Guide
How to Play Killer Sudoku
Killer Sudoku is sudoku with sum cages. Fill every row, column, and 3×3 box with 1–9 while making the digits in each dotted cage add up to the small number in its corner.
Play Killer SudokuThe objective
Complete the grid so every cage adds up
Killer Sudoku — also known as sumdoku or sudoku killer — is played on the same 9×9 grid as classic sudoku, divided into nine 3×3 boxes. Every row, column, and box must contain the digits 1 to 9 exactly once. What changes is the clues: the grid starts empty, and dotted cages group the cells. The small number in the top-left of a cage is the sum of the digits inside it, and a digit may not repeat within a cage.
Because there are no starting digits, every deduction comes from arithmetic. That makes Killer Sudoku feel like a cross between sudoku and kakuro — and every puzzle on this site has exactly one solution that can be reached by logic alone. When you are ready, open the board.
Step by step
Four steps from empty grid to solved
- 1
Read the grid
The board is a standard 9×9 sudoku with nine 3×3 boxes. Unlike classic sudoku it starts completely empty. Instead, dotted outlines group the cells into cages, and a small number in the corner of each cage tells you what its digits must add up to.
- 2
Fill rows, columns, and boxes
Every row, every column, and every 3×3 box must contain each digit from 1 to 9 exactly once — the same rule as ordinary sudoku. A digit that repeats in any row, column, or box is wrong.
- 3
Make every cage add up
The digits inside a cage must add up to the cage total, and no digit may appear twice in the same cage. A two-cell cage marked 3 can only be 1 and 2; a three-cell cage marked 24 can only be 7, 8, and 9. These forced combinations are how you break in.
- 4
Solve without guessing
Every puzzle here has exactly one solution and can be solved by logic alone. Use pencil notes to track candidates, lean on the 45 rule for the totals of rows, columns, and boxes, and watch the mistake counter — three wrong digits end the run.
Rules
The rules in full
The objective
Fill all 81 cells so every row, column, and 3×3 box holds the digits 1–9 once and every cage adds up to its printed total. The puzzle is solved the moment the last correct digit is placed.
Cage rules
A cage is a group of orthogonally connected cells inside a dotted outline. The small number is the sum of the digits in the cage. A digit can never repeat within a cage, even when two cells of the cage are far apart in the grid. A single-cell cage simply tells you that cell’s digit.
No starting numbers
Killer Sudoku normally gives no pre-filled digits. All the information is in the cage sums, which is why single-cell cages, very small cages, and very high or low totals are the natural first moves.
Mistakes
Each digit is checked against the unique solution as soon as you place it. A wrong digit turns red and costs one of your three mistakes; a third mistake ends the run. Correct digits lock in place. Hints never count as mistakes.
Timer, best times, and saving
The clock runs while you play and pauses when the tab is hidden. Your best time and win streak are stored per difficulty on your device, and an unfinished puzzle is saved automatically so a refresh does not lose it.
Cheat sheet
Cage totals with only one combination
Learning these ten forced combinations is the fastest way to get better. Each set of digits is the only way to reach that total in a cage of that size — the order inside the cage is still up to the rest of the puzzle.
| Cells in cage | Total | Only possible digits |
|---|---|---|
| 2 | 3 | 1 + 2 |
| 2 | 4 | 1 + 3 |
| 2 | 16 | 7 + 9 |
| 2 | 17 | 8 + 9 |
| 3 | 6 | 1 + 2 + 3 |
| 3 | 7 | 1 + 2 + 4 |
| 3 | 23 | 6 + 8 + 9 |
| 3 | 24 | 7 + 8 + 9 |
| 4 | 10 | 1 + 2 + 3 + 4 |
| 4 | 30 | 6 + 7 + 8 + 9 |
Controls
Tap, click, or use the keyboard
Everywhere
Tap or click a cell, then a number
Select a cell, then press a digit on the number pad (or your keyboard) to place it. The row, column, box, and cage of the selected cell are highlighted, and digits already fully placed dim on the pad.
Everywhere
Notes toggle
Switch Notes on, then tap digits to pencil small candidate marks into the selected cell. Placing a correct digit automatically clears that mark from its row, column, box, and cage.
Desktop
Keyboard
1–9 place a digit, arrow keys move the selection, Backspace or Delete erases, N toggles notes, U or Ctrl+Z undoes, H reveals a hint, and M mutes the sound. Every on-screen button also has a visible focus ring.
Difficulty
Easy to Expert
The difficulty setting controls the largest cage the puzzle generator will use. Bigger cages have more possible digit sets, so they give you less to hold onto at the start and reward the 45 rule and elimination. Every level is guaranteed a single solution.
Cages of up to 3 cells
Easy
Small cages and plenty of forced pairs such as 3, 4, 16, and 17. A good place to learn cage combinations without needing any advanced technique.
Cages of up to 4 cells
Medium
Four-cell cages appear, so you will start combining cage totals with row, column, and box constraints, and pencil notes become useful.
Cages of up to 5 cells
Hard
Larger cages mean more candidate combinations per cage. The 45 rule and comparing neighbouring cages become the main way forward.
Cages of up to 6 cells
Expert
The largest, most irregular cages. Expect to chain innies and outies, eliminate combinations across several cages, and keep careful notes.
Avoid these
Common mistakes
Repeating a digit inside a cage
The most common error: a cage of 10 over three cells is not 3+3+4. Digits never repeat inside a cage, so always check that your combination uses distinct digits.
Guessing instead of eliminating
With a three-mistake limit, a 50/50 guess is expensive. When two digits remain, look for another cage or region that separates them before committing.
Forgetting the row, column, and box
A cage total can be satisfied and still put two 7s in one row. Always check the sudoku constraints alongside the cage arithmetic.
Skipping notes on harder levels
On Hard and Expert the cage combinations overlap in ways that are impossible to hold in your head. Keep notes on and let auto-clearing prune them as you place digits.
Tips
Habits that solve puzzles faster
- Start with the cages that have only one possible combination: 3, 4, 16, and 17 in two cells; 6, 7, 23, and 24 in three.
- Every row, column, and box adds up to 45 — use it to find a missing cell when all but one cell of a region is caged.
- Turn on notes and pencil in the digits a cage allows; the row, column, and box then remove the rest.
- A digit cannot repeat inside a cage, so a 3-cell cage totalling 8 can only be 1+2+5 or 1+3+4.
- When a cage spans two rows or columns, compare it with the 45 total to isolate a single leftover cell.
- Single-cell cages are free digits — place them first and let them shrink neighbouring cages.
For the full technique guide, see Killer Sudoku strategy and tips.
Devices
Play anywhere, saved on your device
Desktop browsers
Chrome, Edge, Firefox, and Safari all run the game with no plugin. Click a cell and type a digit, use the arrow keys to move, and press N for notes, U to undo, and H for a hint.
Phones and tablets
The layout is touch-first. The 9×9 grid fits the width of a phone in portrait with the number pad and tools directly beneath it, so no scrolling is needed mid-puzzle.
Progress on this device
Best time, games played, win streak per difficulty, and your current unfinished puzzle live in local browser storage. They stay until you reset them or clear site data, and they do not sync between devices.