limits instead. Kakuro has no rows, columns or boxes in the sudoku sense. A digit may appear twice in one row if the two cells belong to different runs, so it plays like a pure sum puzzle.
Sum Sudoku is just another name for Killer, not a wider family. Kakuro is a separate puzzle rather than a sudoku with sums bolted on, so we do not group it with Killer as a hybrid. The shared skill is combinations: learning which digit sets can make a given total, which carries over from Killer to Kakuro and helps in the others.
The six levels use the same rating bands as classic, so Easy, Medium, Hard, Expert, Master and Legendary mean the same thing in every mode. The bands and what each demands are laid out in sudoku difficulty levels. Killer adds one thing you can feel from the start: cage arithmetic does some of the work that patterns do in classic.
The key fact for planning your play: Expert, Master and Legendary Killer puzzles have no given digits at all. The cages alone fix the solution, so there is no safe digit to start from. Easy to Hard keep some givens, which is why the first Killer puzzles you meet are gentler than the grid suggests.
Killer needs nine techniques that classic never uses. They sit in our technique ladder in the order below, with the project rating each carries. The first three are basic-tier and the other six are tough-tier. Innies and outies is the one to learn first, because the 45 rule it builds on is the first tool most Killer solvers reach for. For the cage-sum lookup that feeds all of them, keep the combinations chart open.
| Technique | Tier | Project rating |
|---|---|---|
| Killer Sudoku Cage Combinations | Basic | 1.5 |
| Innies and Outies | Basic | 2.0 |
| Captured Candidates | Basic | 2.7 |
| Hard Killer Combinations | Tough | 2.5 |
| Cage and Unit Overlap | Tough | 2.7 |
| Cage Splitting | Tough | 3.2 |
| Multi-Cell Innies and Outies | Tough | 3.4 |
| Cage Compare | Tough | 3.8 |
| Rule of Parity | Tough | 4.0 |
The ratings are our own killer ladder, not Sudoku Explainer rules, so read them as an order of difficulty. Tier follows the ladder, not the number alone: Hard Killer Combinations (2.5) is filed as tough although it rates below Captured Candidates (2.7), which is basic. Each technique has its own article with a worked example; pick the next one when your current set stops producing moves.
By rating, not much at the typical level and noticeably at the top. Our averages across the same six bands:
| Measure | Classic 9x9 | Killer 9x9 |
|---|---|---|
| Easy average rating | 1.26 | 1.40 |
| Legendary average rating | 10.26 | 10.43 |
| Hardest puzzle in the catalogue | 11.1 | 12.8 |
| Givens at Expert, Master, Legendary | 21 to 29, 19 to 28, 20 to 31 | none |
A Killer puzzle of a given level asks for a similar depth of reasoning as a classic one, with a slightly higher average. The extreme end is where they part: our hardest Killer is rated 12.8 against 11.1 for classic. What changes more is the feel. Classic rewards scanning for patterns across the grid. Killer rewards counting: sums, combination sets and the 45 rule, first inside one cage, then across several.
So the answer depends on your habits. If you are strong at patterns but dislike arithmetic, expect Killer to feel slower at first. If you enjoy sums, the cages give you a new kind of foothold. Start from Easy, learn the rules in how to play Killer Sudoku, and move up once cage combinations feel automatic.
Killer is a full mode on Sudoku69 with all six levels, notes and cage outlines. The other variants in the table are not available here.