Killer Sudoku
Fill the grid so every row, column, and 3x3 box contains 1 through 9. Digits inside each cage must sum to the target number, and no digit repeats within a cage.
Killer Sudoku
20–30 givens. Small cages (2–3 cells). Learn cage sums.
Standard play. Timer runs. Hints available.
What is killer sudoku?
Killer sudoku (sometimes called sumdoku or sum sudoku) combines the placement rules of standard sudoku with arithmetic. The grid is still 9x9. Every row, column, and 3x3 box still needs digits 1 through 9 with no repeats. The difference is cages: groups of cells outlined with dashed borders, each labeled with a target sum. The digits you place inside a cage have to add up to that target, and they can't repeat within the cage.
Most killer sudoku puzzles give you zero pre-filled digits. The cages and their sums are your only starting information. That sounds harder than regular sudoku, and it is — but the cage constraints also give you more to work with. A 2-cell cage with sum 3 can only be 1+2. A 3-cell cage summing to 24 can only be 7+8+9. These locked combinations are free placements that standard sudoku never offers.
If you already know standard sudoku, the transition is straightforward: you're adding one constraint (cage sums) and gaining one tool (combination elimination). The rules page walks through everything step by step. If you want to jump straight in, pick a difficulty above and go.
How to play
Tap a cell to select it, then tap a number to place it. Dashed outlines show the cage boundaries, and the small number in the top-left corner of each cage is the target sum. When you select a cell, its entire cage highlights so you can see which cells belong together.
Use notes mode to jot down candidate digits when you're working through combinations. The auto-candidates feature is cage-aware — it excludes digits that would break the cage sum or repeat within the cage. If you get stuck, the hint system will point you toward a cell you can solve right now, with an explanation of the technique.
Play modes
Classic
Timer runs up. Up to 3 hints. Undo available. The default way to play.
Timed Trial
Beat the countdown. Time limit scales with difficulty: 15 min for easy, 6 min for einstein.
Challenge
No hints. No undo. Every digit placement is permanent.
How to solve killer sudoku
Technique by technique, beginner to advanced.
Start with small cages
Two-cell cages have the fewest valid combinations, so they're the easiest to fill. A sum of 3 is always 1+2. A sum of 4 is always 1+3. A sum of 16 is always 7+9. A sum of 17 is always 8+9. Memorizing the forced 2-cell combinations (sums 3, 4, 16, 17) gives you instant placements at any difficulty.
Three-cell cages work similarly when the sum is extreme. A sum of 6 is always 1+2+3. A sum of 24 is always 7+8+9. For mid-range sums, you'll have multiple valid combinations, but you can often narrow them down using row and column constraints. Check the combinations reference when you need a quick lookup.
Cage-sum elimination
When only one cell in a cage is empty, you can calculate its value by subtracting the filled digits from the cage sum. If a 3-cell cage has sum 15 and two of the cells contain 3 and 5, the third cell must be 7. This works regardless of what the row and column constraints say.
Even when multiple cells are empty, listing the valid combinations and crossing out those that conflict with existing digits in the row, column, or box narrows your options fast. If a cage has two empty cells and only one combination survives the elimination, both cells are solved.
The 45 rule
Every row sums to 45. So does every column and every 3x3 box. This is the single most useful technique in killer sudoku, especially from hard difficulty onward.
When most of a row's cages sit entirely within that row, add up their target sums. Subtract from 45. The difference equals the combined value of any cage cells that poke out of the row. If only one cell pokes out (or in), you get its exact value. These cells that extend outside a unit are called outies, and cells of external cages that extend into a unit are innies.
Example: row 1 contains three cages with sums 12, 14, and 10, all fully within the row, plus one cage with sum 15 that has two cells in row 1 and one in row 2. The three full cages sum to 36. The two row-1 cells of the last cage must sum to 45 - 36 = 9. Since the cage total is 15 and the row-1 portion is 9, the row-2 cell is 15 - 9 = 6.
Standard sudoku techniques
All the standard sudoku methods carry over. Naked singles, hidden singles, pointing pairs, box/line reduction, X-Wings. The cage constraints don't replace standard methods — they work alongside them. You'll often alternate between cage-based reasoning and position-based reasoning within the same puzzle.
Naked singles and hidden singles are especially productive after you've done a round of cage combination elimination. Removing candidates via cage constraints frequently creates singles elsewhere on the grid. Always re-scan after any cage-based elimination.
Working with cage combinations
For each cage, write out all valid combinations that produce the target sum without repeating digits. Then eliminate any combination that includes a digit already placed in the same row, column, or box as a cage cell.
If all surviving combinations contain a particular digit, that digit is guaranteed to appear in the cage. If they all place it in the same cell, that cell is solved. This is one of the most productive mid-game techniques, especially for 4- and 5-cell cages where the combination count starts high but drops quickly after a few eliminations.
Cage interaction chains
When two cages share cells in the same row, column, or box, their digit sets interact. If cage A must contain 1, 2, and 3 in row 5, those digits are off-limits for any other cage in that row. If cage B also sits in row 5 and its only valid combination requires a 2, you can eliminate all combinations of cage A that don't include 2, because cage B must have it.
These interactions chain. Narrowing cage A's combinations might limit cage B, which limits cage C, which solves a cell in cage A. Expert and Einstein puzzles rely on these chains. They're the killer sudoku equivalent of chain deductions in standard sudoku.
Common mistakes
Forgetting the no-repeat-in-cage rule. Standard sudoku allows the same digit in two cells of the same cage (as long as they're not in the same row, column, or box). Killer sudoku does not. A 3-cell cage with sum 9 cannot be 1+4+4 even if the 4s are in different boxes.
Ignoring the 45 rule. Many players try to power through hard puzzles using only cage combinations and standard techniques. It works sometimes, but the 45 rule provides information that no other technique can reach. It's worth checking every row and box for innies/outies before declaring yourself stuck.
Guessing. Every puzzle we publish has a unique logical solution. If you feel like you need to guess, re-scan the grid. There's a deduction waiting — you just haven't found it yet.