How to play
Killer Sudoku keeps the rules of ordinary Sudoku and adds one more layer. Fill the 9×9 grid so that every row, column and 3×3 box contains the digits 1 to 9 once each. The grid is also split into cages drawn with dashed lines. The small number in the top-left corner of a cage is the sum of its digits, and a digit may not repeat inside a cage.
Choose a level and press Start. Easy puzzles begin with four to nine digits already placed and use cages of two to four cells. Medium puzzles start from an empty grid. Hard puzzles also start empty and have fewer, larger cages of up to six cells. The clock stops while the game is paused or the tab is hidden, and progress is saved in this browser.
Controls
- Select a cell by clicking or tapping it, or move with the arrow keys.
- Enter a digit with the keys 1–9 or the number buttons; the small number under each button shows how many of that digit are still missing. Backspace, Delete, 0 or Erase clears the cell.
- Notes: N or the Notes button switches between digits and pencil marks, and Shift plus a digit writes a note directly. Placing a digit removes that note from every cell in the same row, column, box and cage.
- Undo and redo: the buttons, U and R, or Ctrl+Z and Ctrl+Y.
- Check (C) crosses out every digit that disagrees with the solution. Hint (H) fills in one cell: the selected one if it is empty or wrong, otherwise a wrong cell, otherwise the empty cell with the fewest possible digits.
- P or Esc pauses, F switches to full screen, and the New puzzle (plus) button opens the level choice.
How it works
Every row, column and box holds 1 + 2 + … + 9 = 45. That one fact drives most Killer Sudoku solving. If the cages lying completely inside a row add up to 38, the other cells of that row must add up to 45 − 38 = 7. Solvers call this the rule of 45, and it also works for boxes and for blocks of neighbouring rows or columns.
The second tool is the list of combinations. Two cells adding up to 3 can only be 1 and 2, and a sum of 17 can only be 8 and 9. Three cells adding up to 6 must be 1, 2 and 3, and four cells adding up to 30 must be 6, 7, 8 and 9. Middle sums are looser: 15 in three cells has eight possible sets. The helper next to the board lists every set of different digits that fits the selected cage. It crosses out a set when it lacks a digit you have entered in the cage, or when its other digits cannot be fitted into the empty cells because those cells already see them in their row, column or box.
Conflict marking, which you can switch off, underlines a digit in red when it repeats in a row, column, box or cage, and turns a cage sum red when a full cage has the wrong total or its digits make the sum impossible to reach. It follows only the rules and never gives away the solution; Check compares your digits with the solution.
Every puzzle has exactly one solution. The collection was built offline: fill a grid, cut it into tiny cages, then keep merging neighbouring cages while a solver still finds only one solution. A second, independent solver confirmed every puzzle afterwards. The level comes from the steps a logical solver needed: Easy needs only single candidates and cage combinations, Medium also needs a step such as the rule of 45 on one row, column or box, or a pair, and Hard needs a harder step such as the rule of 45 over several rows or columns, or a triple. No puzzle requires guessing. A puzzle may also appear rotated or mirrored, which keeps its solution unique.
Where to start
- Start with cages that have only one combination, such as 3 or 4 in two cells, or 6, 7, 23 or 24 in three cells.
- Add up the cages inside one row, column or box and use 45 to find the total of the cells left over.
- If a two-cell cage of sum 3 lies in one row, 1 and 2 are used there, so cross them off the rest of that row.
- Write notes for big cages and watch the helper strike out sets as you fill digits in.
- Solve a Medium puzzle without hints, then try to beat your best time.
Frequently asked questions
Do I need to be good at arithmetic?
Only small additions. Most of the work is recognising combinations, and the helper lists them for you.
Why do Medium and Hard puzzles start empty?
The cage sums carry all the information. A Killer Sudoku can have a single solution without any starting digits, which is the classic form of the puzzle.
Does a hint count against me?
Hints are shown on the result card. A puzzle solved with a hint is still solved, but its time does not count as a best time.