Killer Sudoku

How to use it

• Play — a puzzle at the chosen level, one you have not played yet. Choosing another level starts a game at it.
• Daily — one puzzle per date, numbered from 21 October 2026, harder through the week: Monday Easy, Tuesday Medium, Wednesday Medium, Thursday Hard, Friday Hard, Saturday Expert, Sunday Medium. A daily you are playing at midnight stays on the page until you finish it. Opened on a later day, the page shows that day's puzzle instead, and an earlier one you had started counts as played, not won.

Select a cell, then a digit on the pad or the keyboard. The arrow keys move around the board, Backspace, Delete or 0 clears a cell, N switches notes on and off, and Shift with a digit adds a note. Ctrl+Z undoes and Ctrl+Y redoes; the Undo and Redo buttons do the same on a phone.

Notes are pencil marks. Placing a digit clears it from the notes of its row, column, box and cage. Auto fills every empty cell with the digits its cage's sets allow, less those its row, column, box and cage already hold. A digit that repeats in a row, column, box or cage, or a full cage whose digits do not add to its sum, shows in red and underlined.

The combination helper beside the board (under the pad on a phone) lists every set of different digits that makes the selected cell's cage, and the digits every set still open holds. A set is struck through once the grid rules it out: it lacks a digit already in the cage, or its digits cannot spread over the cage's notes and what their rows, columns and boxes hold. Tap a set to cross it out yourself.

Hint first points out a digit that is wrong. Otherwise it finds the simplest next step from your correct digits and explains it; the second press applies it. Where no technique finds a step, it offers to reveal a cell. Show mistakes marks every wrong digit as you go.

The timer starts with the first move and stops while the tab is hidden or the game is paused; pausing hides the grid. Stats are kept per level: games played, wins, current and best streak, best and average time. A game finished after a hint, a revealed cell or with mistakes shown counts as played, not won, and ends your streak, and so does one left for a new game once it holds a digit or a note.

Print puzzles prints 1, 2, 4 or 6 puzzles a page, up to 5 pages, on A4 or Letter, with the dotted cages and their sums, then their solutions, 12 to a page, when Solutions page is ticked.

How to solve Killer Sudoku

Cage combinations. A cage's digits are all different, so its sum and size allow only certain sets. A 2-cell cage of 3 can only be 1 + 2, and a 3-cell cage of 23 only 6 + 8 + 9. Where several sets fit, the digits around the cage strike some out: a 2-cell cage of 10 can be 1 + 9, 2 + 8, 3 + 7, 4 + 6, but with both cells in a row that already holds a 9, only 2 + 8, 3 + 7, 4 + 6 remain.

The 45 rule. Every row, column and box holds 1 to 9 once, which adds to 45. Add up the cages wholly inside a row (or several rows, a column or a box), and the cells left over, the innies, make up the rest. Cages that reach out of it add to 45 plus the cells outside, the outies. One innie or outie is then a digit; two or three are a small cage of their own.

The puzzles come from a fixed bank of 1,000 per level. Each starts from a random solved grid cut into cages of one or two cells, and neighbouring cages merge one pair at a time, each merge kept only while the puzzle can still be solved by logic within its level. A solving path that fills the grid by logic alone also proves that the puzzle has only one solution. The largest cage is 4 cells at Easy and 6 at Expert, and an Easy puzzle has at most 2 single-cell cages, an Expert one none.

Grading solves a puzzle the way a person does: it takes the simplest step any technique finds, applies it, and starts again from the simplest. The level is the hardest technique that path needs, and Hint follows the same path from wherever you are.

Worked hints

Quick reference

Frequently asked questions

See also

• Sudoku — for the classic puzzle with givens, at six levels.
• Kakuro — for sums across and down with no rows or boxes.
• Calcudoku — for cages with +, −, × and ÷ on smaller grids.