Killer Sudoku — Play Free Online

Killer Sudoku is classic Sudoku with the clues taken away and arithmetic put in their place. There are no given digits at all. Instead the grid is divided into dotted outlines called cages, each printed with the total its squares must add up to — and every puzzle here still has exactly one answer, reachable without guessing.

The rules, in full

Everything true of ordinary Sudoku is still true: each row, each column and each 3×3 box holds the digits 1 to 9 once each. Killer adds two rules on top, and they are the entire game.

• The digits inside a cage must add up to the small number printed in its top-left corner.
• A digit cannot repeat inside a cage — even where the cage bends across two rows or two boxes, which ordinary Sudoku would allow.

That second rule is the one newcomers forget, and it is doing more work than the arithmetic. A three-cell cage totalling 7 is not merely "three digits that sum to 7"; it is three different digits, which leaves only 1+2+4.

Where to start when the board is empty

A blank grid with no givens looks unapproachable, and the first move is always the same: find the cages whose totals leave no choice. A total is only interesting when it is extreme for the number of squares it covers — a two-cell cage adding to 3 has to be 1 and 2, while one adding to 10 could be any of four pairs and tells you almost nothing.

Those are not the only forced cages, but they are the ones worth memorising. Mark the two or three digits into the squares as pencil notes, and the ordinary Sudoku techniques take over from there — the moment a cage settles a digit, it starts eliminating that digit from a row, a column and a box, exactly as a given would.

The rule of 45

Every row, column and box contains 1 through 9, so every one of them totals 45. That turns the cage sums into a second, independent way of reading the board. If the cages sitting wholly inside one box add up to 38, the squares left over in that box must hold 7 between them. If a single square of a cage pokes out of a box, you can total the box's cages, subtract 45, and read that square's digit straight off.

Killer players call those leftovers innies and outies, and on a hard grid they are often the only way in. It is the one technique with no equivalent in ordinary Sudoku, and it is worth practising deliberately.

What the four levels mean here

Difficulty in Killer is carried almost entirely by cage size, so that is what these levels change. Larger cages mean more combinations to hold in your head and fewer totals that force a digit outright.

• Easy — cages of at most two squares. Almost every total is a pair, so the combination table above solves much of the grid.
• Medium — up to three squares. Triples bring in the rule of 45.
• Hard — up to four. Fewer forced cages, so the opening takes real searching.
• Expert — up to five squares, and the fewest cages on the board. Expect to work the innies and outies before a single digit is certain.

There are 8 boards at each level. Every one was checked to have exactly one solution before it was published — twice, in fact, once while it was being built and again before it was saved. In Killer that matters more than in classic Sudoku: with no givens to cross-check against, a grid with two answers would not be an interesting ambiguity, it would simply be unfinishable.

Killer and the rest of the site

The elimination techniques are shared, so the technique guides apply here once the arithmetic has given you a foothold — naked pairs and box/line reductions behave identically on a Killer grid. If you are new to Sudoku generally, start with how to play and the easy puzzles before coming back to this.

Scores here are kept off the leaderboard on purpose. A Killer board has all 81 squares to fill where a classic grid has roughly half, so its totals are not comparable with the rest of the site's and would make every classic result look poor against a different game.

Last reviewed 8 November 2026